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Title: Comparative Ecology of Pinyon Mice and Deer Mice in Mesa Verde National Park, Colorado
Author: Douglas, Charles L.
Language: English
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Transcriber's Note

Italic text is presented as _Text_.


  Volume 18, No. 5, pp. 421-504

  August 20, 1969

  Comparative Ecology of Pinyon Mice
  and Deer Mice in
  Mesa Verde National Park, Colorado




  Editors of this number: Frank B. Cross, Philip S. Humphrey,
  J. Knox Jones, Jr.

  Volume 18, No. 5, pp. 421-504
  Published August 20, 1969

  Lawrence, Kansas






  INTRODUCTION                                                    424
    Physiography                                                  425
    Vegetation and Climate                                        427

  ACKNOWLEDGMENTS                                                 427


  HOME RANGE                                                      435
    Calculations of Home Range                                    437
    Analysis by Inclusive Boundary Strip                          439
    Analysis by Exclusive Boundary Strip                          440
    Adjusted Length of Home Range                                 440
    Distance Between Captures                                     441

  VEGETATIONAL ANALYSIS OF HABITATS                               446

  MICROCLIMATES OF DIFFERENT HABITATS                             450

  HABITAT PREFERENCE                                              459

  NESTING AND NEST CONSTRUCTION                                   461

  REPRODUCTION                                                    465

  GROWTH                                                          469

  PARENTAL BEHAVIOR                                               471
    Transportation of Young                                       472

  CHANGES OWING TO INCREASE IN AGE                                475

  ANOMALIES AND INJURIES                                          476
    Losses Attributed to Exposure in Traps                        477
    Dental Anomalies                                              478
    Anomalies in the Skull                                        478

  FOOD HABITS                                                     479

  WATER CONSUMPTION                                               482

  PARASITISM                                                      491

  PREDATION                                                       493

  DISCUSSION                                                      495
    Factors Affecting Population Densities                        497
    Adaptations to Environment                                    499

  LITERATURE CITED                                                501


Centuries ago in southwestern Colorado the prehistoric Pueblo
inhabitants of the Mesa Verde region expressed their interest in mammals
by painting silhouettes of them on pottery and on the walls of kivas.
Pottery occasionally was made in the stylized form of animals such as
the mountain sheep. The silhouettes of sheep and deer persist as
pictographs or petroglyphs on walls of kivas and on rocks near
prehistoric dwellings. Mammalian bones from archeological sites reveal
that the fauna of Mesa Verde was much the same in A. D. 1200, when the
Pueblo Indians were building their magnificent cliff dwellings, as it is
today. One of the native mammals is the ubiquitous deer mouse,
_Peromyscus maniculatus_. The geographic range of this species includes
most of the United States, and large parts of Mexico and Canada.

Another species of the same genus, the pinyon mouse, _P. truei_, also
lives on the Mesa Verde. The pinyon mouse lives mostly in southwestern
North America, occurring from central Oregon and southern Wyoming to
northern Oaxaca. This species generally is associated with pinyon pine
trees, or with juniper trees, and where the pinyon-juniper woodland is
associated with rocky ground (Hoffmeister, 1951:vii).

_P. maniculatus rufinus_ of Mesa Verde was considered to be a mountain
subspecies by Osgood (1909:73). The center of dispersion for _P. truei_
was in the southwestern United States, and particularly in the Colorado
Plateau area (Hoffmeister, 1951:vii). The subspecies _P. truei truei_
occurs mainly in the Upper Sonoran life-zone, and according to
Hoffmeister (1951:30) rarely enters the Lower Sonoran or Transition
life-zones. _P. maniculatus_ and _P. truei_ are the most abundant of the
small mammals in Mesa Verde National Park, which comprises about
one-third of the Mesa Verde land mass.

Under the auspices of the Wetherill Mesa Archeological Project, the
flora of the park recently was studied by Erdman (1962), and by Welsh
and Erdman (1964). These studies have revealed stands of several
distinct types of vegetation in the park and where each type occurs.
This information greatly facilitated my study of the mammals inhabiting
each type of association. The flora and fauna within the park are
protected, in keeping with the policies of the National Park Service,
and mammals, therefore, could be studied in a relatively undisturbed

Thus, the abundance of these two species of _Peromyscus_, the botanical
studies that preceded and accompanied my study, the relatively
undisturbed nature of the park, and the availability of a large area in
which extended studies could be carried on, all contributed to the
desirability of Mesa Verde as a study area.

My primary purpose in undertaking a study of the two species of
_Peromyscus_ was to analyze a number of ecological factors influencing
each species--their habitat preferences, how the mice lived within their
habitats, what they ate, where they nested, what preyed on them, and how
one species influenced the distribution of the other. In general, my
interest was in how the lives of the two species impinge upon each other
in Mesa Verde.


The Mesa Verde consists of about 200 square miles of plateau country in
southwestern Colorado, just northeast of Four Corners, where Colorado,
New Mexico, Arizona and Utah meet. In 1906, more than 51,000 acres of
the Mesa Verde were set aside, as Mesa Verde National Park, in order to
protect the cliff dwellings for which the area is famous.

The Mesa Verde land mass is composed of cross-bedded sandstone strata
laid down by Upper Cretaceous seas. These strata are known locally as
the Mesaverde group, and are composed, from top to bottom, of Cliff
House sandstone, the Menefee formation, the Point Lookout sandstone, the
well known Mancos shale, and the Dakota sandstone, the lowest member of
the Cretaceous strata. The Menefee formation is 340 to 800 feet thick,
and contains carbonaceous shale and beds of coal.

There are surface deposits of Pleistocene and Recent age, with gravel
and boulders of alluvial origin; colluvium composed of heterogeneous
rock detritus such as talus and landslide material; and alluvium
composed of soil, sand, and gravel. A layer of loess overlays the
bedrock of the flat mesa tops in the Four Corners area. The earliest
preserved loess is probably pre-Wisconsin, possibly Sangamon in age
(Arrhenius and Bonatti, 1965:99).

The North Rim of Mesa Verde rises majestically, 1,500 feet above the
surrounding Montezuma Valley. Elevations in the park range from 8,500
feet at Park Point to about 6,500 feet at the southern ends of the
mesas. The Mesa Verde land mass is the remnant of a plateau that erosion
has dissected into a series of long, narrow mesas, joined at their
northern ends, but otherwise separated by deep canyons. The bottoms of
these canyons are from 600 to 900 feet below the tops of the mesas.

The entire Mesa Verde land mass tilts southward; Park Headquarters, in
the middle of Chapin Mesa (Fig. 1), is at about the same elevation as is
the entrance of the park, 20 miles by road to the north.

    [Illustration: FIG. 1: Map of Mesa Verde National Park and vicinity,
       showing major trapping localities from 1961-1964. Trapping
       localities are designated in the text as follows: 1) North End
       Wetherill Mesa 2) Rock Springs 3) Mug House 4) Bobcat Canyon
       Drainage 5) North of Long House 6) Juniper-Pinyon-Bitterbrush
       Site 7) Navajo Hill 8) West of Far View Ruins 9) South of Far
       View Ruins, also general location of trapping grid 10) M-2
       Weather Station 11) East Loop Road Site 12) Big Sagebrush Stand,
       Southern end Chapin Mesa 13) Grassy Meadow, Southern end Moccasin
       Mesa 14) Bedrock Outcroppings, Southern end Moccasin Mesa
       15) 1/4 mi. SE Park Entrance 16) Meadow, 1 mi. SE Park Entrance
       17) Morfield Ridge.]

Vegetation and Climate

Mesa Verde is characterized by pinyon-juniper woodlands that extend
throughout much of the West and Southwest. Although the pinyon-juniper
woodland dominates the mesa tops, stands of Douglas fir occur in some
sheltered canyons and on north-facing slopes. Thickets of Gambel oak and
Utah serviceberry cover many hillsides and form a zone of brush at
higher elevations in the park. Aspens grow in small groups at the base
of the Point Lookout sandstone and at a few other sheltered places where
the supply of moisture suffices. Individual ponderosa pine are scattered
through the park, and stands of this species occur on some slopes and in
the bottoms of some sheltered canyons.

Tall sagebrush grows in deep soils of canyon bottoms, and in some burned
areas, and was found to be a good indicator of prehistoric occupation

The climate of Mesa Verde is semi-arid, and most months are dry and
pleasant. Annual precipitation has averaged about 18.5 inches for the
last 40 years. July and August are the months having the most rainfall.
Snow falls intermittently in winter, and may persist all winter on
north-facing slopes and in valleys. In most years, snow is melting and
the kinds of animals that hibernate are emerging by the first of April.

Because of the great differences in elevation between the northern and
southern ends of the mesas, differences in climate are appreciable at
these locations. Winter always is the more severe on the northern end of
the park, owing to persistent winds, lower temperatures, and more snow.
The northern end of the park is closer to the nearby La Platta Mountains
where ephemeral storms of summer originate. They reach the higher
elevations of the park first, but such storms dissipate rapidly and are
highly localized. The northern end of the park therefore receives much
more precipitation in summer and winter than does the southern end.

The difference in precipitation and the extremes in weather between the
northern and southern ends of the mesas affect the distribution of
plants and animals. Species of mammals, plants, and reptiles are most
numerous on the middle parts of the mesas, as also are cliff-dwellings,
surface sites, and farming terraces of the prehistoric Indians.

Anderson (1961) reported on the mammals of Mesa Verde National Park, and
Douglas (1966) reported on the amphibians and reptiles. In each of these
reports, earlier collections are listed and earlier reports are

I lived in Mesa Verde National Park for 28 months in the period July
1961 to September 1964, while working as Biologist for the Wetherill
Mesa Archeological Project, and the study here reported on is one of the
faunal studies that I undertook.


This study could not have been completed without the assistance and
encouragement of numerous persons. I am grateful to Dr. Olwen Williams,
of the University of Colorado, for suggesting this study and helping me
plan the early phases of it.

Mr. Chester A. Thomas, formerly Superintendent, and Mrs. Jean Pinkley,
formerly Chief of Interpretation at Mesa Verde National Park, permitted
me to use the park's facilities for research, issued collecting permits,
and in 1965 appointed me as a research collaborator in order that I
might complete my studies.

Dr. H. Douglas Osborne, California State College, Long Beach, formerly
Supervisory Archeologist of the Wetherill Mesa Project, took an active
interest in my research and provided supplies, transportation and
laboratory and field assistance under the auspices of the Wetherill
Project. His assistance and encouragement are gratefully acknowledged.

Mrs. Marilyn A. Colyer of Mancos, Colorado, ably assisted in analyzing
vegetation in the trapping grid; Mr. Robert R. Patterson, the University
of Kansas, assisted me in the field in October of 1963 and in August of
1965. Mr. James A. Erdman, United States Geological Survey, Denver,
formerly Botanist for the Wetherill Mesa Project, and Dr. Stanley L.
Welsh, Brigham Young University, identified plants for me in the field,
and checked my identifications of herbarium specimens. I owe my
knowledge of the flora in the park to my association with these two
capable botanists.

I am grateful to the following persons for identification of
invertebrates: D. Eldon Beck, fleas and ticks; Paul Winston, mites; V.
Eugene Nelson, mites; William Wrenn, mites; Wayne W. Moss, mites;
William B. Nutting, mites (_Desmodex_); Marilyn A. Colyer, insects; John
E. Ubelaker, endoparasites; Veryl F. Keen, botflies. George A. King,
Architect, of Durango, Colorado, prepared the original map for Figure 1.

Mr. Harold Shepherd of Mancos, Colorado, Senior Game Biologist, Colorado
Department of Fish, Game and Parks, obtained permission for me to use
the department's trapping grid near Far View Ruins, and provided me with
preserved specimens of mice.

Mr. Fred E. Mang Jr., Photographer, National Park Service, processed
large numbers of photomicrographs of plant epidermis. Dr. Kenneth B.
Armitage, The University of Kansas, offered valuable suggestions for the
study of water consumption in the two species of _Peromyscus_, and
permitted me to use facilities of the Zoological Research Laboratories
at The University of Kansas. Dr. Richard F. Johnston, The University of
Kansas, permitted me to house mice in his controlled-temperature room at
the Zoological Research Laboratories. I am grateful to all of the above
mentioned persons for their aid.

I acknowledge with gratitude the guidance, encouragement, and critical
assistance of Professor E. Raymond Hall throughout the course of the
study and preparation of the manuscript. I also extend my sincere thanks
to Professors Henry S. Fitch, Robert W. Baxter, and William A. Clemens
for their helpful suggestions and assistance.

To my wife, Virginia, I am grateful for encouragement and assistance
with many time-consuming tasks connected with field work and preparation
of the manuscript.

Travel funds provided by the Kansas Academy of Science permitted me to
work in the park in August, 1965. The Wetherill Mesa Project was an
interdisciplinary program of the National Park Service to which the
National Geographic Society contributed generously. I am indebted to the
Society for a major share of the support that resulted in this report.
This is contribution No. 44 of the Wetherill Mesa Project.


Trapping was begun in September of 1961 in order to analyze the
composition of rodent populations within the park. I used the method of
trapping employed by Calhoun (1948) in making the Census of North
American Small Mammals (N. A. C. S. M.). It consisted of two lines of
traps, each 1,000 feet long having 20 trapping stations that were 50
feet apart. The lines were either parallel at a distance of 400 feet
from each other, or were joined to form a line 2,000 feet long. Three
snap traps were placed within a five-foot radius of each station, and
were set for three consecutive nights. More than a dozen areas were
selected for extensive trapping (Fig. 1). Some of these were retrapped
in consecutive years in order to measure changes in populations.

One circular trapline of 159.5 feet radius was established in November
1961, and was tended for 30 consecutive days to observe the effect of
removing the more dominant species (Calhoun, 1959).

Other mouse traps and rat traps were set in suitable places on talus
slopes, rocky cliffs, and in cliff dwellings. Most of these traps were
operated for three consecutive nights.

In order to test hypotheses concerning habitat preferences of each of
the species of _Peromyscus_, several previously untrapped areas that
appeared to be ideal habitat for one species, but not for the other,
were selected for sampling. In the summers of 1963 and 1964 snap traps
were set along an arbitrary line through each of these areas. Traps were
placed in pairs; each pair was 20 feet from the adjacent pairs.

A mixture of equal parts of peanut butter, bacon grease, raisins, roman
meal and rolled oats was used as bait. Rolled oats or coarsely ground
scratch feed was used in areas where insects removed the mixture from
the traps.

Rodents trapped by me were variously prepared as study skins with
skulls, as flat skins with skulls, as skeletons, as skulls only, or as
alcoholics. Representative specimens were deposited in The University of
Kansas Museum of Natural History. In the course of my study, traps were
set in the following areas:

_Morfield Ridge_

In July 1959 a fire destroyed more than 2,000 acres of pinyon-juniper
forest (_Pinus edulis_ and _Juniperus osteosperma_) in the eastern part
of the park. The burned area extends from Morfield Canyon to Waters
Canyon, encompassing several canyons, Whites Mesa, and a ridge between
Morfield Canyon and Waters Canyon that is known locally as Morfield
Ridge (Fig. 1). Beginning on September 4, 1961, three pairs of traplines
were run on this ridge at elevations of 7,300 to 7,600 feet.

Vegetation in the trapping area consisted of dense growths of grasses
and herbaceous plants, which had covered the ground with seeds. In this
and in the following accounts, the generic and specific names of plants
are those used by Welsh and Erdman (1964). The following plants were
identified from the trapping area on Morfield Ridge:

  _Lithospermum ruderale_
  _Chenopodium pratericola_
  _Achillea millefolium_
  _Artemisia tridentata_
  _Aster bigelovii_
  _Chrysothamnus depressus_
  _Chrysothamnus nauseosus_
  _Helianthus annuus_
  _Helianthella_ sp.
  _Lactuca_ sp.
  _Lepidium montanum_
  _Quercus gambelii_
  _Agropyron smithii_
  _Bromus inermis_
  _Bromus japonicus_
  _Oryzopsis hymenoides_
  _Calochortus nuttallii_
  _Linum perenne_
  _Sphaeralcea coccinea_
  _Polygonum sawatchense_
  _Solidago petradoria_
  _Wyethia arizonica_
  _Nicotiana attenuata_
  _Fendlera rupicola_
  _Penstemon linarioides_

Only _Peromyscus maniculatus_, _Perognathus apache_ and _Reithrodontomys
megalotis_ were taken in this area (Table 1). Many birds inhabit this
area, including hawks, ravens, towhees, jays, juncos, woodpeckers,
doves, sparrows and titmice. Rabbits, badgers and mule deer also live in
the area. Only two reptiles, a horned lizard and a collared lizard, were

_South of Far View Ruins_

Two parallel trap lines were established on October 4, 1961, in the area
immediately south of Far View Ruins (Fig. 1). In altitude, latitude and
geographical configuration the area is similar to that trapped in the
Morfield burn, but the Chapin Mesa site had not been burned.

Canopy vegetation is pinyon-juniper forest. A dense understory was made
up of _Amelanchier utahensis_ (serviceberry), _Cercocarpos montanus_
(mountain mahogany), _Purshia tridentata_ (bitterbrush), and _Quercus
gambelii_ (Gambel oak). The ground cover consisted of small clumps of
_Poa fendleriana_ (muttongrass), and _Koeleria cristata_ (Junegrass),
intermingled with growths of one or more of the following:

  _Artemisia nova_
  _Solidago petradoria_
  _Sitanion hystrix_
  _Astragalus scopulorum_
  _Lupinus caudatus_
  _Eriogonum alatum_
  _Penstemon linarioides_
  _Eriogonum racemosum_
  _Eriogonum umbellatum_
  _Polygonum sawatchense_
  _Amelanchier utahensis_
  _Purshia tridentata_
  _Comandra umbellata_

Seeds of _Cercocarpos montanus_ covered the ground under the bushes in
much of the trapping area, and large numbers of juniper berries were on
the ground beneath the trees. Individuals of _P. truei_ and _P.
maniculatus_ were caught in this area (Table 1).

Several deer, rabbits, one coyote, and numerous birds were seen in the
area. No reptiles were noticed, but they were not searched for. A
mountain lion was seen in this general area two weeks after trapping was

_West of Far View Ruins_

Three pairs of traplines were run west of Far View Ruins in an area
comparable in vegetation, altitude, general topography, and
configuration to the area previously described. The elevations concerned
are typical of the middle parts of mesas throughout the park. This area
differs from the trapping area south of Far View Ruins and the one on
Morfield Ridge in being wider and on the western side of the mesa.

The woody understory was sparse in most places, and where present was
composed of _Cercocarpos montanus_, _Purshia tridentata_, _Fendlera
rupicola_ (fendlerbush), _Amelanchier utahensis_, _Quercus gambelii_,
and _Artemisia tridentata_ (sagebrush). The herbaceous ground cover was
dominated by _Solidago petradoria_ (rock goldenrod), and
grasses--including _Poa fendleriana_, _Oryzopsis hymenoides_, and
_Sitanion hystrix_. Other herbaceous species were as follows:

  _Echinocercus coccineus_
  _Achillea millefolium_
  _Aster bigelovii_
  _Wyethia arizonica_
  _Lepidium montanum_
  _Lupinus caudatus_
  _Yucca baccata_
  _Linum perenne_
  _Eriogonum racemosum_
  _Eriogonum umbellatum_
  _Polygonum sawatchense_
  _Delphinium nelsonii_
  _Penstemon linarioides_

Fresh diggings of pocket gophers were observed along the trap lines.
Badger tunnels were noted in numerous surface mounds that are remnants
of prehistoric Indian dwellings, but no badgers were seen. Numerous deer
and several rabbits were present. Juncos, two species of jays, and
woodpeckers were seen daily. No reptiles were observed.

Both _Peromyscus maniculatus_ and _P. truei_ were caught in this area
(Table 1).

_Big Sagebrush Stand, South Chapin Mesa_

A circular trapline, 1,000 feet in circumference, was established on
November 16, 1961, in a stand of big sagebrush, and was operated for 30
consecutive nights.

The vegetation of the trapping area was predominantly _Artemisia
tridentata_ (big sagebrush), interspersed with a few scattered seedlings
of pinyon and juniper. This stand was burned in 1858 (tree-ring date by
David Smith) and some charred juniper snags still stood. The deep sandy
soil also supported a variety of grasses and a few other small plants.
The following species were common in this area:

  _Bromus inermis_
  _Oryzopsis hymenoides_
  _Poa fendleriana_
  _Sitanion hystrix_
  _Solidago petradoria_
  _Orthocarpus purpureo-albus_

The 15 to 20 acres of sagebrush were surrounded by pinyon-juniper
forest. The trapping station closest to the forest was approximately 100
feet from the edge of the woodland. More _P. truei_ than _P.
maniculatus_ were caught here (Table 1).

_East Loop Road, Chapin Mesa_

The trapping area lies north of Cliff Palace, eastward of the loop road,
at elevations of 6,875 to 6,925 feet. Two pairs of traplines were run
from January 9, 1962, to January 12, 1962, and from February 13 to 15,

Vegetation was pinyon-juniper woodland with an understory of mixed
shrubs. One to four inches of old snow covered the ground during most of
the trapping period, but the ground beneath trees and shrubs was
generally clear, providing suitable location for traps.

Numerous juncos and jays were seen in this area; deer and rabbits also
were present.

Individuals of _P. truei_ and of _P. maniculatus_ were taken (Table 1).

_Navajo Hill, Chapin Mesa_

Navajo Hill is the highest point (8,140 feet) on Chapin Mesa. The top of
the hill is rounded and the sides slope gently southward and westward
until they level out into mesa-top terrain at elevations of 7,950 to
8,000 feet. The northern and eastern slopes of the hill drop abruptly
into the respective canyon slopes of the East Fork of Navajo Canyon and
the West Fork of Little Soda Canyon. The gradually tapering southwestern
slope of the hill extends southward for one mile and is bisected by the
main highway, which runs the length of the mesa top.

Heavy growths of grasses cover the ground; _Amelanchier utahensis_,
_Cercocarpos montanus_, and _Fendlera rupicola_ comprise the only tall
vegetation. Trees are lacking on this part of the mesa, except on the
canyon slopes, where _Quercus gambelii_ forms an almost impenetrable

Four traplines were run from May 4-7, 1962, and from May 9-12, 1962. _P.
maniculatus_ was taken but _P. truei_ was not present here in 1962, or
in 1964 or 1965 when additional trapping was performed as a check on
populations (Table 1).

Other species trapped include the montane vole, long-tailed vole, and
Colorado chipmunk. Mule deer and coyotes were abundant in the area.
Striped whipsnakes, rattlesnakes and gopher snakes are known to occur in
this vicinity (Douglas, 1966).

_North End Wetherill Mesa_

In 1934 a widespread fire deforested large areas of pinyon-juniper
woodland on the northern end of Wetherill Mesa. The current vegetation
consists of shrubs with a dense ground cover of grasses. Many dead trees
still remain on the ground, providing additional cover for wildlife.

The trapping area was a wide, grassy meadow, three and a half miles
south of the northern end of the mesa. A pronounced drainage runs
through this area and empties into Rock Canyon. Four traplines were run
parallel to each other. The first lines were established on May 23,
1962, and the second pair on June 3, 1962.

Another pair of lines was run in a grassy area two miles south of the
northern escarpment of Wetherill Mesa. This area was one and a half
miles north of the above-mentioned area. These lines ran along the
eastern side of a drainage leading into Long Canyon. The vegetation was
essentially the same in both areas, and they will be considered

The vegetation was composed predominantly of grasses. _Quercus gambelii_
and _Amelanchier utahensis_ were the codominant shrubs. _Artemisia
tridentata_ and _Chrysothamnus depressus_ (dwarf rabbitbrush), were
common. Plants in the two areas included the following:

  _Juniperus scopulorum_
  _Symphoricarpos oreophilus_
  _Artemisia ludoviciana_
  _Sitanion hystrix_
  _Stipa comata_
  _Astragalus scopulorum_
  _Artemisia tridentata_
  _Chrysothamnus depressus_
  _Helianthus annuus_
  _Tetradymia canescens_
  _Quercus gambelii_
  _Bromus tectorum_
  _Poa fendleriana_
  _Lupinus caudatus_
  _Yucca baccata_
  _Sphaeralcea coccinea_
  _Eriogonum umbellatum_
  _Amelanchier utahensis_
  _Fendlera rupicola_
  _Lomatium pinatasectum_

Individuals of _P. maniculatus_ and of _Reithrodontomys megalotis_ were
caught (Table 1).

    TABLE 1--Major Trapping Localities in Mesa Verde National Park,

    Vegetational Key as Follows: 1) Pinyon-Juniper-Muttongrass
       2) Pinyon-Juniper-Mixed Shrubs 3) Juniper-Pinyon-Bitterbrush
       4) Juniper-Pinyon-Mountain Mahogany 5) Grassland with Mixed Shrubs
       6) Big Sagebrush 7) Pinyon-Juniper-Big Sagebrush 8) Grassland.

  Column headings:

  A: Date
  B: No. trap nights
  C: _P. truei_
  D: _P. man._
  E: Type of vegetation

          Locality        |      A      |   B   |  C  |  D  | E
  Morfield Ridge          | Sept. 1961  |  1080 |   0 |  83 | 5
                          | Oct. 1963   |   360 |   0 |  13 | 5
                          |             |       |     |     |
  S. of Far View          | Oct. 1961   |   360 |  10 |  13 | 2
                          |             |       |     |     |
  W. of Far View          | Oct. 1961   |  1080 |  22 |  17 | 2
                          |             |       |     |     |
  South Chapin Mesa       | Nov.-Dec.   |  3600 |  16 |   9 | 6
                          |    1961     |       |     |     |
                          |             |       |     |     |
  East Loop Road          | Jan. 1962   |   720 |   6 |   2 | 2
                          |             |       |     |     |
  Navajo Hill             | May 1962    |   720 |   0 |  18 | 5
                          | Aug. 1964   |    20 |   0 |   2 | 5
                          | Aug. 1965   |    50 |   0 |   8 | 5
                          |             |       |     |     |
  N. Wetherill Mesa       | May-June    |  1080 |   0 |  57 | 5
                          |    1962     |       |     |     |
                          |             |       |     |     |
  Bobcat Canyon Drainage  | June 1962   |   360 |   0 |   0 | 6
                          |             |       |     |     |
  N. of Long House        | June 1962   |  1080 |   3 |   4 | 1
                          |             |       |     |     |
  Mug House--Rock Springs | Aug. 1962   |   720 |   8 |  14 | 4
                          | Aug. 1963   |   720 |   9 |   7 | 4
                          |             |       |     |     |
  S. Wetherill Mesa       | Aug. 1962   |   720 |   0 |   5 | 3
                          |             |       |     |     |
  1 mi. SE Park Entr.     | June 1963   |    50 |   0 |  16 | 7
                          |             |       |     |     |
  1/4 mi. SE Park Entr.   | July 1963   |   100 |   0 |   7 | 8
                          |             |       |     |     |
  M-2 Weather Sta.        | May 1964    |    25 |   2 |   0 | 1
                          |             |       |     |     |
  8 mi. S North Rim       |             |       |     |     |
      Moccasin Mesa       | Aug. 1964   |   100 |   0 |   3 | 8
                          |             |       |     |     |
  10 mi. S North Rim      |             |       |     |     |
      Moccasin Mesa       | Aug. 1964   |    25 |   2 |   0 | 2

_Bobcat Canyon Drainage_

Bobcat Canyon, a large secondary canyon on the eastern side of Wetherill
Mesa, is a major drainage for much of the mesa at its widest part. The
mesa top drains southeast into a pour-off at the head of Bobcat Canyon.
A stand of big sagebrush, _Artemisia tridentata_, grows in the sandy
soil of the drainage, and extends northwest for several hundred yards
from the pour-off. The sagebrush invades the pinyon-juniper forest at
the periphery of the area.

Two traplines were set in the drainage, with trapping stations at
intervals of 25 feet. The lines traversed elevations of 7,000 to 7,100
feet, and were run from June 26 to 29, 1962.

Grasses are the most abundant plants in the ground cover. _Artemisia
dracunculus_ is common in the drainage, and _A. nova_ grows around the
periphery of the drainage. Other species occurring in this stand

  _Aster bigelovii_
  _Tetradymia canescens_
  _Tragopogon pratensis_
  _Bromus tectorum_
  _Poa fendleriana_
  _Sitanion hystrix_
  _Stipa comata_
  _Lupinus argenteus_
  _Calochortus gunnisonii_
  _Sphaeralcea coccinea_
  _Phlox hoodii_
  _Eriogonum umbellatum_
  _Peraphyllum ramosissimum_
  _Purshia tridentata_
  _Penstemon linarioides_

No mice were caught in three nights of trapping (360 trap nights), and
only one mammal, a _Spermophilus variegatus_, was seen.

_North of Long House, Wetherill Mesa_

Pinyon-juniper forest with a dominant ground cover of _Poa fendleriana_
was described by Erdman (1962) as one of the three distinct types of
pinyon-juniper woodland on Wetherill Mesa. Such a woodland occurs
adjacent to the Bobcat Canyon drainage, and is continuous across the
Mesa from above Long House to the area near Step House. Plants in the
ground cover include:

  _Cryptantha bakeri_
  _Opuntia rhodantha_
  _Chrysothamnus depressus_
  _Solidago petradoria_
  _Koeleria cristata_
  _Lupinus argenteus_
  _Yucca baccata_
  _Phlox hoodii_
  _Eriogonum racemosum_
  _Eriogonum umbellatum_
  _Cordylanthus wrightii_
  _Pedicularis centranthera_
  _Penstemon linarioides_
  _Penstemon strictus_

Two traplines were run from July 9 to 12, 1962, in the area south of the
Bobcat Canyon drainage at an elevation of 7,100 feet. No mice were
caught in three nights of trapping. Four additional lines were
established on July 24, 1962, and were run for three nights, in the area
north of the Bobcat Canyon drainage at elevations of 7,100 to 7,150

_P. maniculatus_ and _P. truei_ were caught here (Table 1). This
vegetational association may have few rodents because there is a
shortage of places where they can hide. Although _Poa fendleriana_ is
abundant, the lack of shrubs leaves little protective cover for mammals.

_Mug House--Rock Springs_

A juniper-pinyon-mountain mahogany association extends from the area of
Mug House to Rock Springs, on Wetherill Mesa. On that part of the ridge
just above Mug House, the understory is predominantly _Cercocarpos
montanus_ (mountain mahogany), but northward toward Rock Springs the
understory changes to _Fendlera rupicola_, _Amelanchier utahensis_,
_Cercocarpos_, and _Purshia tridentata_. The ground cover is essentially
the same as that in the pinyon-juniper-muttongrass association described

Four traplines were run from July 31 to August 2, 1962, and from August
13 to 15, 1963. These lines ran northwest-southeast, starting 1,000 feet
southeast of, and ending 3,000 feet northwest of, Mug House. The lines
traversed elevations of 7,225 to 7,325 feet. Individuals of _P.
maniculatus_ and _P. truei_ were caught here (Table 1).

Deer and rabbits inhabit the trapping area. Bobcats have been seen, by
myself and by others, near Rock Springs. Lizards of the genera
_Cnemidophorus_ and _Sceloporus_, as well as gopher snakes were seen in
this area.


Three pairs of traplines were run from August 7-9, 1962, in a
juniper-pinyon-bitterbrush stand on the southern end of Wetherill Mesa,
starting 200 yards southwest of Double House (Fig. 1).

The forest on the southern end of the mesas consists of widely-spaced
trees, which reflect the low amounts of precipitation at these lower
elevations. Juniper trees are more numerous than pinyons, and both
species are stunted in comparison to trees farther north on the mesa.
_Purshia tridentata_ (bitterbrush) is the understory codominant.
_Artemisia nova_ (black sagebrush) is present and grasses are the most
abundant plants in the ground cover. Herbaceous species in the sparse
ground cover include the following:

  _Opuntia polyacantha_
  _Solidago petradoria_
  _Lathyrus pauciflorus_
  _Penstemon linarioides_
  _Lupinus caudatus_
  _Yucca baccata_
  _Phlox hoodii_

Only _P. maniculatus_ was caught in this stand; all mice were caught in
the first night of trapping.

Five areas were selected for trapping in the summers of 1963 or 1964, in
order to test hypotheses concerning habitat preferences of each of the
species of _Peromyscus_. Four of these areas appeared to be ideal
habitat for one species, but not for the other. The fifth area was
expected to produce both species of _Peromyscus_. Each of these areas is
discussed below.

_One Mile Southeast of Park's Entrance_

A small stand of _Artemisia tridentata_, occurring one mile southeast of
the entrance to the park, is bordered to the north and northeast by a
grassy meadow, discussed in the following account. Kangaroo rats have
been reported in this general area, and I wanted to determine whether
_P. maniculatus_ and _Dipodomys_ occurred together there. Fifty trap
nights in this sagebrush, on June 20, 1963, yielded only _P.
maniculatus_ (Table 1).

_Meadow, One-Quarter Mile Southeast of Park's Entrance_

A grassy meadow lies just to the east of the highway into the park,
one-quarter of a mile southeast of the park's entrance. On July 30,
1963, one hundred traps were placed in two lines through the meadow, and
were run for one night. Only individuals of _P. maniculatus_ were caught
(Table 1).

_M-2 Weather Station, Chapin Mesa_

The M-2 weather station of the Wetherill Mesa Archeological Project was
on the middle of Chapin Mesa at an elevation of 7,200 feet. This site
was in an old C. C. C. area, about one mile north of the park's U. S.
Weather Bureau station. The vegetation surrounding the M-2 site was a
pinyon-juniper-muttongrass association. It was thought that both species
of _Peromyscus_ would occur in this habitat.

On May 10, 1964, 25 traps were placed in this area and were run for one
night. Only individuals of _P. truei_ were caught (Table 1).

_Grassy Meadow, Southern End Moccasin Mesa_

This large meadow is located eight miles south of the northern rim of
Moccasin Mesa. The meadow lies in a broad, shallow depression that forms
the head of a large drainage (Fig. 1). To the south of the meadow the
drainage deepens, then reaches bedrock as it approaches the pour-off.

On August 23, 1964, one hundred traps were set in pairs in a line
through the middle of the meadow; adjacent pairs were 20 feet from each
other. Only individuals of _P. maniculatus_ were caught (Table 1).

Grasses are dominant in the ground cover, and _Sphaeralcea coccinea_
(globe mallow) is codominant. The abundance of globe mallow is due to
the present and past disturbance of this meadow by a colony of pocket
gophers. Trees are absent in the meadow. Species of plants include the

  _Opuntia polyacantha_
  _Chenopodium_ sp.
  _Artemisia ludoviciana_
  _Chrysothamnus nauseosus_
  _Koeleria cristata_
  _Poa pratensis_
  _Lupinus ammophilus_
  _Calochortus gunnisonii_
  _Erigeron speciosus_
  _Gutierrezia sarothrae_
  _Tetradymia canescens_
  _Tragopogon pratensis_
  _Bromus tectorum_
  _Sphaeralcea coccinea_
  _Eriogonum racemosum_
  _Polygonum sawatchense_
  _Comandra umbellata_
  _Penstemon strictus_

_Bedrock Outcroppings, Southern End Moccasin Mesa_

Two miles south of the preceding site, much of the mesa is a wide
expanse of exposed bedrock, which extends approximately 100 feet inward
from the edges of the mesa. Pinyon-juniper-mixed shrub woodland adjoins
the bedrock.

On August 23, 1964, 25 traps were placed along the bedrock, near the
edge of the forest. Only two mice, both _P. truei_, were caught. (Table


In order to learn how extensively mice of different ages travel within
their habitats, whether their home ranges overlap, and how many animals
live within an area, it was necessary to determine home ranges for as
many mice, of each species, as possible (Hayne, 1949; Mohr and Stumpf,
1966; Sanderson, 1966).

In 1961, the Colorado Department of Fish, Game and Parks established a
permanent trapping grid in the area south of Far View Ruins (Fig. 1). The
grid was constructed and used by Mr. Harold R. Shepherd, Senior Game
Biologist, and his assistant, in the summers of 1961 and 1962, in a
study concerning the effect of rodents on browse plants used by deer.
The Department of Fish, Game and Parks allowed me to use the grid during
1963 and 1964, and also permitted me to use its Sherman live traps.

The grid is divided into 16 units, each with 28 stations (Fig. 2). Traps
at four stations (1a, 1b, 1c, 1d) are operated in each unit at the same
time, with two traps being set at each station. The traps are moved each
day in a counter-clockwise rotation to the next block of four stations
(2a, 2b, 2c, 2d) within each unit. The stations are arranged so that on
any given night, traps in adjacent units are separated by at least 200
feet. As a result, animals are less inclined to become addicted to
traps, for even within one unit they must move at least 50 feet to be
caught on consecutive nights.

    [Illustration: FIG. 2: Diagram of trapping grid for small mammals,
       showing units of subdivision. Trapping stations were numbered
       in each unit as shown in unit A.]

Traps were carefully shaded and a ball of kapok was placed in each trap
to provide protection against the killing temperatures that can develop
inside. In spite of these precautions, mice occasionally succumbed from
heat or cold. The traps were baited with coarsely-ground scratch feed.

Mammals trapped in the grid were inspected for molt, sexual maturity,
larvae of botflies, anomalies, and other pertinent data. Each animal was
marked by toe- and ear-clipping and then released. Four toes were used
on each front foot, and all five toes were used on each hind foot; two
toes were clipped on the right front foot to signify number nine. The
tip of the left ear was clipped to signify number 100, and the tip of
the right ear was clipped to signify 200. If 300 or more animals had
been captured, the tip of the tail would have been clipped to represent
number 300. A maximum of 799 animals could have been marked with this
system, which was used by Shepherd. I continued with it, starting my
listings with number one.

Only two mice were caught that had been marked in the previous season by

Live traps were operated in the trapping grid from July 9 through
October 25, 1963, and from June 25 through August 21, 1964. Traps were
rotated through all stations five different times (35 days) in 1963, and
twice (14 days) in 1964. Approximately three man hours were required
each day to service and rotate the traps to the next group of stations.
By the autumn of 1964, a total of 282 mice had been captured, marked and
released; these were handled 817 times. In 1963, 235 mice were caught
for an average of 20 captures per day; in 1964, 47 mice were caught for
an average of 9 captures per day.

Calculations of Home Range

A diagrammatic map of the trapping grid was drawn to scale with one
centimeter equal to 100 linear feet. Trapping stations were numbered on
the diagram to correspond with stations in the field. An outline of this
drawing also was prepared to the same scale, but station numbers were
omitted. Mimeographed copies of such a form could be placed over the
diagrammatic map and marks made at each station where an animal was
caught. A separate form was kept for each animal that was caught four or
more times.

In calculating home range, it was assumed that animals would venture
half-way from the peripheral stations, at which they were caught, to the
next station outside the range. A circle having a scaled radius of 25
feet (half the distance between stations) was inscribed around each
station on the periphery of the home range by means of a drafting
compass. The estimated range for each animal was then outlined on the
form by connecting peripheries of the circles. Both the inclusive
boundary-strip method and the exclusive boundary-strip method (Stickel,
1954:3) were used to estimate the ranges. The area encompassed within
the home ranges was measured by planimetering the outline of the
drawing. At least two such readings were taken for each home range; then
these planimeter values were converted into square feet.

The customary practice in delimiting home ranges on a scaled map of a
grid is to inscribe squares around the peripheral stations at which the
animal was trapped, and then to connect the exterior corners of these
squares (Stickel, 1954:3). If the distance between stations was 50 feet,
such squares would have sides 50 feet long. An easier method is to
inscribe a circle having a scaled radius of 25 feet around the
peripheral stations by means of a drafting compass. To my knowledge this
method has not been used previously and consequently has not been tested
by experiments with artificial populations.

To test the accuracy of this method, a "grid of traps" was constructed
by using 8-1/2 by 11 inch sheets of graph paper with heavy lines each
centimeter. The intersects of the heavier lines were considered as trap
stations. A "home range" of circular shape, 200 feet (4 cm.) in
diameter, with an area of 31,146 square feet (0.71 acre), was cut from a
sheet of transparent plastic. Another "home range" was made in an oblong
shape with rounded ends. This range measured 2 by 65 centimeters (100 by
325 feet) and had an area of 32,102 square feet (0.74 acre). Each
plastic range was tossed at random on sheets of graph paper for fifty
trials each. The range was outlined on the graph paper, then circles
having a scaled radius of 25 feet were inscribed around each "trap
station" within the range. The peripheries of the inscribed circles were
then connected and the estimated home range was delimited by the
exclusive boundary-strip method. The estimated range was measured by
planimetering, and the data were compared with the known home range
(Table 2).

It was found that when calculated by the exclusive boundary-strip
method, the circular home range was overestimated by 2.22 per cent. The
oblong home range was overestimated by only 1.50 per cent. Stickel
(1954:4) has shown that the exclusive boundary-strip method is the most
accurate of several methods of estimating home ranges, and in her
experiments this method gave an overestimate of two per cent of the
known range. Thus, my method of encircling the peripheral stations
yields results that are, on the average, as accurate as the more
involved method of inscribing squares about the trap stations, and saves
a great deal of time as well. My method probably yields better accuracy;
a perfect circle is easily drawn by means of a compass, whereas a
perfect square is more difficult to construct without a template.

It is generally understood that the estimated home range of an animal
tends to increase in size with each additional capture; this increase is
rapid at first, then slows. Theoretically, the more often an animal is
captured, the more reliable is the estimate of its home range. Most
animals, however, rarely are captured more than a few times. The
investigator must decide how many captures are necessary before the data
seem to be valid for estimating home ranges.

An animal must be trapped at a minimum of three stations before its home
range can be estimated, and even then the area enclosed in the triangle
will be much less than the actual home range. Some investigators have
plotted home ranges from only three captures (Redman and Selander,
1958:391), whereas others consider that far more captures are needed to
make a valid estimate of range (Stickel, 1954:5).

    TABLE 2--Summary of Data from Experiments in Calculating Home Ranges
    for an Artificial Population.

         |        |         |          | Actual |  Calculated    |
         |   No.  |  Trap   |  Shape   |  area  | area of range  |
  Series |   of   | spacing |    of    |   of   |  by exclusive  | ± S. D.
         | trials | in ft.  |  range   | range  | boundary-strip |
         |        |         |          | in ft. |     method     |
  A      |   50   |   50    | Circular | 31,146 |     31,782     |  9,600
  B      |   50   |   50    |  Oblong  | 32,102 |     32,583     |  9,466

In my study, 161 individuals of _P. truei_ were caught from one to 13
times each. The estimated home ranges of 10 individuals of _P. truei_,
each caught from eight to 13 times, were plotted and measured after each
capture from the fourth to the last. The percentage of the total
estimated range represented by the fourth through tenth captures was,
respectively, 52, 65, 73, 85, 88, 93, and 96 per cent.

Ninety-seven individuals of _P. maniculatus_ were caught from one to 10
times each. For five individuals that were each caught from seven to 10
times, the percentage of total estimated range represented by the fourth
through seventh captures was, respectively, 59, 69, 85, and 93 per cent.

The above percentages do not imply that the true home range of
individuals of these species can be reliably estimated after seven or 10
captures; the average percentages do, however, indicate a fairly rapid
increase in known size of home ranges between the fourth and tenth
captures. The estimated home ranges of _P. maniculatus_ tended to reach
maximum known size at about seven captures, whereas the estimated ranges
of _P. truei_ tended to attain maximum known size after nine or more
captures. The controversy over the number of captures of an individual
animal required for a reliable estimate of its home range was not
settled by my data.

I initially decided to estimate home ranges for animals caught five or
more times and at three or more stations. Of the 282 animals caught and
marked, only 48 were caught five or more times. Because of the small
numbers of _P. maniculatus_ that were caught five or more times, I
wanted to determine whether mice caught four times had an estimated
range that was significantly smaller than that of mice caught five
times. Eight individuals of _P. maniculatus_ were caught four times
each, and it seemed desirable to use the data from these mice if such
use was justified. Data from the 48 mice caught five or more times were
used for this testing.

By means of a T-test, I compared the estimated ranges of those 48 mice
following their fourth capture with ranges estimated after the fifth
capture. The results did not demonstrate significant differences between
the two sets of estimates; therefore, I decided to use data resulting
from four or more captures, and at three or more stations.

Table 3 shows estimations of the home ranges of males and females of
each species of _Peromyscus_. When the inclusive boundary-strip method
is used, the area encompassed by the range tends to be larger than the
area of the same range when estimated by the exclusive boundary-strip
method. Stickel (1954:4) has shown that the inclusive boundary-strip
method overestimates the home range by about 17 percent.

Analysis of Home Range by Inclusive Boundary-Strip Method

When all age groups were considered, the ranges of 16 males of _P.
truei_ averaged 20,000 to 80,000 square feet (ave. 47,333; S. D.
19,286). The sizes of home ranges were not significantly different (P >
0.05) between adult and subadult (including juveniles and young) males.

All females of _P. truei_ (22) had ranges encompassing 16,666 to 83,333
square feet (ave. 40,666; S. D. 17,566). Sizes of home ranges between
adult and non-adult females did not differ significantly. The mean range
of adult males of _P. truei_ did not differ from that of adult females
(P > 0.05).

Fifteen males of _P. maniculatus_ had ranges of 16,666 to 66,666 square
feet (ave. 34,222; S. D. 16,000); six adult males had ranges of 33,333
to 53,333 square feet (ave. 38,666). Sizes of home ranges of adult and
non-adult males of this species did not differ significantly.

Five females of _P. maniculatus_ had ranges of 33,333 to 76,666 square
feet (ave. 51,333; S. D. 15,913); of these, four adults had ranges of
33,333 to 53,333 square feet (ave. 45,000). Sizes of home ranges of
adult males of this species did not differ (P > 0.05) from those of
adult females.

The ranges of adult males of _P. truei_ were compared with ranges of
adult male of _P. maniculatus_; likewise the ranges of adult females of
each species were compared. In each case no difference was demonstrable
in sizes of ranges between the species.

The largest home range of any _P. truei_ was that of animal number 18, a
young male with an estimated home range of 133,333 square feet. This
animal was caught only five times, and his home range appeared
unusually large in relation to that of other young males of this
species; hence some of the widely-spaced sites of capture probably
represent excursions from the animal's center of activity, rather than
the true periphery of his range. These data were, therefore, not used in
further computations. Stickel (1954:13) pointed out the advisability of
removing such records from data to be used in calculations of home

Number eight had the largest home range of any female of _P. truei_; she
was captured ten times, and had a range of 83,333 square feet. The
vegetation within her range was pinyon-juniper woodland with
understories of _Amelanchier_, _Artemisia nova_ and _Purshia_. Most of
her home range was in the western half of unit H, but extended into
parts of units D, I, G and N.

The largest home range for adult males of either species was number
three of _P. truei_; he had a range of 80,000 square feet. The largest
range for an adult of _P. maniculatus_ was 66,666 square feet (Table 3).

Analysis of Home Range by Exclusive Boundary-Strip Method

Stickel (1954:4) has shown that under theoretical conditions the
exclusive boundary-strip method is the most accurate of several methods
of estimating home range. This method overestimates the known range by
only two percent.

Table 3 shows a comparison of home range calculations obtained for each
species, when calculated by inclusive and exclusive boundary-strip

The data for males and for females of each species were compared in the
same manner as in the inclusive boundary-strip method. The ranges of 16
male individuals of _P. truei_ encompassed 14,000 to 56,666 square feet
(ave. 34,333; S. D. 13,266); of these, the ranges of 10 adult males were
from 23,333 to 53,333 square feet (ave. 39,733). Twenty-two females of
this species had ranges of 13,333 to 50,000 square feet (ave. 27,199; S.
D. 8,820). Eighteen adult females had the same extremes, but the average
size of range, 28,000 square feet, was larger. Sizes of home ranges of
males and females did not differ significantly.

The ranges of fifteen males of _P. maniculatus_ encompassed 13,333 to
46,666 square feet (ave. 26,666; S. D. 10,180). Of these, six adults had
the same extremes in range, but an average size of 31,440 square feet.

The ranges of five females of _P. maniculatus_ varied from 28,000 to
53,333 square feet (ave. 37,199; S. D. 10,140). All but one of these
females were adults. The sizes of home ranges of males and females did
not differ significantly. No differences were found when ranges of adult
males, or adult females, of both species were compared.

Adjusted Length of Home Range

The adjusted length of the range also can be used as an expression of
home range. In this method, one-half the distance to the next trapping
station is added to each end of the line drawn between stations at
either end of the long axis of the range (Stickel, 1954:2).

The average length of home range for 15 males of _P. truei_ was 363 feet
(S. D. 105 ft.); for 22 females of this species 326 feet (S. D. 94 ft.);
for 14 males of _P. maniculatus_ 286 feet long (S. D. 94 ft.); and for
four females of this species 347 feet (S. D. 83 ft.). The mean lengths
of range of males and females differed significantly in _P.
maniculatus_, but not in _P. truei_. However, no difference was
demonstrable in mean sizes of ranges between males, or between females,
of the two species.

Distance Between Captures

The distance between captures has been used by several investigators as
an index of the extent of home range. More short than long distances
tend to be recorded when traps are visited at random, and when inner
traps of the range are more strongly favored (Stickel, 1954:10).

    TABLE 3--Summary of Data for Estimated Home Ranges of Mice from a
       Wild Population.

                  |                  |     |      | Estimated  |
      Type of     |     Species      | Sex |  No. | home range | ± S. D.
      Estimate    |                  |     |      | in sq. ft. |
  Inclusive       |    _P. truei_    |  M  |  16  |   47,333   | 19,286
  boundary-strip  |      "    "      |  F  |  22  |   40,666   | 17,566
                  |                  |     |      |            |
                  | _P. maniculatus_ |  M  |  15  |   34,222   | 16,000
                  |  "       "       |  F  |   5  |   51,333   | 15,913
  Exclusive       |    _P. truei_    |  M  |  16  |   34,333   | 13,266
  boundary-strip  |     "    "       |  F  |  22  |   27,199   |  8,820
                  |                  |     |      |            |
                  | _P. maniculatus_ |  M  |  15  |   26,666   | 10,180
                  |  "       "       |  F  |   5  |   37,199   | 10,140
  Adjusted Length |    _P. truei_    |  M  |  16  |      363   |    105
                  |     "    "       |  F  |  22  |      326   |     94
                  |                  |     |      |            |
                  | _P. maniculatus_ |  M  |  14  |      286   |     94
                  |  "       "       |  F  |   4  |      347   |     83

It is important to know approximately how far mice travel in one night.
The distances traveled between captures on successive nights were
calculated for all mice. Even animals caught most frequently usually
were caught only once or twice on successive nights. Data from animals
caught less than four times, and hence not usable for calculations of
home range, could be used in calculating the distance between captures
on successive nights. Thus the data were sampled in a more or less
random manner for each species.

The mean distance traveled between captures on successive nights was
determined for adult and non-adult animals (juvenile, young and
subadult) of both sexes. Adult males of _P. maniculatus_ traveled an
average of 151.66 feet (n = 24); young males of this species traveled an
average of 134.28 feet (n = 7). Adult females of _P. maniculatus_
traveled 170.00 feet (n = 4); no data were available for young females.

Adult males of _P. truei_ traveled an average of 169.47 feet (n = 38);
and young males traveled 159.44 feet (n = 18). Adult females of this
species traveled 155.71 feet between captures (n = 35), while young
females traveled 140.66 feet (n = 15).

The means were tested for differences in the distances traveled between
young and adult males and between young and adult females of each
species, as well as between males and between females of opposite
species. In all cases, there were no demonstrable differences in the
distance traveled between captures.

One of the more striking journeys between captures was that of number
59, a juvenal male of _P. maniculatus_, which traveled 1,070 feet
between captures on July 16 and 17, 1963. The route between the two
capture sites was over the most rugged part of the trapping grid. This
datum was excluded from further calculations. The only other animal that
approached this distance was a young female _P. truei_ that traveled 750
feet between captures.

Figure 3 shows the distribution of distances traveled by mice of each
species between successive captures. Since there were no demonstrable
differences between age groups or sexes in the distances traveled, these
data represent a composite of the ages and sexes of each species. They
show 101-125 feet to be the most prevalent of the distances traveled by
both species, and 51-75 feet to have a higher percentage of occurrence
among _P. maniculatus_. These distances indicate that if an animal was
trapped on successive nights, it tended to be trapped within the same
unit of the grid. It would have been necessary for an animal to travel
200 feet or more in order to be caught in traps in an adjoining unit of
the grid.

The distance between captures also was calculated by the more customary
method of averaging the distances between sites of capture, regardless
of the time intervening between captures.

Only data from mice caught four or more times were used because these
individuals probably had home ranges in the study area, whereas those
caught fewer than four times may have been migrants.

The mean distance between captures (n = 95) for 15 males and five
females of _P. maniculatus_ was 161 feet. Sixteen males and 22 females
of _P. truei_ traveled an average of 143 feet between captures (n =
248). For purposes of comparison, these average distances between
captures could be considered as radii of the estimated home ranges. When
the range for each species is calculated by considering average distance
between captures as the radius of the estimated home range, the average
range of _P. truei_ is 64,210 square feet, and that of _P. maniculatus_
is 81,392 square feet. Both of these estimations are larger than those
made by the inclusive and exclusive boundary-strip method (Table 3), and
smaller than those calculated by using adjusted length of range as the

Since it is known that ranges of some animals tend to be longer than
wide (Mohr and Stumpf, 1966), calculations of estimated range based on
average distance between captures probably are more accurate than those
based on adjusted length of range.

Usually the estimated home ranges were not symmetrical, and did not
resemble oblongs or circles in outline. Rather, the ranges tended to
follow parts of vegetational zones. Since trapping grids are geometrical
in form, there is a tendency among investigators to consider home ranges
of animals as conforming to geometrical design. This may or may not be
the true situation; telemetric studies on larger animals indicate that
home ranges do not conform to geometrical design. At present there is a
poverty of knowledge concerning methods for determining the precise home
ranges of small mammals. Telemetry appears to offer an unlimited
potential for studies of this kind.

    [Illustration: FIG. 3: Graphs showing the distribution of distances
       between stations at which mice were captured on successive nights
       in Mesa Verde National Park. Graphs for each species represent
       records of both males and females.]

Individuals of _P. truei_ and _P. maniculatus_ usually do not have
mutually exclusive home ranges. When the home ranges for all females or
males of one species are drawn on a single map of the trapping grid,
almost every one of their ranges overlaps with the range of at least one
other mouse. In some instances, the home range of an individual overlaps
ranges of several other individuals. In extreme cases an animal's range
lies completely within the estimated boundaries of another individual's
range. Such an enclosed range was always that of a juvenile or of a
young animal. However, an adult may have more than half of its range
overlapping with that of another adult of the same sex and of the same,
or different, species.

In general, the two species tended to be restricted to certain areas of
the trapping grid where the respective habitats were more favorable for
their needs. Figure 4 shows the parts of the trapping grid utilized by
each species. Of course there is overlap in the areas utilized by each
species; a few individuals of _P. maniculatus_ may be found in what
appears to be _P. truei_ habitat, and _vice versa_. In such cases, an
inspection of the vegetation usually reveals an intermediate type of
habitat--for example, an open sagebrush area in pinyon-juniper
woodland--that is habitable for either or both species.

The ranges of _P. truei_ tend to be clustered in the western half of the
trapping grid, whereas the ranges of _P. maniculatus_ are clustered in
the eastern half of the grid (Fig. 4). The vegetation of the grid and
the preferred habitats of each species are discussed in following

On the basis of the sizes of estimated home ranges, it is possible to
compute the approximate number of individuals of each species that occur
in each acre of appropriate habitat.

    [Illustration: FIG. 4: Diagram of trapping grid south of Far View
       Ruins, showing the preferred habitats of _P. truei_ and
       _P. maniculatus_.]

On the basis of an average home range of 30,206 ± 25,545 square feet
(one standard deviation) for both male and female individuals of _P.
truei_, there should be approximately 0.781 to 9.345 individuals of this
species per acre of pinyon-juniper woodland. An average home range of
29,400 ± 24,570 square feet for males and females of _P. maniculatus_
indicates that the density of this species is between 0.807 and 9.018
animals per acre in mixed shrub or shrub and sagebrush types of

Figure 4 shows that approximately 10 of the 16 units of the trapping
grid are suitable habitat for _P. truei_; the remaining six units are
habitat of _P. maniculatus_. From the preceding calculations of density
one could expect to find between seven and 90 individuals of _P. truei_,
and between five and 54 individuals of _P. maniculatus_ as residents
within the 22.95 acres of the trapping grid. The higher estimates of
density appear to be large enough to compensate for any overlapping of
home ranges.

The calculation of density of each species within the trapping grid is
dependent upon the precision with which the home ranges of individuals
can be estimated. At this time, home ranges of small rodents can not be
measured with great precision, therefore any such calculations are, at
best, only approximations. This does not imply that estimations of home
range are of little value; however, calculations of density, using home
ranges as a basis, tend to amplify the variance inherent in the data.
This amplification is reflected in the wide range between low and high
limits of the densities for each species within the trapping grid.

In order to check on the accuracy of the above calculations, an estimate
of density was made for each species on the basis of trapping data.
Trapping records kept for each animal were checked for the year 1963.
More data on home ranges were obtained in that year due to higher
population densities than in 1964. If an animal was caught four or more
times in 1963, it was considered to be a resident; animals caught in
both 1963 and 1964 were considered to be residents even if caught fewer
than four times. Mice caught three times, with at least a month elapsing
between the first and third captures, were considered to be probable
residents. Other animals caught three or fewer times were considered to
be migrants.

In 1963, 15 individuals of _P. truei_ were caught four or more times, or
in both years, and considered to be residents; six other mice were
classed as probable residents. Of _P. maniculatus_, 18 individuals were
classed as residents, and two as probable residents. Thus the trapping
data for 1963 indicate that 21 individuals of _P. truei_ and 20 of _P.
maniculatus_ were residents of the trapping grid. These estimates lie
well within the estimated limits of density of each species, as
calculated from data on home range while taking into account the
relative proportions of available habitat for each species within the
trapping grid. Analyses of trapping data indicate that the density of
each species probably is overestimated by calculations of density based
on home range data.

Males and females of both species of _Peromyscus_ appeared to be highly
individualistic in the amount of area they utilized. Some adult males of
_P. truei_ covered large areas, whereas others were relatively
sedentary. The same was true of young males of _P. truei_, although the
younger males tended to have smaller ranges than adult males. Most
pregnant or lactating females, of both species, tended to use smaller
areas for their daily activities than did non-pregnant or non-lactating
females. There were notable exceptions to this generality, for some
lactating females had exceptionally large home ranges.

Size of home range apparently was not influenced by the location of an
animal's range within the grid. Far more data would be needed to
correlate minor differences in vegetational associations with sizes of
ranges in different parts of the grid.

It is surprising that adults of _P. truei_ do not have larger home
ranges than adults of _P. maniculatus_. _P. truei_ is the larger, more
robust animal, capable of rapid running and occasional saltatorial
bounding; individuals of this species can traverse large areas with
ease. The semi-arboreal nature of _P. truei_ may explain why individuals
of this species do not have larger ranges than individuals of _P.
maniculatus_. _P. truei_ has a three-dimensional home range, whereas _P.
maniculatus_ has a range that is two-dimensional only (excluding the
relatively minor amount of burrowing done by each species).


Detailed maps of vegetation within the trapping grid were needed to aid
in analyzing distribution of mice within the grid. In preparing such
maps, I recorded all plants within a 25 foot radius of each trapping
station. The dominant and codominant plants in the overstory (trees or
shrubs) were noted at each station. Next the three most abundant plants
other than the dominant and codominants were rated for each station,
where possible. Finally a listing was made of all remaining species of

On the basis of this analysis, four vegetational maps were prepared. One
shows associations of dominant overstory and understory plants.
Individual maps are devoted to the first, second and third most abundant
plants in the ground cover within the trapping grid (Figs. 5-8).
Approximately seven man-hours were required to analyze each trapping
unit, and 112 man-hours to analyze the entire grid.

The home range grid encompasses approximately one million square feet.
At least four different vegetational stands occur within the grid: 1)
pinyon-juniper woodland with various associations in the understory; 2)
_Artemisia tridentata_ (big sagebrush), or _A. nova_ (black sagebrush);
3) _Quercus gambelii_ (Gambel oak); and 4) mixed shrubs--_Fendlera
rupicola_ (fendlerbush), _Amelanchier utahensis_ (Utah serviceberry),
and _Cercocarpos montanus_ (mountain mahogany).

Flora in the ground cover is regulated, at least in part, by the canopy
cover; hence different associations of pinyon-juniper woodland and each
of the stands mentioned above have different plants, or a different
distribution of the same kinds of plants, in their ground cover.

Units A, B, E, and parts of D and G in the western third of the grid are
in pinyon-juniper woodland (Fig. 5). A relatively pure understory of
_Poa fendleriana_ (muttongrass), is typical of such woodland on the
middle parts of the mesas. Woodland on the western third of the grid
differs somewhat in that, when the area occupied by each plant is
considered, _Artemisia tridentata_ is codominant there with _Poa
fendleriana_. As far as individual plants are concerned, _Poa_ far
outnumbers _Artemisia_. The next most abundant plants in the ground
cover are _Solidago petradoria_ (rock goldenrod), _Chrysothamnus
depressus_ (dwarf rabbitbrush), and _Penstemon linarioides_ (penstemon),
in that order.

In unit E there is a large depression, about 200 by 60 feet, created by
removal of soil (Fig. 8). _Artemisia nova_ grows there, and pioneering
plants adapted to early stages of succession are present.

A zone of woodland, where _Artemisia nova_ replaces _A. tridentata_ as
an understory codominant with _Poa fendleriana_, borders the
pinyon-juniper-muttongrass community to the east. The next most abundant
plants in the ground cover are _Solidago petradoria_, _Penstemon
linarioides_ and _Comandra umbellata_ (bastard toadflax). _Koeleria
cristata_ (Junegrass) is as abundant as _Comandra_, but probably is less
important as a source of food for mice.

A small strip of the pinyon-juniper-muttongrass community with an
understory of _Artemisia nova_ and _Purshia tridentata_ (bitterbrush)
adjoins the above area to the east (Figs. 5-8). _Solidago petradoria_,
_Balsamorrhiza sagittata_ (balsamroot), and _Comandra umbellata_ are the
three most abundant plants in the ground cover. The terrain slopes
eastward from this zone into a large drainage.

    [Illustration: FIG. 5: Diagram showing the major associations of
       understory and overstory vegetation in a trapping grid located
       south of Far View Ruins, Mesa Verde National Park, Colorado.]

As the forest floor begins to slope into the drainage, the ground
becomes rocky and shrubs assume more importance in the understory. Most
of this shrubby zone is on the slope; on the western side this zone
abuts pinyon-juniper woodland, and on the eastern side is bordered by
_Artemisia tridentata_ in the sandy bottom of the drainage. Shrubs
become more abundant and pinyon and juniper trees become less abundant
as one approaches the drainage. In the vegetation maps, this brushy zone
is delimited on the east by a heavy line passing vertically through the
middle of the grid (Figs. 5-8). The codominant shrubs in the understory
of this zone are _Amelanchier utahensis_, _Artemisia nova_ and _Purshia
tridentata_. The three most abundant plants on the ground are _Artemisia
ludoviciana_, _Chrysothamnus depressus_ and _Penstemon linarioides_.

The drainage occupies most of unit N and parts of Units I, J and M. Unit
N is at the head of the drainage; the ground slopes rapidly southward
and the bottom of the drainage in unit J is approximately 50 feet lower
than in unit N. The canopy cover of the drainage is _Artemisia
tridentata_ (Fig. 5). The same three plants that are most abundant in
the ground cover of the slope are also most abundant in the drainage.

    [Illustration: FIG. 6: Diagram showing the most abundant species of
       plants in the ground cover of the trapping grid south of Far View

The eastern slope of the drainage is covered with oak chaparral
(_Quercus gambelii_); this zone occupies parts of units J, L, M, and P.
_Artemisia ludoviciana_, _Solidago petradoria_, and _Viguiera
multiflora_ (goldeneye), are the most abundant plants of the ground

Mixed shrubs (_Amelanchier_, _Cercocarpos_, and _Fendlera_) form large
islands in the oak chaparral, in units K, L and P. The brushy areas of
oak and mixed shrub give way at the top of the slope to pinyon-juniper
forest with an understory of _Artemisia nova_ and _Purshia tridentata_.
The three most abundant plants in the ground cover of the shrub zones
are _Solidago petradoria_, _Balsamorrhiza sagittata_, and _Comandra
umbellata_. The eastern part of unit O has _Amelanchier utahensis_ in
the understory, in addition to _Artemisia nova_ and _Purshia tridentata_
(Fig. 5). The northeastern corner of unit O is in pinyon-juniper
woodland with an understory of _Cercocarpos montanus_.

    [Illustration: FIG. 7: Diagram showing the second most abundant
       species of plants in the ground cover of the trapping grid south
       of Far View Ruins.]

There are two relatively pure stands of sagebrush in the grid: one is in
unit N, and the other in unit F and part of unit G. As figures 5 to 8
show, unit N has a relatively pure stand of _Artemisia tridentata_ (big
sagebrush), with _Artemisia ludoviciana_, _Agropyron smithii_ (western
wheatgrass), and _Koeleria cristata_ (Junegrass), being most abundant in
the ground cover. _Artemisia tridentata_ and _Artemisia nova_ form the
overstory in unit F and part of G. The three most abundant plants in the
ground cover there are _Chrysothamnus depressus_, _Solidago petradoria_,
and _Penstemon linarioides_ (Figs. 6-8).

    [Illustration: FIG. 8: Diagram showing the third most abundant
       species of plants in the ground cover of the trapping grid south
       of Far View Ruins.]


Four microclimatic stations were established in units D, F, L and M of
the trapping grid to record air temperatures and relative humidities at
ground level. These sites were chosen as being representative of larger
topographic or vegetational areas within the grid. Belfort
hygrothermographs were installed on June 10, 1964, and were serviced
once each week through October 31, 1964, at which time the stations were
dismantled. Each station consisted of a shelter 18 by 9 by 11.5 inches,
having a false top to minimize heating (Fig. 9). The shelters were
painted white. Several rows of holes, each one inch in diameter, were
drilled in all four sides of each shelter, to provide circulation of
air. The holes were covered by brass window screening to prevent entry
of insects and rodents. Preliminary tests with several U. S. Weather
Bureau maximum and minimum thermometers, suspended one above the other,
from the top to the bottom of the shelter, revealed that there was no
stratification of air within the shelters. Nevertheless, each shelter
was placed so that the sun did not strike the sensing elements of the
hygrothermograph inside it.

    [Illustration: FIG. 9: (above) Photograph of microclimatic shelter
       built to house hygrothermograph. False top minimizes heating, and
       ventilation holes are covered with screening. (below) Photograph
       showing shelter in use.]

Accuracy of the hair elements was checked by means of a Bendix-Friez
battery driven psychrometer, in periods when humidity conditions were
stable (on clear days the relative humidity is at its lowest limits and
is "stable" for several hours during early afternoon).

The four microclimatic stations were in the following places: 1) a stand
of big sagebrush near Far View Ruins; 2) a pinyon-juniper-muttongrass
association; 3) a stand of big sagebrush at the head of a drainage; and
4) a stand of Gambel oak on a southwest-facing slope of the drainage.
Table 4 shows monthly averages of maximum and minimum air temperatures
and relative humidities at each of the four sites. Vegetation and
microclimates of the sites are discussed below.

_Far View Sagebrush Site, 7,650 feet elevation_

The shelter housing the hygrothermograph was next to the stake of
station F4a in the trapping grid (Fig. 10), in a stand of big sagebrush
on the flat, middle part of the mesa top, approximately 100 yards
southwest of Far View Ruins. The sagebrush extends approximately 200
feet in all directions from the station (Fig. 5). Pinyon pine and Utah
juniper trees are encroaching upon this area, and scattered trees are
present throughout the sagebrush. This area is one of the habitats of
_P. maniculatus_.

Sagebrush tends to provide less shade for the ground than pinyon-juniper
woodland, and therefore the surface temperatures of the soil rise
rapidly to their daily maximum. In mid-June, air temperatures rise
rapidly from 6 A. M. until they reach the daily maximum between 2 and 4
P. M. Shortly after 4 P. M. the air temperatures decrease rapidly and
reach the daily low by about 5 A. M.

Relative humidities follow an inverse relationship to air temperatures;
when air temperatures are highest, relative humidities approach their
lowest values. Thus, on clear days, humidities decrease during the day,
reaching a minimum slightly later than air temperatures attain their
maximum. Unless it rains, the highest humidities of the day occur
between midnight and 6 A. M.

_Drainage Site, 7,625 feet elevation_

This site was in the bottom of the drainage that runs through the
eastern side of the trapping grid, and through parts of units M, N, I,
and J. The site was at station M4d on a level bench at the head of the
drainage (Fig. 11). Southward from the station the drainage deepens
rapidly, and the bottom loses approximately 25 feet in elevation for
every 200 feet of linear distance. _P. maniculatus_ lives here.

The microclimate of the drainage differs markedly from that of other
stations. The major difference is attributable to the topography of the
drainage itself. Nocturnal cold air flows from the surrounding mesa top
to lower elevations. A lake of cold air forms in the bottom of the
drainage; the depth of the lake depends in part upon the depth of the
drainage. The same phenomenon occurs in canyons and causes cooler night
time temperatures on the floor of canyons than on adjacent mesa tops
(Erdman, Douglas, and Marr, in press). Drainage of cold air into lower
elevations affects both nocturnal air temperatures and relative
humidities. Table 4 shows that maximum air temperatures in the drainage
did not differ appreciably from those at other stations. Mean minimum
temperatures, however, were considerably lower in the drainage than at
the other sites. This phenomenon is reflected also in the mean air
temperatures at this station.

    [Illustration: FIG. 10: (above) Photograph of microclimatic station
       at the Far View Sagebrush Site, at trapping station F4a in the
       grid south of Far View Ruins. Dominant vegetation is _Artemisia

    [Illustration: FIG. 11: (below) Photograph of microclimatic station
       at the Drainage Site, in the bottom of a shallow drainage at
       trapping station M4d of the grid south of Far View Ruins.]

The drainage site had the highest humidities of all stations each month
in which data were collected (Table 4). Relative humidities of 90 to 100
per cent were common in the drainage, but occurred at other stations
only in rainy periods. For example, in the month of August, 26 of the
daily maximum readings were between 95 and 100 per cent at the drainage
site, but at the other stations relative humidities were above 95 per
cent for an average of only nine nights. Minimum humidities were about
the same for all stations, since they are affected by insolation
received during the day, and not by the drainage of cold air at night.

_Oak Brush Site, 7,640 feet elevation_

The station was in an oak thicket at trapping station L4a, 250 feet
south and 50 feet east of the drainage site on a southwest-facing slope
of about 30 degrees (Fig. 12). The station was on the lower third of the
slope, approximately 15 feet higher than M4d, the station in the bottom
of the drainage. _P. truei_ and _P. maniculatus_ occur together in this

Air temperatures and relative humidities at this station did not differ
appreciably from mean temperatures and humidities at the other stations.
The unusual feature is the lack of evidence of cold air drainage. The
lake of cold air in the bottom of the drainage apparently is too shallow
to reach this station. This site is near the head of the drainage, and
the cold, nocturnal air probably moves rapidly down slope into the
deeper parts of the canyon, rather than piling up at the shallow head of
the drainage.

In spite of the shade afforded the ground by the oak brush, temperatures
reached the same maximum values as at the drainage site, owing to the
orientation of the slope. South-facing slopes receive more direct
insolation throughout the day and throughout the year than north-facing
slopes and mesa tops (Geiger, 1965:374). In Mesa Verde, south-facing
slopes tend to be more arid; snow melts rapidly, and most of this
moisture evaporates. As a consequence, south-facing slopes have less
soil moisture and more widely-distributed vegetation than north-facing
slopes where snows often persist all winter and melt in spring. (For a
detailed discussion of climates on northeast-versus-southwest-facing
slopes in Mesa Verde, see Erdman, Douglas, and Marr, in press.)

_Pinyon-Juniper-Muttongrass Site, 7,600 feet elevation_

The station was in the trapping grid at D5b (Fig. 13). The
pinyon-juniper woodland surrounding this site resembles much of the
woodland on the middle part of the mesa. The forest floor is well shaded
by the coniferous canopy, and muttongrass is the dominant plant in the
ground cover. _P. truei_ lives in this habitat.

The climate at this site is moderate. Shade from the canopy greatly
moderates the maximum air temperatures during the day; minimum air
temperatures, however, are about the same as at the other stations
(Table 4). Mean temperatures are somewhat lower at this site than at the
others because of the lower maximum temperatures. Relative humidities do
not differ markedly from those at other stations.

Figure 14 shows hygrothermograph traces at all stations for a typical
week. An interesting phenomenon is illustrated by several of these
traces. By about midnight, air temperatures have cooled to within a few
degrees of their nightly low. At this time, heat is given up by the
surface of the ground in sufficient quantities to elevate the air
temperature at ground level. This release of reradiated energy lasts
from one to several hours, then air temperatures drop to the nightly low
just before sunrise. A depression in the percentage of relative humidity
accompanies this surge of warmer air. On some nights winds apparently
disturb, or mix, the layers of air at ground level. On such nights the
reradiation of energy is not apparent in the traces of the thermographs.
Reradiation of energy is restricted to ground level, and traces of
hygrothermographs in standard Weather Bureau shelters, approximately
four feet above the ground surface, at other sites on the mesa top did
not record it.

    [Illustration: FIG. 12: (left) Photograph of microclimatic station
       at the Oak Brush Site, at trapping station L4a of the grid south
       of Far View Ruins. (right) General view of the stand of Gambel
       oak in unit L of the trapping grid.]

    [Illustration: FIG. 13: Photograph of microclimatic station at the
       Pinyon-Juniper-Muttongrass Site, at trapping station D5b of the
       grid south of Far View Ruins. Grass in the foreground is
       muttongrass, _Poa fendleriana_.]

The instruments used in this study were unmodified Belfort
hygrothermographs containing as sensing units a hair element for
relative humidity and a Bourdon tube for air temperatures. The hair
element, especially, does not register changes in humidity at precisely
ground level; rather, it reflects changes in the layer of air from about
ground level to about a foot above. Thus data from these instruments
give only approximations of the conditions under which mice live while
they are on the ground.

Climatic conditions greatly influence trapping success. Larger numbers
of mice generally were caught on nights when humidities were higher than
average. Rain in part of the evening almost invariably resulted in more
mice of each species being caught. This was probably due to increased
metabolism, by the mice, to keep warm. Apparently the mice began
foraging as soon as the rains subsided; mice were always dry when caught
after a rain. Few mice were caught if rains continued throughout the
night and into the daylight hours.

    TABLE 4--Monthly Averages of Daily Means for Maximum, Minimum, and
       Mean Air Temperatures and Relative Humidities at Four Sites in
       Mesa Verde National Park, Colorado.

         Site        |     Maximum Temps.     |     Maximum R. H.
                     |  J    J    A    S    O |  J    J    A    S    O
                     |                        |
  Far View Sagebrush | 89   91   86   77   74 | 68   84   82   88   71
  Drainage           | 86   91   85   78   78 | 87   94   93   96   84
  Oak Brush          | 86   88   82   76   81 | 57   78   80   80   66
  Pinyon-Juniper-Poa | 75   80   74   66   64 | 59   83   82   88   58
                     |                        |
                     |     Minimum Temps.     |     Minimum R. H.
                     |  J    J    A    S    O |  J    J    A    S    O
                     |                        |
  Far View Sagebrush | 42   53   50   42   31 | 18   24   25   29   21
  Drainage           | 36   48   45   38   26 | 21   26   27   29   30
  Oak Brush          | 42   52   50   42   32 | 19   25   30   31   21
  Pinyon-Juniper-Poa | 44   54   50   42   34 | 22   30   29   32   25
                     |                        |
                     |      Mean Temps.       |       Mean R. H.
                     |  J    J    A    S    O |  J    J    A    S    O
                     |                        |
  Far View Sagebrush | 66   72   68   60   52 | 43   54   54   48   46
  Drainage           | 61   70   65   58   52 | 54   60   60   62   52
  Oak Brush          | 64   70   66   59   56 | 38   51   55   56   44
  Pinyon-Juniper-Poa | 60   67   62   54   49 | 41   56   55   60   42

    [Illustration: FIG. 14: Diagram of hygrothermograph traces showing
       daily progressions of air temperatures and relative humidities at
       each of four microclimatic stations, from the morning of July 1
       through the morning of July 8, 1964. Slanting vertical lines on
       each chart designate midnight (2400 Hrs.) of each day.]

Nights of high trapping success usually were associated with days having
solar insolation below the average. Insolation was measured with a
recording pyrheliometer at a regional weather station (M-2) on the
middle of Chapin Mesa, at an elevation of 7,150 feet (Erdman, Douglas,
and Marr, in press). This station was approximately one mile south of
the trapping grid; isolation at this site would have been essentially
the same as that received by the trapping grid. Below-average isolation
for one day indicates cloudy conditions, which are accompanied by
increased humidity, but may or may not be accompanied by precipitation.
Trapping on nights preceded and followed by days of average or above
average isolation with average humidities--indicative of clear days and
clear moonlit nights--did not yield appreciably higher catches of mice
than other nights. Hence there was no evidence that mice tended to
avoid, or to seek out, traps on clear moonlit nights.

On cold, humid nights in autumn numerous mice caught in Sherman live
traps succumbed from exposure, even though nesting material (kapok or
cotton) and food were in the traps. Occasionally mice succumbed to heat
when traps were inadvertently exposed to too much sunlight. Apparently
little heat is required to kill individuals of either species. Traps in
which animals died due to excessive heat usually were not hot to the
touch; in most instances the traps were checked before 9:00 A. M.,
several hours before the sun caused maximum heating. Such individuals
may have licked the fur of their chests in an attempt to lower their
body temperatures. Although mice characteristically salivate before
succumbing from heat, these individuals had moist fur over the entire
chest and upper parts of the front legs, indicating licking. Mice killed
by exposure to heat or cold usually were juveniles or young; subadult
and adult individuals of both species were more tolerant. Older animals
would be expected to have better homeostatic controls than younger


In Mesa Verde _P. truei_ and _P. maniculatus_ occur together only at the
fringes of the pinyon-juniper woodland, where ecotonal areas provide
less than optimum habitats for both species. Almost all individuals of
_P. truei_ occur only in pinyon-juniper woodland, whereas _P.
maniculatus_ occurs only in more open habitats, such as grassy meadows
and stands of sagebrush.

Pinyon mice were abundant in a variety of associations within the
pinyon-juniper woodland. The highest population densities were in
pinyon-juniper woodland having an understory of mixed shrubs. In such an
association, _Poa fendleriana_ usually is the dominant grass in the
ground cover. _P. truei_ was especially abundant along brushy slopes
where mixed shrubs (_Amelanchier_, _Cercocarpos_ and _Fendlera_) were
codominant with pinyon pines and Utah junipers. The pinyon-juniper-mixed
shrub area west of Far View Ruins was almost optimum habitat for _P.

_P. truei_ was abundant on the rocky ridge of Wetherill Mesa near Mug
House; the pinyon-juniper woodland here has a _Cercocarpos_ understory,
and appears to provide close to optimum conditions for this species.

Not all associations of the pinyon-juniper woodland support large
numbers of _P. truei_. Pinyon-juniper woodland having a ground cover of
_Poa fendleriana_, and no shrubs, supports few mice; the woodland on
Wetherill Mesa near Long House is an example. Juniper-pinyon woodland
having a _Purshia tridentata_ understory also supports only a few mice.
Such areas occur on the southern ends of the mesas and are
characterized by widely-spaced trees and little ground cover--a
reflection of the relatively low amounts of precipitation received by
the southern end of the park.

_P. truei_ was not found in grasslands on Navajo Hill, or in meadows at
the southern end of Moccasin Mesa. The old burned areas on the northern
end of Wetherill Mesa and on Morfield Ridge now support numerous grasses
and shrubs, but _P. truei_ appears not to live there.

_P. truei_ tends to avoid stands of sagebrush, or grasslands, lacking
pinyon or juniper trees. _P. truei_ may venture into such areas while
feeding. This species is found in thickets of Gambel oak and in areas
with an overstory of mixed shrubs only when a living pinyon-juniper
canopy is present, or when a woodland adjoins these areas.

Rocky terrain apparently is not a requirement for _P. truei_, since much
of the pinyon-juniper woodland that is free of rocks supports large
numbers. Optimum habitat, however, had a rocky floor. In such places,
rocks probably are of secondary importance, whereas the shrubs and other
plants growing on rocky soils are important for food and cover. Rocks
likely provide additional nesting sites, and allow a larger population
to live in an area than might otherwise be possible.

In Mesa Verde the deer mouse, _P. maniculatus_, prefers open areas
having dense stands of grasses, or brushy areas adjoining open terrain.
This species lives in stands of big sagebrush; in grassy areas having an
oak-chaparral or mixed-shrub-overstory; and in grasslands without
shrubs, such as on the southern end of Moccasin Mesa. Pure stands of
sagebrush did not support large numbers of mice unless there was
additional cover nearby in the form of shrubs or oak brush.

Optimum habitats for _P. maniculatus_ were on Navajo Hill, in the burned
areas on Morfield Ridge, on the northern end of Wetherill Mesa, and in
the grassy areas near the entrance of the park. The trapping areas in
the first three mentioned had heavy growths of grass and an overstory of

Some individuals of _P. maniculatus_ ventured into pinyon-juniper
woodland and entered traps. Such animals usually were found in places
having a heavy understory of sagebrush, or in disturbed places within
the woodland.

_P. maniculatus_, but not _P. truei_, was taken in the arid
pinyon-juniper-bitterbrush stand on the southern end of Wetherill Mesa.
_P. maniculatus_ also was present, in about equal numbers with _P.
truei_, in a pinyon-juniper-muttongrass stand north of Long House. Both
of these localities supported only a few mice.

_P. maniculatus_ is found more frequently in pinyon-juniper woodland
when the population density is high, and when such woodlands adjoin
grasslands or sagebrush areas. As mentioned earlier, _P. truei_ and _P.
maniculatus_ occur together in ecotonal areas between the forest and
grassy or brushy areas. In Mesa Verde the deer mouse inhabits exposed
grassy areas that have mostly shrubs in the open canopy.

_P. maniculatus_ is the first to colonize areas that have been burned;
this species invades such areas as soon as primary successional
vegetation becomes established. It can be stated that in general, _P.
maniculatus_ will be found in the harsher, more arid habitats. If the
habitat is so inhospitable that only a few mice can survive there, _P.
maniculatus_ will be present. _P. truei_ apparently requires the more
moderate conditions found in the pinyon-juniper forest, and this species
does not venture far from the edge of the forest.


Ten individuals of _P. truei_ and three of _P. maniculatus_ were
followed to their nesting places. Photographs were taken of the nesting
sites before and after uncovering. Plants or other materials used in
their construction and any commensal arthropods present were saved and
later identified.

Nests of _P. truei_ usually were associated with juniper trees. Dead
branches and trunks of juniper trees decay from the inside, and the
resulting hollows are favored sites for the nests. Pinyon pine trees
tend to decay from the outside and were not used as nesting sites by _P.
truei_. Nests of _P. truei_ were found in hollow trunks and branches of
otherwise healthy juniper trees, and in hollow logs lying on the ground.
The heartwood apparently rots rapidly in juniper trees, but the sapwood
remains intact for many years--even after the tree is lying on the
ground. For example, a part of the pinyon-juniper woodland on the
southern end of Chapin Mesa was burned in 1858, and the hollow trunks of
junipers were still standing in 1966. Almost all of the pinyon pine
trees that were killed by that fire have since decayed; their former
presence is verified only by the crumbling remnants of their trunks that
lie on the ground throughout the burned area.

The following accounts illustrate the preferences of the two species of
mice in selection of nesting sites:

No. 105, _P. truei_, adult. On July 22, 1964, after being released from
a trap, this female ran to a serviceberry bush 10 feet south of station
I4d, preened herself, ate a berry from the bush, and disappeared under a
large rock at the base of the bush. Subsequent excavation revealed a
large nest composed of grasses (_Poa fendleriana_, _Sitanion hystrix_,
_Agropyron smithii_, _Koeleria cristata_), and a few leaves of
serviceberry. There were three entrances to the nest, one on each side
of the rock.

This mouse was captured again on August 12, 1964, released and followed
to a hollow juniper log 15 feet south of station C7b, and 245 feet from
the above nest. This log was dismantled, but no nest was found. A large
number of chewed juniper seeds around the log indicated that this mouse,
or others, had frequented the area.

On August 20, 1964, this female was followed to a large juniper log 20
feet northeast of station I4b. A small nest of shredded juniper bark was
found inside the log, and there were numerous nuts of pinyon pine and
seeds of Utah juniper that had been gnawed open. This site was about 320
feet from that at C7b, and about 240 feet from station I4d (Fig. 15).

No. 118, _P. truei_, young. On August 29, 1963, this male ran into a
hollow branch of a partly dead juniper tree 15 feet south of station
C5d. Part of this branch had been sawed off at some earlier time, and a
hole about one-and-a-half inches in diameter was present in the center
of the remaining part. The branch was not dissected, but probing
revealed that the hole extended far into the branch and enlarged as it
approached the trunk.

No. 177, _P. truei_, adult. This lactating female ran into the hollow
trunk of a juniper 10 feet north of station G7a. Both lateral branches
of the main trunk were rotten and hollow, but the tree appeared to be
healthy. Chewed juniper seeds were present in the trunks and around the
base of the tree.

This female later ran to a juniper log 30 feet north of station N4d.
Apparently there was no permanent nest at this site (Fig. 15).

No. 178, _P. truei_, adult. This female ran into a hollow juniper tree
10 feet south of station H3c. Hundreds of old juniper seeds, with their
embryos chewed out, were present at the base of the tree. The tree was
not cut down.

No. 238, _P. truei_, adult. This male ran into a dead juniper log 10
feet south of station O4b. Chewed juniper seeds were present on the
ground, but no nest was found in the log.

    [Illustration: FIG. 15: Diagrams showing estimated home ranges of
       six individuals of two species of _Peromyscus_, and location of
       these ranges in the trapping grid. Nesting or hiding places are
       described in the text, and are indicated on each diagram by an
       X. Shaded areas represent home ranges estimated from trapping
       records for 1963; outlined, unshaded areas represent estimated
       home ranges for 1964.]

No. 241, _P. truei_, adult. This male ran into a small hole at the base
of a juniper tree 25 feet south of station G7c. The hole was at the fork
of the tree, four inches above the ground, and led to a large
subterranean chamber in the basal part of the trunk.

This male later ran into a dead juniper log lying on the ground 20 feet
southwest of station N3b. No nest was found in the log.

After another capture, this mouse ran to a small juniper log 40 feet
southeast of station G3d. There was a nest of shredded juniper bark and
many juniper seeds inside the log (Figs. 15-17).

No. 245, _P. truei_, adult. This female ran into a large, hollow juniper
log 20 feet northwest of station D4d. No nest was seen, but chewed
juniper seeds were noted in and around the log (Fig. 15).

No. 251, _P. truei_, juvenile. This female ran into a dead juniper log
beside station P4b. Chewed cones of pinyon pine and chewed juniper seeds
were on the ground. A small nest of shredded juniper bark, and a few
leaves of serviceberry, were found inside the log. Chewed pinyon nuts
and juniper seeds also were present in the nest.

    [Illustration: FIG. 16: (above) Photograph of juniper log at station
       G3d, which contained the nest of _P. truei_ # 241.]

    [Illustration: FIG. 17: (below) Photograph of dissected juniper log
       at station G3d, showing the nest of _P. truei_ # 241, at end of
       mattock handle. The nest of shredded juniper bark contained
       chewed seeds of juniper trees.]

No. 267, _P. truei_, juvenile. This male ran into a fallen juniper log
40 feet southwest of station P7a and then disappeared into a hole
leading under an adjacent rock. Dissection of the log revealed many
chewed juniper seeds inside and beneath the log, but no nest. I did not
overturn the large rock or excavate under it.

No. 268, _P. truei_, adult. This pregnant and lactating female ran into
a hollow branch of a partly-dead juniper tree 10 feet south of station
O7d. The limb and base of the tree were hollow, and there were large
numbers of chewed juniper seeds nearby. Because of time limitations, the
branch was not dissected.

No. 74, _P. maniculatus_, juvenile. This female ran into a small
circular hole in the ground 13 feet north of station J3a. Excavation
revealed that this hole led into the abandoned tunnel of a pocket gopher
(_Thomomys bottae_). The tunnel was followed for about four feet, but no
nest was found and the tunnel led under a thicket of oak brush which
made further excavation impractical (Fig. 15).

No. 247, _P. maniculatus_, adult. This male was followed to a large nest
situated at the base of a stump and under a juniper log lying beside the
stump, five feet from station I2c. This large nest was built on the
ground and was constructed of grasses (_Poa fendleriana_, _Stipa
comata_, and _Koeleria cristata_), and contained a few leaves of Gambel
oak. It was the largest nest found. Chewed pinyon nuts were in the nest.
(Fig. 15).

No. 276, _P. maniculatus_, juvenile. This male ran into a small hole at
the base of a dead juniper tree 40 feet north of station O2c. It would
have been necessary to cut the tree down to uncover the nest, and this
was not deemed to be worthwhile.

The preceding accounts indicate that, in Mesa Verde, nests of _P. truei_
usually are associated with hollow juniper logs or branches. In one
instance a nest of _P. truei_ was found on the ground, under a rock.
Shredded juniper bark, and, in one case, grasses were the materials most
commonly used for nest building.

Individuals of _P. maniculatus_ did not build nests in trees. One nest
was found under a stump and adjacent log. Another site was in the
abandoned tunnel of a pocket gopher, and a third was under a large rock.
The only nest that was unquestionably built by a _P. maniculatus_ was
constructed of grasses and a few leaves.

It seems unlikely that competition for nesting sites between the two
species of _Peromyscus_ affects the local distribution of each species.
The analysis of nesting sites suggests that _P. truei_ is restricted, in
Mesa Verde, by the availability of fallen logs, hollow branches, or
hollow trunks of juniper trees. My observations lead me to think that
within the pinyon-juniper woodland there is a surplus of nesting sites
for individuals of _P. truei_. Many juniper trees have dead branches,
and hollow juniper logs are abundant throughout the forest. It is
inconceivable to me that the population of _P. truei_ could reach
densities sufficient to saturate every nesting site available to them in
the trapping grid.

Sagebrush areas, or brushy zones adjacent to the pinyon-juniper woodland
usually do not contain juniper logs; when hollow juniper trees or logs
are not available, _P. truei_ is not found as resident of such areas. As
mentioned earlier, individuals of _P. truei_ may venture into such areas
to feed if they are adjacent to pinyon-juniper woodland.

An individual of _P. truei_ may have more than one nest within its home
range (for example Nos. 105 and 241 cited above). Each mouse probably
has refuges, each containing a nest, strategically located in its home
range. Thus, if a mouse is chased by a predator, or by another mouse, it
need not return to its main nest, but can seek refuge in one of its
secondary nests. These secondary nests were small and were invariably
constructed from shredded juniper bark. Some of these nests were little
more than a scant handful of shredded bark that formed a platform to sit
upon. Other nests were larger and ball-shaped, with one opening on the
side. All of the secondary nests that were found were inside hollow
juniper logs. The bark used in construction of the nests had, in each
case, been transported from nearby living trees. The logs had previously
lost their bark through decay.

The evidence indicates that these secondary refuges are prepared with
considerable care. Not only is the bark transported for some distance,
but it is shredded into a soft mass of fibers. When a mouse first
establishes itself in a new area, perhaps it begins several such nests
before settling upon the most favorable site. The less desirable sites,
if still within the animal's range, are then available (barring
competition by a new inhabitant) for outlying refuges.

My data do not indicate whether individuals of _P. maniculatus_ use a
similar arrangement of nests within their home ranges. The population of
_P. maniculatus_ was sparse in the trapping grid, and the habitat these
mice occupied was such as to make following them extremely difficult.

In captivity, both species constructed nests that were indistinguishable
to me, when the mice were given cotton, kapok, or pieces of burlap as
building material. The cotton or kapok was used directly, but the burlap
was shredded into a fine mass of fluffy fibers. The burlap seemed to me
to be the best building material, for it maintained its shape best.

Both species constructed nests that resembled inverted bowls. Solitary
mice naturally built smaller nests than those built by females with

The entrance to the closed nests varied; often the female would bolt
through the side of the nest where there was no opening. Sometimes the
mice would exit and enter through the top of the nest. In some cases it
appeared that the entire nest was closed; probably the occupant had
closed the entrance. Such a closed nest would have the advantage of
greatly moderating the microenvironment within the nest, and would allow
the animal within to remain comfortable with a minimum expenditure of
energy. The larger nests found in the trapping grid resembled those
built by captives. Nests built of grasses were always larger than those
built of juniper bark. Juniper bark is as easily worked into nests as
are grasses, in my judgment. Therefore, difficulty of construction of
nests from this material probably does not account for the smaller size
of the nests composed of bark. I think the difference in insulating
characteristics between the two materials probably accounts for the
difference in size of the nests.


In Mesa Verde, _Peromyscus_ reproduces from April through September.
Reproduction is greatly reduced in the autumn, and most females complete
reproduction before October.

Ten of the 20 females of _P. maniculatus_, taken in May, contained
embryos; five others were lactating. Lactating and pregnant females were
collected on May 5, 1962, indicating that reproduction in some females
began in early April. In September, 15 of 34 females were pregnant or
lactating, whereas in October only two out of 15 females of _P.
maniculatus_ were reproducing. Only one female of _P. maniculatus_ was
found to contain embryos in October. This large adult was taken on
October 3, 1963, and had six embryos, each five millimeters long. She
probably would have produced a litter later in October, and would have
been nursing into November. A report of October breeding in
north-central Colorado described nine of 23 females of _P. maniculatus_
as being in a reproductive state; seven were lactating and one was
pregnant between October 26 and 31, 1952 (Beidleman, 1954:118).

In the Museum of Natural History, the University of Kansas, there are 35
females of _P. maniculatus_ more than 144 millimeters in total length
taken from Mesa Verde in November, 1957 (Anderson, 1961:53). None of
these contained embryos, and no pregnant females have been taken from
the park in November.

_P. truei_ and _P. maniculatus_ reproduce at about the same time. A
female of _P. truei_ prepared as a specimen on May 10, 1964, contained
four embryos, each 20 millimeters long, indicating a breeding time in
mid-April. Svihla (1932:19) reported the gestation period for
non-lactating _P. truei_ to be 25 to 27 days and for lactating
individuals, 40 days. Lactation tends to increase the gestation period
of other _Peromyscus_ by about five days (Asdell, 1964:266). The
gestation period of nine non-lactating females of _P. m. rufinus_ was
reported by Svihla to be 23 to 24 days. Lactation increased the length
of the period of gestation in this subspecies to between 23 and 32 days
(mean for seven females 26.57 ± 0.73, Svihla, 1932:19).

Females of _P. truei_ were observed in various stages of reproduction
from June through September. Ten of the 20 females of _P. truei_ taken
in September were reproducing; four contained embryos and the other six
were lactating. In October, only one of 17 females caught in snap traps
was lactating. Lactating females were caught in live-traps as late as
October 23, although most females had ceased reproduction by then. No
pregnant or lactating females were observed in November.

In _P. maniculatus_, puberty has been placed at 32 to 35 days for
females weighing 13 grams, and in males at from 40 to 45 days, at
weights of 15 to 16 grams (Jameson, 1953:45). In _P. truei_, the weight
of the testes is reported to rise in March and diminish through
September, with accessory organs following the same cycle (Asdell,
1964:267). Young of _P. truei_ nurse for about one month, although some
litters may not be weaned until 40 days of age. Young of _P.
maniculatus_ are weaned between 22 and 37 days of age (Svihla, 1932:30).

Twenty-six pregnant females of _P. maniculatus_, taken in the breeding
seasons of 1961-1964, contained from one to eight embryos each; the mean
was 4.65 ± 1.67. Other investigators have found similar mean values in
this species (Asdell, 1964:266).

Thirteen females of _P. truei_ taken in the breeding seasons of
1961-1964, contained from three to six embryos each; the mean was 4.0 ±
.912. Svihla (1932:25) reported litter sizes, at birth, of two to five
and a mean of 2.84, in 19 litters. Other investigators have reported
litter sizes of one to five with a mean of 3.4, and one to six with a
mean of 3.6 (Asdell, 1964:268). Apparently _P. truei_ does not have more
than six young per litter.

In captivity, females of both species began reproduction in early
February. These captives had been kept for several months at a
temperature of 21 degrees Centigrade, and on a daily photoperiod of 15
hours. Some captive males had enlarged, scrotal testes in January; the
extended photoperiod and warm temperature probably influenced the
breeding condition. In both species testes of wild males caught in
autumn after late September and on through the winter were abdominal,
except for one male of _P. maniculatus_ which had enlarged, scrotal
testes on October 15.

Dates at which different animals arrived at breeding condition varied,
in part owing to subadults (young of the year) appearing in the catch
from early summer to late autumn. Some adult females appeared to be
pregnant or lactating throughout much of the summer and early autumn,
whereas other females, that were caught a number of times, apparently
reproduced only once in the summer.

Some females may fail to breed even though they are mature enough to do
so. One female of _P. truei_ captured eight times (August 30 to
September 20) was a juvenile when first caught, and was classed as young
(in postjuvenal molt) on September 10. She did not reproduce in her
first breeding season, unless she did so after September 20, which is
unlikely. Another female of _P. truei_ was an adult when first caught,
and was caught 12 times (August 21 to October 25). At no time were her
mammae enlarged and she was not lactating or pregnant. It is improbable
that she reproduced earlier in the season, for teats of mice that have
reproduced earlier usually are enlarged to such a degree that previous
parturition is clearly indicated. It was surprising to catch a female,
of any age, 12 times in two months without sign of reproductive

Only one female of _P. maniculatus_ did not show reproductive activity.
She was a juvenile on July 19 when first caught; a subadult on August 28
when caught the third time, and an adult on October 23 when caught the
fifth time.

Burt reported a rest period of a month or more in the summer, in
Michigan, during which many females of _P. leucopus_ did not reproduce.
They began to breed again in late summer at about the time when young of
the year began reproducing (Burt, 1940:17, 19). Abundant mast was
correlated with reproductivity in autumn, according to Jameson
(1953:54), who thought that "food is a basic determinant of the autumn
reproduction" of _P. leucopus_.

Little has been written about the length of time males remain in
breeding condition. Difficulties in determining breeding condition are
many. Fertility customarily is determined by sectioning testes and
noting the presence or absence, and relative abundance, of sperm. This
procedure necessarily sacrifices the individual and indicates the
breeding condition at only one moment and for only the individuals
sacrificed. My observations of males caught a number of times in live
traps shed some light on the breeding condition of males, but the
investigator is likely to err in extrapolating physiological data from
morphology when he notes whether the testes are abdominal or scrotal and
whether they are enlarged or small. It was assumed that testes that have
not descended, and that lie within the abdominal cavity, are not capable
of producing viable sperm. This is the condition in most juveniles, and
in all males during winter. As the breeding condition is attained,
testes descend into the scrotum. Soon the testes and their accessory
organs enlarge and are readily apparent.

Howard (1950:320) reported that numerous males of _P. leucopus_ sired
litters when their testes appeared to be abdominal, and therefore
questioned whether the criterion of descended testes is valid as an
indicator of breeding condition. My captive males of _P. maniculatus_
and _P. truei_ did not sire litters when their testes were abdominal,
even though such males were left with adult females for as long as four
to five months (August through December). Captive pairs of both species
yielded no evidence of reproductive activity until January when, as
mentioned earlier, some of the males had scrotal testes. Young were born
first in early February, although their parents had been confined
together since the preceding August. Jameson reported the testes of
fecund males of _P. maniculatus_ as almost always 8.0 millimeters or
larger (Jameson, 1953:50). Testes that are at least partly scrotal must
be considered as being capable of producing motile sperm, even though
this may not be the case for all individuals.

Toward the beginning and end of the breeding season the testes and
accessory organs of wild mice were small and probably produced few if
any sperm. At these times some males apparently were so frightened by
being handled that the testes were retracted into the inguinal canals.
It would have been easy to consider such males as having abdominal
testes when in fact they did not. In such cases the scrotum usually was
noticeably enlarged; it was found also that in many cases the testes
returned to the scrotal position if the mouse was held gently for a few
minutes. Careful handling of animals was found to prevent, or at least
retard, retraction of the testes. Retraction of the testes from the
scrotum was not a problem at the height of the breeding season when the
testes were engorged.

I had originally assumed that all adult males would be fertile
throughout the breeding season, and that any males with abdominal testes
would be subadults or young of the year. This assumption was an
oversimplification; all adult males did not reach breeding condition at
the same time of year. My data do not support a firm conclusion, for it
is difficult to follow non-captive individuals throughout a breeding
season, owing to sporadic appearance of animals in traps. Nevertheless,
observations of mice that were trapped a number of times indicated the

1) Some adult males that had abdominal testes in the middle of July
reached breeding condition as late as late August and even late

2) Some juvenal males had scrotal testes at the time their postjuvenal
molt was just beginning to be apparent on their sides. Most juvenal
males did not have scrotal testes, and many juveniles that appeared
repeatedly in traps from mid-July through late October did not attain
breeding condition. A mouse that was a juvenile in mid-July must have
been born in mid-June.

3) Apparently animals born early in the breeding season may reproduce
later in that season, whereas those born later in the breeding season
tend not to breed until the following year.

Possibly cooler evening temperatures in July and August, due to the
relatively larger amounts of precipitation in those months, inhibit
reproductive development of late-born young. Most plants have ceased
vegetative growth and have produced seeds by this time; but the
interrelationships between growing seasons, climatic conditions, and
reproductive physiology are unknown.

Only one adult of each species had scrotal testes after late September;
the _P. truei_ had scrotal testes on October 24, 1963, and the _P.
maniculatus_ had scrotal testes on October 15 of that year.


Growth of captive _P. maniculatus_ and _P. truei_ is discussed in
several reports. One of the most complete is that of McCabe and
Blanchard (1950) on _P. m. gambelii_ and _P. t. gilberti_ in California.
A detailed discussion of the dentition in _P. truei_ and wear of the
teeth in different age groups is given by Hoffmeister (1951). Molt in
these species has been considered by a number of authors (Collins, 1918;
McCabe and Blanchard, 1950; Hoffmeister, 1951; Anderson, 1961). The
report by McCabe and Blanchard is valuable because molt is compared
between the two species from the first to the twenty-first week of
postnatal development.

    [Illustration: FIG. 18: Scatter diagram of postnatal growth of
       captive mice, showing increase in length of bodies from birth to
       70 days of age. The records for _P. truei_ represent 11
       individuals of five litters; those for _P. maniculatus_ represent
       17 individuals of four litters.]

The thoroughness of the above-mentioned studies is readily apparent to
those who have worked with mice of the genus _Peromyscus_. Nevertheless,
the ecology of local populations of _P. maniculatus_ and _P. truei_ as
reported for the San Francisco Bay area (McCabe and Blanchard, 1950) has
little relationship to the ecology of mice of other subspecies of these
species, in southwestern Colorado. Indeed, the preferred habitats, and
to some extent the behavior, differ strikingly in Colorado and

    [Illustration: FIG. 19: Graphs showing postnatal growth of solitary
       captive individuals of _P. truei_ and _P. maniculatus_,
       representing the only young in each of two litters.]

Figures 18 and 19 show that some litters grow appreciably faster than
others, but the end results are about the same. Since the young were
measured at irregular intervals, statistical procedures for calculating
confidence limits of the curves were not applicable.

Solitary young reared by one female of each species, attained maximum
size more rapidly than animals having litter mates (Fig. 19).
Nevertheless, solitary individuals and individuals from litters all
reach essentially the same size 50 days after birth.

The gestation time of _P. truei_ is several days longer than that of _P.
maniculatus_, and the young of _truei_ are fewer and heavier than those
of _maniculatus_. As would be expected, _truei_ remains in the nest
longer and nurses longer than _maniculatus_.

Young of each species grow rapidly for the first month, and attain, in
that time, the largest percentage of their adult size; they grow rapidly
up to sometime between the thirtieth and fiftieth days. Thereafter the
rate of growth diminishes and the animals begin to gain weight rather
than continuing to extend the lengths of the body and appendages.

Figure 19 reveals that the appendages of young _maniculatus_ attain most
of their length about a week earlier than those of _truei_. Young
_truei_ acquire mobility and coordination somewhat later than young
_maniculatus_, but both species are seemingly equal in these respects by
about the end of the second week.

Length of gestation period, number and size of embryos, amount of time
spent in the nest, and time required for bodily growth are all of major
importance in determining the relative success of _truei_ and
_maniculatus_. These parameters will be considered further in the


In the laboratory, pregnant females were supplied with either kapok,
cotton, or a piece of burlap with which to make a nest. The kapok or
cotton was used directly by the mice in constructing a hollow, compact,
moundlike nest. When burlap was used for nest building, the female first
completely frayed the cloth by chewing it into a fluffy mass of fibers.

When the top of a nest was opened to inspect young, the female would
attempt to pull the nesting material back into shape by means of
forefeet and teeth. The mother's defensive posture was to cover the
young with her body, often lying over them and facing upward, toward the
investigator. In this semi-recumbent position, the female would attack
the investigator's fingers with her forefeet and teeth. Often the female
would stand bipedally and use the forefeet and teeth to mount the
attack. If at this time a young chanced to wander away from the mother,
she would quickly pick it up and place it in the nest at her feet.

When disturbed, females of both species, but especially _P.
maniculatus_, often dove headlong under their nest or into the wood
shavings on the floor of the cage. This type of retreat was most often
used when young were nursing. Time is required even by the mother to
disengage nursing young, and this mode of escape is the most expedient.
The mother disengaged nursing young by licking around their faces and
pushing with her paws.

Nursing females of both species tolerated the male parent in the nest. A
male and female often sat side by side in the nest and by means of their
bodies participated in covering the young. Males were not observed to
attempt any defense of the nest, or of the young. Females were tolerant
of older young in the nest when another litter was born and was being
nursed. In one nest, a female of _P. truei_ gave birth to a litter of
three when her older litter was 29 days old. The three older young
continued to nurse until they were 37 days old, at which time they were
removed from the cage. The female appeared tolerant of this nursing by
members of the older litter, but appeared to give preference to the
wants of the younger offspring.

One female of _P. truei_ lost or killed all but one young of her litter;
at about the same time, a _P. maniculatus_ and all but one of her young
inexplicably died. Since the remaining young _maniculatus_, a male, was
just weaned and was considered expendable, I placed him in the cage with
the female _truei_ and her 33-day-old, male offspring. The reaction to
the newcomer was unexpected. The female immediately covered the _P.
maniculatus_ and her own young and prepared to defend them against me.
Later, when the _P. maniculatus_ was disturbed, he had only to emit a
squeak and the female _truei_ would run to cover and protect him. When
the young male of _P. truei_ was 69 days old the female kept him out of
the nest, but still kept the male _maniculatus_ in the nest with her.
Although the female was somewhat antagonistic to her own young, she did
not injure him, but only kept him out of the nest. The male _truei_ was
left in the cage with his mother and the _P. maniculatus_ from September
23 to December 10. None of the mice had any apparent cuts on the ears or
tail to indicate fighting. As much as seven months after the _P.
maniculatus_ was introduced into the cage, the female _truei_ continued
to cover him with her body whenever there was a disturbance. The male
_maniculatus_ not only tolerated this attention, but ran under the
female _truei_ when frightened. "Adoption" of young of another species
has been reported for a number of animals, but, without further
evidence, it is not possible to postulate that such adoptions occur
between species of _Peromyscus_ in nature.

Young males are tolerated by their mothers after weaning. One young male
_maniculatus_ was left in the cage with his mother from the time of his
birth in autumn until late February of the following year. A litter was
born on February 24. A young male _P. truei_ was also left in the cage
with his mother until he had acquired most of his postjuvenal pelage;
the female and male usually sat together in the cage.

Females of both species sometimes eat their young when the young die
shortly after birth. One female of each species killed three of her four
young, and ate their brains and viscera. In one of these cases, the
female, of _P. maniculatus_, also died; the female of _P. truei_ was the
same one that adopted the surviving _P. maniculatus_. The female _truei_
continued to nurse her one remaining young for at least several days
after killing three of his litter mates. A reason for this cannibalism
might have been that I had fed these mice for several weeks on a mixture
of grains low in protein content. Inadequacy of this diet for nursing
females may have caused them to become cannibalistic. The feed of all
captives was changed to Purina Laboratory Chow after the young were

Transportation of Young

Females of both species transported their young either by dragging them
collectively while the young were attached to mammae, or by carrying
them one at a time in the mouth. Since mice of the subgenus _Peromyscus_
have three pairs of nipples, they probably transport only six young
collectively. Svihla (1932:13) has stated that both pectoral and
inguinal teats are used in transporting young, in contrast to Seton's
reputed assertion that only inguinal nipples were used. But Svihla
neglected to cite Seton's complete statement. Seton (1920:137) recorded
a litter of three as using only the inguinal mammae, but on the
following page recorded the use of both inguinal and pectoral mammae by
another litter of four. My findings agree with those of Svihla. Nursing
females of both species were removed periodically from cages by lifting
them by the tail. The young would hang onto the mammae and the female
would clutch the young to her with all four feet. Young two weeks old or
older crawled behind the mother while nursing.

The method of transporting young in the mouth has been mentioned by
Seton (1920:136) and described by Lang (1925) and Hall (1928:256). These
authors report that the mother picks the young up in her paws, and
places it ventral-side up in her mouth, with her incisors around it. The
young are not picked up by the skin on the nape of the neck, as are the
juveniles of dogs and cats. I have found that females of both species of
_Peromyscus_ carry their young ventral-side up in their mouth while the
young are small, and sometimes when the young are older. Generally, when
females of _P. truei_ moved young weighing more than 10 grams, the
female grasped the young from the dorsal side, across the thorax just
posterior to the shoulders, and held them with the incisors more or less
around the animal. Perhaps this method was used with older young because
of the observed tendency of the larger young to resist being turned over
and grasped from the ventral side, and because their increased weight
would have made it difficult, if not impossible, for the mother to pick
them up with her paws. The young rarely resisted the efforts of the
mother to move them by this method; when grasped across the thorax by
the mother, the young would remain limp until released. Some females of
_P. truei_ would drag almost fully grown young back into the nest in
this manner. I have not observed older young of a comparable age to be
moved by females of _P. maniculatus_. The females of _P. maniculatus_
appear to be somewhat less concerned than those of _P. truei_ for the
welfare of their young once they are mobile and close to being weaned.

The following listing describes changes in postnatal development of
young, of each species, from birth to nine weeks of age.

         _P. maniculatus_                    _P. truei_
  FIRST WEEK: At birth, young are   | At birth, young are helpless, red
  helpless, red overall, small      | overall, smaller than _P. truei_,
  with wrinkled skin. Pinna of ear  | skin wrinkled. Ear, eyes, and
  folded over and closed; eyes      | digits as in _P. truei_.
  closed; digits not separated      |
  from rest of foot.                |
  Redness diminished by fourth day. | Redness decreases and disappears by
                                    | fourth day.
  Hair apparent by fifth day;       | Hair apparent by fourth day; body
  dorsal one-half or two-thirds of  | bicolored by end of week.
  body more darkly pigmented than   |
  venter by fourth day.             |
  Young squeak loudly and suck;     | Young squeak loudly; sucking more
  sometimes crawl, but drag hind    | pronounced than in _P. truei_; may
  legs.                             | crawl, but drag hind legs.
  SECOND WEEK: Appreciable increase | As in _P. truei_.
  in size; head about 60 percent    |
  larger than at birth, by 14th     |
  day, and still large in           |
  proportion to body.               |
  Toes on hind foot separated more  | As in _P. truei_, but somewhat more
  from foot.                        | advanced.
  Body well haired by end of week;  | Body well haired by end of week;
  dorsum dark gray, venter whitish; | dorsum dark gray with brownish
  tail bicolored in most, but not   | tint; venter whitish; tail
  haired.                           | bicolored in most, but not haired.
  Pinna of ear unfolded and open by | As in _P. truei_, but development
  end of week.                      | somewhat more advanced.
  Through day 10, use hind legs to  | Crawl well by end of week;
  push, but by end of week use legs | difficult to hold, squirm but do
  to crawl; difficult to hold,      | not bite; agile.
  squirm but do not bite.           |
  Walk behind mother while nursing; |
  agile.                            |
  THIRD WEEK: Eyes open on 16th to  | Eyes open on 16th to 20th day,
  21st day.                         | partly open earlier.
  Gray pelage of dorsum brownish.   | Pelage of dorsum brownish; molt
  Apparently there is a molt line   | line across shoulders progressing
  progressing posteriorly from      | posteriorly; browner anterior to
  nose; the molt line has moved to  | line, grayer posterior to it.
  shoulder region by end of week;   |
  pelage anterior to line browner,  |
  grayer posterior to it.           |
  Tail haired and weakly bicolored  | Tail haired and bicolored in all
  in some individuals by end of     | individuals.
  week.                             |
  Young walk and jump well; squirm  | Young walk and jump well; fight and
  but rarely bite.                  |  bite when handled.
  FOURTH WEEK: Begin to eat solid   | Some young eat grain by 24th day;
  foods at 23-29 days, but also     | others continue to nurse.
  nurse.                            |
  Molt line about 3/4 inch          | Juvenal pelage complete; no sign of
  posterior to head. Juvenal pelage | postjuvenal molt.
  completed by end of week. Some    |
  young have brownish hair on front |
  legs.                             |
  Young roll over on backs and use  | As in _P. truei_; also, all jump
  feet to ward off litter mates     | well, and fight fiercely when
  that are dropped into nest, or    | handled.
  into container, with them.        |
  FIFTH WEEK: Young weaned on 30th  | All young weaned before or by end
  to 40th day; some nurse beyond    | of week; none observed to nurse
  30th day if female is lactating.  | beyond 30th day, even if female is
                                    | lactating.
  Juvenal pelage complete and no    | Juvenal pelage complete;
  postjuvenal molt apparent on      | postjuvenal pelage not apparent on
  dorsum.                           | most, but probably present on all,
                                    | and concealed under juvenal pelage.
  SIXTH WEEK: Postjuvenal pelage    | Postjuvenal molt apparent in most
  apparent in most individuals      | young; almost complete in some,
  under juvenal pelage, especially  | except above tail and on flanks.
  along lateral line.               |
  SEVENTH WEEK: Postjuvenal pelage  | Postjuvenal pelage apparent in all
  apparent in most young; in some   | young; less distinct molt line than
  the molt line has progressed well | in _P. truei_.
  up on the sides, but not to       |
  mid-dorsum.                       |
  EIGHTH WEEK: All individuals      | Growth completed in some
  growing; total lengths of 156-170 | individuals; those in larger
  millimeters; weight 17-22 grams.  | litters have total lengths of
                                    | 128-144 millimeters; weight
                                    | 14-17 grams.
  NINTH WEEK: Testes partly scrotal | Scrotum in season usually large,
  in one male on 59th day.          | vaginae open, evidence of coitus
                                    | common. (McCabe and Blanchard,
                                    | 1950:39).
  New brown pelage encroaching on   | Postjuvenal molt completed in some
  saddle and on hind legs;          | individuals by end of week. New
  postjuvenal molt completed in     | pelage tends to be concealed under
  some individuals by eleventh      | juvenal pelage longer than in _P.
  week.                             | truei_.


Increase in length of limb bones, changes in proportion of bones in the
skull, eruption and degree of wear of teeth, and changes in pelage can
be used to ascertain relative age. Different investigators might choose
different limits for the three categories young, subadult, and adult.
Museum specimens were assigned to one of five age groups listed below
mostly on the basis of tooth wear, essentially as described by
Hoffmeister (1951:1).

     Juvenile: M3 just breaking through bony covering of jaw or showing
     no wear whatsoever.

     Young: M3 worn smooth except for labial cusps, and M1 and M2
     showing little or no wear.

     Subadult: M3 worn smooth; labial cusp may persist, but is well
     worn; M1 and M2 having lingual cusps worn, but not smooth; labial
     cusps showing little wear.

     Adult: Lingual cusps worn smooth and labial cusps showing
     considerable wear; labial cusp of M3 may persist.

     Old: Cusps worn smooth; not more than one re-entrant angle per
     tooth discernible, frequently none.

For live animals examined in the field, criteria based on pelage and
breeding condition were used, as follows:

     Juvenile: Only gray, juvenal pelage present.

     Young: Subadult pelage apparent on lateral line or on sides; body
     usually smaller than in adults.

     Subadults: Subadult pelage having mostly replaced juvenal pelage;
     mice often as large as adults; testes of males often abdominal in
     breeding season; gray juvenal pelage may persist on head of some

     Adult: Adult pelage present; body usually largest of all animals in
     population; females may have enlarged mammae from nursing previous
     litters; testes of males usually scrotal in breeding season; gray
     pelage may be present on head of some individuals.

Old individuals in the field could not be distinguished from adults;
hence any animals that appeared older, or more developed, than subadults
were classified as adults.

In _P. truei_, subadult pelage appears first on the lateral line or on
the flanks; new pelage is ochraceous and contrasts markedly with the
gray juvenal coat. In _P. maniculatus_, the subadult pelage contrasts
less with the juvenal coat; the new pelage progresses from anterior to
posterior over the body in the same manner as in _truei_, but replaces
the juvenal coat in a less distinct manner than in _truei_. As a result,
contrast often is lacking between juvenal and subadult pelages in
_maniculatus_ making it difficult to assign an individual to one of
these two age categories when examined in the field. In museum
specimens, the subadult pelage is much more noticeable because it can be
compared with the pelages of other specimens. The subadult pelage in _P.
maniculatus_ is duller than the adult pelage: In _P. truei_ the subadult
and adult pelages appear to have an equal sheen.

In early winter, the postjuvenal pelage acquired by young individuals of
_P. truei_ was thick and luxuriant and indistinguishable from the winter
pelage of adults. My observations lead me to conclude that individuals
born late in the breeding season molt from juvenal summer pelage
directly into winter adult pelage. Technically, this new coat is the
postjuvenal one, yet it cannot be distinguished as such after the molt
is completed.


Anatomical anomalies were rare in the individuals of _Peromyscus_ that I
examined. When anomalies were found they were striking, principally
because of their low rate of occurrence.

One female of _P. truei_, born in captivity, had a congenital defect of
the pinna of the right ear, noted on the fifteenth day after birth.
Closer examination then and later revealed that the pinna was normal in
all respects except that the tip was missing. The tip showed no evidence
of injury. When the mouse was subadult, this defective pinna was
approximately half as long as the normal pinna. The topmost part of the
defective pinna was somewhat more constricted in circumference than the
normal one.

On September 11, 1963, a subadult male of _P. truei_ was captured that
had five functional toes on its right front foot, the only one of more
than 175 individuals caught and handled in the field that exhibited
polydactyly. The front foot was examined closely in the field, but it
could not be determined how or where the extra bones of the sixth toe
articulated. _Peromyscus_ normally has four full-sized toes on each
front foot, and a small inner toe hardly more than an enlarged tubercle,
having no nail.

A few mice of both species had broken toes or claws torn off. Such
injuries were more common on toes of the hind foot. In several instances
the toes were shortened, as if by marking, although the animals
concerned had been marked earlier by clipping toes other than the
injured toes. The reason for these injuries is not apparent, although
they could have been caused by fighting, or from having been caught in
doors of Sherman live traps.

Toes of several mice were swollen and inflamed due to small glochids of
cacti that were stuck in them. Apparently the mice had stepped on the
glochids by chance, for I found no evidence that _Peromyscus_ of either
species eats cacti.

One _P. truei_ had a broken tail; three other individuals had tails
about one-half normal length. One _P. maniculatus_ had a shortened tail.
Some of these injuries probably were caused by the Sherman live traps;
several individuals of _P. truei_ were released after having been caught
by the tail by the spring-loaded door of these traps.

On October 17, 1963, an adult _P. truei_ had a bleeding penis; when this
mouse was recaptured on October 25, the injury was healed.

Losses Attributed to Exposure in Traps

Observations of wild mice caught in live traps suggest that metabolic
maturity is reached later than physical and reproductive maturity. In
such trapping, it became apparent that juvenal and young mice suffered
from exposure to cold and to heat much more than did subadult or adult
mice. Although traps were carefully shaded and ample nesting material
and food provided, some mice died in the traps. An overwhelming majority
of these mice were juveniles and young.

Traps were checked in the morning, both in the summer and autumn, yet
mice died in traps that were barely warm to the touch, in summer, and
cool to the touch in autumn. Older mice frequently were found in traps
that were warm, or even hot, to the touch; yet the older mice rarely
died in such traps. Apparently the tolerance of adults is much greater
to heating and chilling. Greater bulk and perhaps longer pelage in
adults might provide sufficiently better insulation to account for this

Occasionally juvenal mice were found in traps in a sluggish and weakened
condition, especially in autumn when nights were cool. In such cases the
mice were either cupped in the hands and warmed until lively enough to
fend for themselves, or, if especially weakened, were taken to the
laboratory. None of such animals that were returned to the laboratory
lived for more than two weeks. Most of those released in the field did
not reappear in the traps.

I conclude that juvenal and young mice placed under stress by
overheating or cooling die immediately or live only a few days. Subadult
and adult animals tolerate more extreme conditions of overheating or
cooling, presumably because they are able to regulate their internal
temperature better, by either losing or retaining heat more effectively.

Mice found dead in overheated traps had salivated heavily, and may also
have licked the fur on their chests to increase heat dissipation. One
such adult, of _P. truei_, had a wet chest when he was taken from a warm
trap; when released, this mouse ran to a nearby plant of _Comandra
umbellata_, and ate a few of the succulent leaves before running off.
This individual was trapped several times later in the summer, and
apparently suffered no ill effects from the exposure.

Dental Anomalies

Abnormalities in the formation and occlusion, or decay of teeth, are
relatively rare in wild mammals. Of all bodily structures, the teeth
apparently are under the most rigid genetic controls; they form early in
the embryo and follow rigidly specified patterns in their ontogeny.
Apparently any deviation from the normal pattern of tooth formation is
quickly selected against. All specimens of _P. m. rufinus_ and _P. t.
truei_ in the collection of the Museum of Natural History at the
University of Kansas, and in my collection, were examined for dental
anomalies. A total of 317 specimens of _P. m. rufinus_ and 54 specimens
of _P. t. truei_ were examined. The following specimens were found to
have abnormalities:

K. U. 69361, _P. maniculatus_, adult: Small bundles of plant fibers are
lodged between all upper teeth and have penetrated the maxilla anterior
to the left M1. The maxillary bone is eroded away from the roots of all
teeth. The anteriormost roots of both lower first molars are almost
completely exposed, because the dentary has been abraded away.

K. U. 76041, _P. maniculatus_, young: A piece of plant fiber is wedged
between the left M2 and M3. The maxillary bone has eroded away from
around the roots of M3, indicating the presence of an abscess in this

K. U. 69362, _P. maniculatus_, adult: All teeth in the lower right
tooth-row are greatly worn, especially on the lingual side. The labial
half of the right M1 is all that remains; decay is apparent both in the
crown and roots on the lingual side of this tooth.

K. U. 69397, _P. maniculatus_, old: The maxillae have eroded away from
around the anterior roots of each first upper molar, leaving these roots

C. L. D. 231, _P. maniculatus_, old: The teeth in this female are
greatly worn; re-entrant angles are not visible in any teeth. A circular
hole, 0.1 millimeter in diameter, exists in the dentine immediately over
(when viewed from the underside of the skull) the posterior root of the
right M1. The crowns of the teeth are greatly reduced in height, and the
dentine is thin.

Anomalies in the Skull

Wormian bones and other abnormalities in the roofing bones are noted, as

K. U. 76090, _P. maniculatus_, young: The interparietal is divided; the
divided suture is in line with the suture between the parietals. The
interparietal is 7.8 millimeters long.

K. U. 76091, _P. maniculatus_, young: A wormian bone, 0.5 millimeter by
0.2 millimeter, lies between the anterior border of the interparietal
and the posterior border of the left parietal, at a point midway between
the center line of the skull and the posterolateral border of the
parietal bone.

C. L. D. 248, _P. maniculatus_, adult: An oval wormian bone, 1.1
millimeters long and 0.6 millimeter wide, lies between the parietals at
their posterior margin; the long axis of the bone is parallel to the
long axis of the skull.

C. L. D. 246, _P. maniculatus_, juvenal: The interparietal is divided
equally by a suture. An oval wormian bone, 0.3 millimeter long and 0.1
millimeter wide, lies between the frontals, midway between the anterior
and posterior borders of these bones.

C. L. D. 656, _P. maniculatus_, young: A small, rounded wormian bone
lies between the right parietal and interparietal, lateral to the
posterior junction of the suture between the parietals. This bone
extends anteriorly into the parietal bone from the suture of the
interparietal and parietal. This bone is 0.7 millimeter wide, and
extends 0.6 millimeter into the parietal.

C. L. D. 662, _P. maniculatus_, subadult: An elongated, diamond shaped
wormian bone closes the suture between the parietal bones. This bone is
2.3 millimeters long and 0.8 millimeter wide.

K. U. 34735, _P. truei_, old: The anterior one-quarter of the left
parietal bone is slightly depressed; and the posterior one-third of the
left frontal and anterior one-quarter of the left parietal are thin and
sculptured. This malformation of the roofing bones posterior to the
orbit probably is not the result of a break, for the orbital part of the
frontal bone is normal. The frontal-parietal sutures are in the normal
positions on both sides of the skull.

The above-mentioned anomalies do not appear to be correlated with age or
locality at which the specimens were taken. Apparently such anomalies
are present throughout the population, but in a small percentage of


Mice of the genus _Peromyscus_ are known to eat a wide variety of plants
and arthropods, and to be highly opportunistic in selection of food
(Cogshall, 1928; Hamilton, 1941; Williams, 1955, 1959a; Jameson, 1952;
Johnson, 1962). In order to determine possible food preferences, captive
mice of both species were fed plants indigenous to Mesa Verde. Entire
plants were used whenever possible; available seeds also were offered
(Tables 5, 6). All feeding experiments were replicated with at least six
different individuals in order to minimize the trends resulting from
individual preferences or dislikes. The mice of each species tended to
be consistent in their feeding.

The plant species listed in Tables 5 and 6 were those that were eaten or
rejected by a majority of the individuals tested.

Plant material eaten by _P. maniculatus_ and refused by _P. truei_
included only the leaves and stem of _Viguiera multiflora_. Plant
material eaten by _P. truei_ and refused by _P. maniculatus_ included
the leaves of _Calochortus gunnisonii_ and the leaves and stem of
_Erigeron speciosus_.

    TABLE 5--Plants, or Parts of Plants, Eaten by Captive Individuals of
       _P. truei_ in Mesa Verde National Park, Colorado. 0 = not eaten,
       + = eaten, - = not offered.

        Species of Plant       | Leaves | Stem | Flower | Seeds
  _Amelanchier utahensis_      |    -   |   -  |    -   |   +
  _Calochortus gunnisonii_     |    +   |   +  |    -   |   +
  _Chaenactis douglasii_       |    0   |   0  |    -   |   -
  _Chrysothamnus depressus_    |    0   |   0  |    0   |   -
  _Chrysothamnus nauseosus_    |    +   |   0  |    0   |   -
  _Comandra umbellata_         |    +   |   +  |    -   |   -
  _Erigeron speciosus_         |    +   |   +  |    -   |   -
  _Eriogonum alatum_           |    -   |   -  |    -   |   +
  _Juniperus osteosperma_      |    -   |   -  |    -   |   +
  _Lupinus caudatus_           |    0   |   0  |    +   |   -
  _Lithospermum ruderale_      |    0   |   0  |    -   |   0
  _Mellilotus alba_            |    +   |   +  |    +   |   +
  _Mellilotus officinalis_     |    +   |   +  |    +   |   -
  _Orthocarpus purpureo-albus_ |    +   |   +  |    +   |   +
  _Pedicularis centranthera_   |    +   |   +  |    -   |   -
  _Penstemon linarioides_      |    +   |   +  |    -   |   +
  _Pinus edulis_               |    -   |   -  |    -   |   +
  _Polygonum sawatchense_      |    +   |   +  |    -   |   0
  _Solidago petradoria_        |    0   |   0  |    0   |   -
  _Viguiera multiflora_        |    0   |   0  |    0   |   0

Plant material eaten by captives of both species included _Calochortus
gunnisonii_--stem and seeds; _Comandra umbellata_--leaves and stem;
_Eriogonum alatum_--seeds; _Penstemon linarioides_--leaves and stem;
_Pinus edulis_--seeds; and _Juniperus osteosperma_--seeds.

Plant materials refused by both species of mice included the leaves and
stem of _Chaenactis douglasii_, the leaves, stem and seeds of
_Lithospermum ruderale_, and the leaves, stem and flowers of _Solidago

Cricetine rodents chew plant and animal foods thoroughly; contents of
their stomachs appear as finely-particulate fragments. These fragments
invariably contain pieces of epidermis from ingested plants. Due to the
presence of cutin in the cell walls, epidermis is last to be digested.

Microscopic analysis of plant epidermis is useful in helping to
determine food habits of various animals (Dusi, 1949; Williams, 1955,
1959a; Brusven and Mulkern, 1960; Johnson, 1962). The microscopic
analysis of stomach contents provides a practical method of determining
which plants are eaten by rodents. Contents of stomachs and intestines
were removed from mice caught in snap traps, and from preserved
specimens. The contents were placed on a piece of bolting silk, washed
thoroughly with running water, stained with iron-hematoxylin and mounted
on slides, or stored in 70 per cent ethanol (Williams, 1959a; Douglas,

    TABLE 6--Plants, or Parts of Plants, Eaten by Captive Individuals of
       _P. maniculatus_ in Mesa Verde National Park, Colorado. 0 = not
       eaten, + = eaten, - = not offered.

        Species of Plant       | Leaves | Stem | Flower | Seeds
  _Artemisia ludoviciana_      |    0   |   0  |    -   |   -
  _Calochortus gunnisonii_     |    0   |   +  |    -   |   +
  _Chaenactis douglasii_       |    0   |   0  |    -   |   -
  _Comandra umbellata_         |    +   |   +  |    -   |   -
  _Erigeron speciosus_         |    0   |   0  |    -   |   -
  _Eriogonum alatum_           |    -   |   -  |    -   |   +
  _Juniperus osteosperma_      |    -   |   -  |    -   |   +
  _Lappula redowskii_          |    0   |   0  |    -   |   +
  _Lithospermum ruderale_      |    0   |   0  |    -   |   0
  _Orthocarpus purpureo-albus_ |    0   |   0  |    +   |   +
  _Penstemon linarioides_      |    +   |   +  |    +   |   -
  _Pinus edulis_               |    -   |   -  |    -   |   +
  _Purshia tridentata_         |    +   |   +  |    -   |   -
  _Sitanion hystrix_           |    0   |   0  |    -   |   0
  _Solidago petradoria_        |    0   |   0  |    0   |   -
  _Sphaeralcea coccinea_       |    +   |   +  |    -   |   +
  _Stipa comata_               |    0   |   0  |    -   |   +
  _Viguiera multiflora_        |    +   |   +  |    -   |   -

In order to analyze these epidermal fragments, a collection of plants
was made within the park. Slides of the epidermis of these plants were
prepared and analyzed for diagnostic characters (Douglas, 1965:197-199).
Features such as the stomatal arrangement in relation to subsidiary
cells; the types of trichomes, scales and glands; the cellular
inclusions such as starch grains, mucilage and resins are of taxonomic
value (Metcalfe and Chalk, 1950). The configuration of the anticlinal
cell walls is useful in separating species that are similar in other
respects (Douglas, 1965:199).

The following species of plants, and other food items, were identified
in the stomach or intestinal contents of _Peromyscus maniculatus_:

  _Agropyron smithii_
  _Artemisia_ sp.
  _Eriogonum umbellatum_
  _Lupinus ammophilus_
  _Penstemon linarioides_
  _Phlox hoodii_
  _Stipa comata_
  Arachnid legs

Stomach and intestinal contents of _P. truei_ contained the following
food items:

  _Artemisia nova_
  _Artemisia_ sp.
  _Penstemon_ cf. _barbatus_
  _Penstemon_ cf. _linarioides_
  _Poa fendleriana_
  Arachnid legs
  _Eriogonum_ sp.
  _Gutierrezia sarothrae_
  _Yucca_ sp.

Many of the plants eaten by the mice had large numbers of crystals in
the epidermis. Druses were the most abundant, but raphid crystals also
were seen. Every slide contained at least one species of plant which
contained druses. Such crystals are composed mostly of calcium oxalate
(Esau, 1960:41). In Mesa Verde, families of plants having crystals
include: Boraginaceae, Chenopodiaceae, Compositae, Cruciferae,
Leguminosae, Liliaceae, Malvaceae, Ornargraceae, Rosaceae, and
Saxifragaceae. Calcium oxalate is a highly insoluble compound and is
innocuous if it passes through the gastro-intestinal tract without being
absorbed. In rats of the genus _Neotoma_, some calcium oxalate
passes through the intestines unchanged, but large amounts of calcium
are absorbed through the intestine. The urine of pack rats is creamy in
color and contains calcium carbonate. It is not understood how these
rats metabolize the highly toxic oxalic acid, when converting calcium
oxalate to calcium carbonate (Schmidt-Nielsen, 1964:147-148). Apparently
calcium oxalate passes through the intestine unchanged in both species
of _Peromyscus_, for their urine is clear and yellowish.

Although both species of mice appear to prefer plants having soft
leaves, some plants having coarse leaves also are eaten. Many of the
slides contained isolated sclerids. The stomach contents of one
individual of _P. truei_ contained a small fragment of the epidermis of
_Yucca_. This fragment may have come from a young shoot. It is unlikely
that _Peromyscus_ would eat the larger, coarser leaves of _Yucca_.

Pinyon and juniper nuts were found in nests of all mice. Captive mice
were especially fond of pinyon nuts, and these probably provide a
substantial part of the diet of _Peromyscus_ in the autumn and early
winter. The winter staple of _P. truei_ appears to be juniper seeds.
Nesting sites of this mouse often could be located by the mounds of
discarded seeds lying nearby.

Both species eat pinyon and juniper seeds; since _P. truei_ lives in the
forest, it has better access to these foods than does _P. maniculatus_.
Mice remove the embryos of juniper seeds by chewing a small hole in the
larger end of the seed. The seed coats of juniper are extremely hard,
and a considerable amount of effort must be expended to remove the
embryo. Captives discarded the resinous and pithy, outer layers of
juniper berries. Individuals of _P. truei_ are adept climbers. Since
many juniper berries remain on branches throughout the winter, the
ability of these mice to forage in the trees would be especially
advantageous when snow covers the ground.


_Peromyscus maniculatus_ is ubiquitous, occurring in habitats ranging
from mesic boreal forests to arid southwestern deserts. Most subspecies
of _P. maniculatus_ live in moderately mesic or near-mesic environments,
but a few have adapted to arid conditions. It has been assumed that the
success of _P. maniculatus_ in inhabiting such diverse habitats is
associated with its adaptability to different kinds of food and varying
amount of available water (Williams, 1959b:606).

Throughout its range _P. maniculatus_ coexists with one or more other
species of _Peromyscus_ that are more restricted in distribution.
_Peromyscus truei_ is one such species.

Both species live under xeric or near-xeric conditions, for the climate
of Mesa Verde is semi-arid. Other than a few widely-scattered springs,
there are no sources of free water on the top of the Mesa Verde land
mass; thus animals inhabiting the park must rely upon moisture in the
plants and other foods they eat, or upon dew.

Several investigators have studied water consumption in mice of the
genus _Peromyscus_ (Table 7). Dice (1922) did so for the prairie deer
mouse, _P. m. bairdii_, and the forest deer mouse, _P. leucopus
noveboracensis_, under varying environmental conditions. He found that
both species drank about the same amounts of water per gram of body
weight, and that food and water requirements did not differ sufficiently
to be the basis for the habitat differences between these species.
Neither of his samples was from an arid environment. Chew (1951) studied
water consumption in _P. leucopus_, and recently reviewed the literature
on water metabolism of mammals (Chew, 1965). In his studies of five
subspecies of two species of _Peromyscus_, Ross (1930) found significant
differences in water consumption between species but not between
subspecies within a species. One of the subspecies of _P. maniculatus_
tested was from a desert region, whereas the other two were from mesic
areas along the coast of California.

Lindeborg (1952) was the first to measure water consumption of both _P.
m. rufinus_ and _P. t. truei_, the species and subspecies with which my
experiments are concerned. Lindeborg also tested the ability of five
races of _Peromyscus_ to survive reduced water rations. Unfortunately,
the subspecies chosen for these experiments did not include _P. t.
truei_ or _P. m. rufinus_. Lindeborg (1952:25) found that the "amounts
of water consumed by various species of _Peromyscus_ from different
habitats within the same climatic region were not conclusively
different." However, he did find significant differences between some
subspecies from different geographical areas. For example, he found no
significant difference in water consumption between _P. m. bairdii_ from
Michigan and either _P. m. blandus_ or _P. m. rufinus_ from New Mexico,
but he found a highly significant difference between _P. l.
noveboracensis_ from Michigan and _P. l. tornillo_ from New Mexico.
Lindeborg also found that the subspecies of _Peromyscus_ that consumed
the least water, and that were best able to survive a reduced water
ration, were those from the more xeric climatic areas.

Some mammals may be able to change their diets in times of water stress,
and thereby compensate for a shortage of water. At such times,
_Dipodomys_ selects foods with high percentages of carbohydrates and
conserves water by reducing the amounts of nitrogenous wastes to be
excreted (Schmidt-Nielsen _et al._, 1948).

Williams (1959b) found that _P. m. osgoodi_ from Colorado drank more
water on a diet rich in protein than on one rich in carbohydrates. But,
her mice on a high carbohydrate diet used less than a normal amount of
water for a period of only five weeks; at the end of the five weeks they
were drinking about as much as they had been when on the control diet of
laboratory chow. Likewise, mice adjusted to the high protein diet by
consuming more water; but by the end of the fifth week their daily water
consumption approximated the amount drunk when fed on laboratory chow.
Because of these results, Williams questioned the validity of the
assumption that _P. maniculatus_ is able to inhabit a diversity of
habitats because of its adaptability with respect to food and water

I conducted a series of experiments on water and food consumption by
individuals of _P. truei_ and _P. maniculatus_. It was thought that if
there were differences in water or food consumption, or both, knowledge
of them might help to explain the obvious differences in habitat
preferences of these two species in Mesa Verde National Park.

In August of 1965, 30 individuals of _P. truei_ and _P. maniculatus_
were trapped in Mesa Verde National Park at elevations of 7000-8400
feet, and transported to Lawrence, Kansas, where the experiments were
carried out.

Mice were housed in individual metal cages (10 x 7.5 x 5 inches), having
removable tops of wire mesh, and an externally-mounted water bottle that
had a drop-type spout extending into the cage. Cages were on one of five
shelves of a movable tier of shelving, and were rotated randomly, from
one shelf to another, each week. A layer of dry wood shavings covered
the bottom of each cage. A control cage was similarly equipped.

The mice were kept in a room in which temperature and photoperiod were
controlled. The ambient air temperature of this room was 20 to 23
degrees Centigrade throughout the experiments, and averaged 21 degrees.
Humidity was not controlled, but remained low throughout the
experiments. The room was illuminated for eight hours each day, from
about 9 A. M. to 5 P. M.

The animals were fed at least once a week, at which time all remaining
food was weighed and discarded, and the remaining water was measured.
Tap water was used in all of the experiments. The cages were cleaned
each week. Each time the cages containing mice were handled, the control
cage was handled in the same way. The amount of evaporation was
determined each week by measuring the water remaining in the bottle of
the control cage.

Water and food consumption of individuals of _P. maniculatus_ and _P.
truei_ were measured when the mice were fed diets of differing protein
content. To my knowledge, the only other study in which water
consumption was measured for mice of the genus _Peromyscus_ on diets of
different protein contents was by Williams (1959b). Because of the
limited number of animals available, it was decided that the best
results could be obtained by placing all individuals on the same diet
for a predetermined number of weeks, then on a second diet for a certain
period, and so on.

Each mouse was weighed at the beginning, at the mid-point, and at the
end of each experiment. The mice were weighed on the same days, at times
when they were inactive. Because weights of individual mice differ,
water and food consumption was calculated on the basis of the amount
consumed per gram of body weight per day. All foods were air-dry and
contained a negligible amount of water.

First, food and water consumption was measured for nine individuals of
each species on a diet of Purina Laboratory Chow. This chow contains not
less than 23 per cent protein and 4.5 per cent fat, and about 57 per
cent carbohydrate. Since the mice had been maintained on this diet for
several months prior to the experiments, food and water consumption was
measured for a period of only two weeks. Individuals of _P. truei_
consumed more total water and more water per gram of body weight than
individuals of _P. maniculatus_ (Table 7).

Next, 10 mice of each species were placed on a diet of Purina Hog Chow
for a period of four weeks. This chow contains not less than 36 per cent
protein and one per cent fat, and about 42 per cent carbohydrate. Both
species increased their daily water consumption immediately after being
placed on this diet (tables 7 and 11). On the high protein diet, _P.
truei_ again consumed much more water than did _P. maniculatus_ (tables
7 and 9).

    TABLE 7--Food and Water Consumption of _Peromyscus maniculatus_ and
       _P. truei_ When Fed Diets of Different Protein Content. Food and
       Water Consumption Are Determined for the Grams, or Milliliters,
       Consumed per Gram of Body Weight per Day; Daily Totals Are also

                    _Peromyscus maniculatus rufinus_
      Diet    |      | Food          | Total | Water         | Total
    per cent  |  No. | /gram         | grams | /gram         | water
    protein   | mice | /day  ± S. D. | /day  | /day  ± S. D. |  /day
  Lab Chow 23 |   9  | .201     .074 | 4.455 | .262     .183 |  5.751
  Hog Chow 36 |  10  | .238     .060 | 5.232 | .496     .186 | 10.749
  Corn 11     |  11  | .149     .044 | 3.144 | .174     .012 |  3.696

                        _Peromyscus truei truei_
      Diet    |      | Food          | Total | Water         | Total
    per cent  |  No. | /gram         | grams | /gram         | water
    protein   | mice | /day  ± S. D. | /day  | /day  ± S. D. |  /day
  Lab Chow 23 |  10  | .216     .070 | 6.353 | .373     .119 | 10.880
  Hog Chow 36 |  10  | .230     .079 | 6.966 | .653     .189 | 19.571
  Corn 11     |  10  | .158     .010 | 4.318 | .332     .016 |  9.034

The tendency of both species to eat more of the hog chow than they ate
when fed standard laboratory chow may reflect a higher palatability of
the hog chow. Both species consumed similar amounts of food per gram of
body weight, on each of the diets (Table 7). The larger _P. truei_
requires more grams of food per day than the smaller _P. maniculatus_,
but this slight difference in food consumption probably has no effect on
the distribution of these species within Mesa Verde.

The results obtained with the low protein diet were strikingly different
from those of the first two experiments. In this experiment the same
groups of mice were placed on a diet of whole, shelled corn for a period
of six weeks. The corn contained less than 11 per cent protein, about
three per cent fat, and about 80 per cent carbohydrate.

By the end of the first week, on the low protein diet, all mice had
reduced their water intake by about half the amount used per day on the
high protein diet (Table 7). There was not a statistically significant
difference, for either species, between the average amounts of water
drunk in the first and in the sixth weeks of the experiment.

The data in Table 7 show that on all three diets, individuals of _P.
maniculatus_ drank less water per gram of body weight than individuals
of _P. truei_. Variation in water consumption was high; some individuals
of _P. maniculatus_ that drank more than the average amount for the
species, consumed as much water as some individuals of _P. truei_ that
drank less than the average amount. In general, individuals of _P.
maniculatus_ drank about half as much water each day as individuals of
_P. truei_. Individuals of both species were consistent in their
day-to-day consumption.

    TABLE 8--Amounts of Mean Daily Water Consumption as Reported in the
       Literature for Species of _Peromyscus_. Figures in Parentheses are
       Means; Those Not in Parentheses Are Extremes.

  Column headings:

  A: Mean daily ml./gm. wt./day
  B: Water consumption total ml. per day
  C: Temperature
  D: Humidity
  E: Per cent dietary protein
  F: Investigator

                  |     A     |      B      |   C   |   D   | E  |  F
                  |  (.262)   |   (5.70)    |       |       |    |
  _P. m. rufinus_ | .124-.699 | 2.71-15.07  | 20-23 |  low  | 23 | [A]
                  |           |             |       |       |    |
  _P. m. rufinus_ |  (.101)   |   (2.39)    | 20-25 | 24-47 |    | [B]
                  |           |             |       |       |    |
  _P. m. osgoodi_ |  .16-.25  |   3.2-4.3   | 18-22 | 10-20 | 23 | [C]
                  |           |             |       |       |    |
                  |  (.126)   |   (1.74)    |       |       |    |
  _P. m. bairdii_ | .082-.177 |  1.12-2.72  |    21 | 25-68 |    | [D]
                  |           |             |       |       |    |
  _P. m. bairdii_ | .124-.182 | (2.37-3.17) | 20-25 | 24-47 |    | [B]
                  |           |             |       |       |    |
                  |  (.372)   |   (10.80)   |       |       |    |
  _P. t. truei_   | .224-.561 |  7.0-16.92  | 20-23 |  low  | 23 | [A]
                  |           |             |       |       |    |
  _P. t. truei_   |  (.085)   |   (2.77)    | 20-25 | 24-47 |    | [B]
                  |           |             |       |       |    |
  _P. l. nov._    | .057-.117 |  1.36-2.29  |    21 | 25-68 |    | [D]
                  |           |             |       |       |    |
  _P. l. nov._    |           |   (5.36)    |    18 |  62.5 |    | [E]

   [A] Douglas
   [B] Lindeborg, 1952
   [C] Williams, 1959
   [D] Dice, 1922
   [E] Chew, 1951

Table 8 shows average water consumption for several species of
_Peromyscus_ as reported in the literature, and as determined in my
study. It is difficult to compare my results with most of the data in
the literature, because of a lack of information as to protein, fat,
carbohydrate, and mineral contents of foods used in other studies.
Lindeborg (1952) and Dice (1922) fed mice on a mixture of rolled oats,
meat scraps, dry skimmed milk, wheat germ, etc. described by Dice
(1934). Their data on water consumption in _P. maniculatus_ indicate
that this mixture probably is lower in protein content than Purina
Laboratory Chow, that was used in my experiments and those of Williams'
(tables 8 and 9).

The amount of dietary protein consumed under natural conditions is not
known for most wild animals. One index of the minimum amount of protein
necessary is the amount required for an animal to maintain its weight.
At best, this can be only an approximation of the required amount, for
other factors, such as stress, disease, change in tissues during oestrus
or gonadal descent, and changes in constituents of the diet other than
protein, would all be expected to affect the body weight (Chew,

The data in Table 7 show that both species vary their food intake with
changes in diet. Table 10 shows weight changes that took place in
individual mice when fed each of the three diets. A change in weight of
one gram cannot be considered as important, for the weight of an
individual mouse fluctuates depending upon when he last drank, ate,
defecated or urinated.

The only significant changes in weight occurred when mice were fed low
protein food (Table 10). Individuals of _P. truei_ lost 15.72 per cent
and individuals of _P. maniculatus_ lost 10.03 per cent of their total
body weights on this diet. This indicates that food having a protein
content of more than 10 per cent but less than 23 per cent is required
for maintenance of weight in these animals.

Although knowledge of the amount of water consumed, _ad libitum_, by
adult mice is valuable information, maintenance of the population
depends upon reproduction and dispersal of young individuals. My
trapping data indicate that only two to three per cent of the adults
live long enough to breed in consecutive breeding seasons. In spring,
the breeding population is composed largely of mice that were juveniles
or subadults during the latter parts of the breeding season. Therefore,
the critical time for the population may well be the time when the
season's young are being produced. Any unfavorable circumstances, such
as a shortage of food or water, that would affect pregnant or lactating
females would be of primary importance to the integrity of the

    TABLE 9--A Comparison of Mean Daily Water Consumption of Mice on
       High Protein Diets. Numbers in Parentheses Are Average Values;
       All Others Are Ranges of Values.

  Column headings:

  A: Temperature
  B: Relative humidity
  C: Investigator

                  |     Mean daily H_{2}O     |         |       |
                  |        consumption        |         |       |
      Species     +-------------+-------------+    A    |   B   |    C
                  | cc./gm. wt. |  Total cc.  |         |       |
  _P. m. osgoodi_ | (0.27-0.54) |  (4.6-9.3)  | 18-22 C | 10-20 |Williams,
                  |             |             |         |       |  1959
                  |   (0.496)   |   (10.74)   |         |       |
  _P. m. rufinus_ | 0.186-0.764 |  4.54-16.57 | 20-23 C |  low  |Douglas
                  |   (0.653)   |   (19.57)   |         |       |
  _P. t. truei_   | 0.429-1.031 | 13.28-30.28 | 20-23 C |  low  |Douglas

One would assume that pregnant and lactating females require more water
than non-pregnant females. One might also assume that juveniles require
different amounts of water and food than adults. Juveniles have less
dense pelage than adults, and probably are affected more by their
immediate environment because of their relatively poor insulation.
Juveniles might also be in an unfavorable situation insofar as water
conservation is concerned, because they are actively growing, and in
most cases, acquiring new pelage; it is well known that these are times
of stress for the individual.

    TABLE 10--Weights of Mice at Start and Finish of Experiments,
       Showing Changes in Weight and Mean Weights, and Means of Changes
       in Weight (mean delta).

                         _Peromyscus truei truei_
      |      Lab Chow       |       Hog Chow       |         Corn
  No. |Start | End  |[Delta]| Start | End  |[Delta]| Start | End  |[Delta]
   1  | 31.0 | 31.3 |  0.3  |  31.3 | 32.3 |  1.0  |  32.3 | 29.0 |  3.3
   5  | 31.1 | 30.5 |  0.6  |  30.5 | 32.8 |  2.3  |  32.8 | 28.7 |  4.1
   6  | 27.6 | 27.1 |  0.5  |  27.1 | 29.5 |  2.4  |  29.5 | 27.3 |  2.2
   7  | 28.0 | 26.3 |  1.7  |  26.3 | 27.5 |  1.2  |  27.5 | 22.2 |  5.3
  13  | 25.8 | 30.6 |  4.8  |  30.6 | 27.0 |  3.6  |  27.0 | 22.2 |  4.8
  14  | 26.9 | 30.7 |  3.8  |  30.7 | 31.4 |  0.7  |  31.4 | 27.3 |  4.1
  15  | 25.4 | 29.4 |  4.0  |  29.4 | 29.8 |  0.4  |  29.8 | 24.0 |  5.8
  16  | 33.0 | 32.9 |  0.1  |  32.9 | 30.5 |  2.4  |  30.5 | 26.0 |  4.5
  19  | 37.6 | 38.1 |  0.5  |  38.1 | 31.8 |  6.3  |  31.8 | 22.0 |  9.8
  20  | 23.5 | 25.8 |  2.3  |  25.8 | 26.2 |  0.4  |  26.2 | 22.9 |  3.1
  [=Y]| 28.9 | 30.2 |  1.8  |  30.2 | 29.8 |  2.0  |  29.8 | 25.2 |  4.7

                     _Peromyscus maniculatus rufinus_
      |      Lab Chow       |       Hog Chow       |         Corn
  No. |Start | End  |[Delta]| Start | End  |[Delta]| Start | End  |[Delta]
   2  | 23.0 | 20.7 |  2.3  |  20.7 | 21.1 |  0.4  |  21.1 | 18.6 |  2.5
   3  | 22.7 | 23.1 |  0.4  |  23.1 | 23.8 |  0.7  |  23.8 | 20.7 |  3.1
   4  | 22.0 | 21.1 |  0.9  |  21.1 | 21.8 |  0.7  |  21.8 | 21.3 |  0.5
   8  | 26.3 | 28.1 |  1.8  |  28.1 | 15.8 |  2.3  |  25.8 | 23.8 |  2.0
   9  | 21.5 | 24.0 |  2.5  |  24.0 | 25.1 |  1.1  |  25.1 | 21.8 |  3.3
  10  |      |      |       |       |      |       |  22.5 | 20.0 |  2.5
  11  | 21.0 | 22.1 |  1.1  |  22.1 | 20.8 |  1.3  |  20.8 | 19.0 |  1.8
  12  | 22.3 | 23.2 |  0.9  |  23.2 | 21.3 |  1.9  |  21.3 | 20.4 |  0.9
  17  | 18.9 | 20.0 |  1.1  |  20.0 | 19.2 |  0.8  |  19.2 | 19.4 |  0.2
  18  | 17.0 | 17.5 |  0.5  |  17.5 | 19.5 |  2.0  |  19.5 | 17.3 |  2.2
  21  | 18.9 | 18.1 |  0.8  |  18.1 | 20.2 |  2.1  |  20.2 | 17.3 |  2.9
  [=Y]| 21.4 | 21.8 |  1.2  |  21.8 | 21.8 |  1.3  |  21.9 | 19.9 |  2.2

Lindeborg (1950:76) found that 15 days before parturition, pregnant and
non-pregnant females of _P. m. bairdii_ drank about the same amounts of
water, that females consumed more water after the young were born and
until they were weaned, and that water consumption increased with an
increase in weight in young, growing individuals. He found that in the
later stages of pregnancy, females of _P. m. bairdii_ required 36 per
cent more water than non-breeding females; at 14 days after parturition,
nursing females required 111 per cent more water than non-breeding
females, and at weaning time, 158 per cent more water. Dice (1922:35)
reported a 217 per cent increase in drinking of _P. m. bairdii_ before
parturition, and 171 per cent increase while nursing.

Several females of both species were bred prior to the start of the
experiments described herein. As a consequence, it was possible to
determine water and food consumption for lactating females of each
species, and later, for their litters. Pregnant and lactating females,
and newly-weaned litters, were fed laboratory chow throughout this
experiment. The litters were separated from their mothers as soon as the
young were observed to be eating, or no later than 33 days after birth.

Table 11 shows the amounts of water and food consumed by two females of
each species while they were either in the later stages of pregnancy, or
were nursing. Although the data in Table 11 do not cover the full
developmental time of the litters involved, it is obvious that both
lactating females of _P. truei_ and one female of _P. maniculatus_
consumed more water than the average for their species (Table 7). Water
and food consumption was measured for both females of _P. truei_ while
they were nursing. The female that gave birth to litter A was left in
the cage with the male for several days after the litter was born,
resulting in another litter being born about 27 days after the first.
Therefore, the record of this female represents an extreme case of
stress (probably a common occurrence in nature) in which a female is
nursing one litter while she is pregnant with a second.

The record of the female of _P. truei_ that gave birth to litter B is
the most complete, including data from the fifth day after parturition
until the young were weaned on the thirty-third day after parturition.
The record of the female of _P. maniculatus_ that gave birth to litter C
covers the last 10 days of nursing before the young were weaned. After
being separated from her litter, this female drank more than the average
amounts of water, on both high and low protein diets. Although the food
and water were lost several times for the female of _P. maniculatus_
with litter D, the period of time covered by the 14 days when water and
food consumption were measured includes times just prior to parturition
and to weaning of the young.

    TABLE 11--Water and Food Consumed by Nursing Females of _P. truei_
       and _P. maniculatus_. Consumption Is Calculated on the Basis of
       Amount (Milliliters or Grams) Consumed per Gram of Body Weight
       per Day, as well as Total Amounts Used per Day.

  Column headings:

  A: Water used
  B: No. days
  C: Average weight
  D: ml. H_{2}O/gm./day
  E: Total water/day
  F: No. in litter
  G: Food used
  H: gms. food/gm./day
  I: Total food/day

         Female        |   A   | B  |   C   |  D   |   E   | F
  _P. truei_ (A)       |  447  | 17 | 33.00 | .796 | 26.29 | 3
  _P. truei_ (B)       |  676  | 28 | 32.70 | .738 | 24.14 | 3
  _P. maniculatus_ (C) |  191  | 10 | 19.45 | .983 | 19.10 | 5
  _P. maniculatus_ (D) |  133  | 14 | 24.35 | .224 |  5.46 | 6
         Female        |   G   | B  |   C   |  H   |   I   | F
  _P. truei_ (A)       | 214.7 | 26 | 33.00 | .250 |  8.26 | 3
  _P. truei_ (B)       | 120.5 | 24 | 32.70 | .153 |  5.02 | 3
  _P. maniculatus_ (C) |  47.8 | 10 | 19.45 | .246 |  4.78 | 5
  _P. maniculatus_ (D) | 180.1 | 21 | 27.42 | .312 |  8.58 | 6

It is interesting that the female of _P. maniculatus_ with litter C used
much more than the average amount of water for the species, and even
more per gram of body weight than lactating females of _P. truei_.
Conversely, water consumption of the female with litter D was within one
standard deviation of the mean for all adults of _P. maniculatus_. I
infer that at least some lactating females of _P. maniculatus_ are
better adapted to aridity than are some lactating females of _P. truei_.

Table 11 also shows food consumption of the four females discussed
above. All females, with the exception of the female with litter D,
consumed amounts of food that lie within one standard deviation of the
means for their species. The female with litter D had the most young,
consumed the most food but drank the least water of the four females.
Later, when separated from her litter and placed on the low protein
diet, this female drank only .046 milliliters of water per gram of body
weight per day. This figure is less than one-third of the average amount
(.174) for this species (Table 7).

The records of water and food consumption for litters A, C, and D are
given in Table 12; the mice in litter B persisted in placing wood
shavings in the opening of the spout on their water bottle, causing loss
of the water. The data show that mice in all three litters had an
average water and food consumption within one standard deviation of the
mean for adults of their respective species (Tables 7 and 12). It is
interesting that juveniles of both species require no more food and
water per gram of body weight than adults. This indicates that if a
young animal survives the rigors of postnatal life until it is weaned,
it is then at no disadvantage as far as food and water consumption are
concerned. This would be greatly advantageous to the species, as a
population, for the young could disperse immediately upon weaning, and
go into any areas that would be habitable for adults of the species.

    TABLE 12--Food and Water Consumed by Young Mice in Litters, After
       Weaning. Consumption Is Calculated on the Basis of the Amount
       (Milliliters or Grams) Consumed per Gram of Litter Weight per
       Day; Total Amounts Are Shown and Can Be Divided by Litter Size
       for Average Individual Consumption. Litter Sizes Are as
       Follows: A=3; C=5; D=6.

                       | Total |         |     | Average |  ml.   | Total
         Litter        | water |  Total  | No. |  total  |H_{2}O/ | water
                       | used  |corrected|days |  weight |gm./day |  /day
  _P. truei_ (A)       | 1207  |  1120   | 57  |  58.30  |  .337  | 19.64
  _P. maniculatus_ (C) | 1427  |  1340   | 57  |  76.14  |  .308  | 23.50
  _P. maniculatus_ (D) |  700  |   670   | 31  |  58.80  |  .367  | 21.61
                               |  Total  |     | Average |  Gms./ | Total
             Litter            |  food   | No. |  total  |gms. wt.|  food
                               |  used   |days |  weight |  /day  |  /day
  _P. truei_ (A)               |  651.2  | 50  |  58.30  |  .223  | 13.02
  _P. maniculatus_ (C)         |  743.8  | 57  |  76.14  |  .171  | 13.04
  _P. maniculatus_ (D)         |  471.1  | 31  |  58.80  |  .258  | 15.19

The young of pregnant and lactating females are the animals in the
population most likely to be affected by a deficient supply of water.
Drought could reduce the water content of the vegetation to such a level
that pregnant or lactating females might find it difficult, if not
impossible, to raise litters successfully. If such a drought persisted
throughout an entire breeding season, the next year's population would
be reduced in numbers, for even under normal climatic conditions it is
almost exclusively the juveniles that survive from one breeding season
to the next. If such a hypothetical drought occurred, lactating females
of _P. truei_ would be in a more critical position than lactating
females of _P. maniculatus_.

In order to determine how much water was available to mice in the peak
of the breeding season, samples of the three most common plants in the
study area were collected each week for analysis of their moisture
content. Plants were placed in separate plastic bags that were sealed in
the field. About a dozen plants of each species were used in each
determination. Only the new tender shoots of the plants were collected,
for it was assumed that mice would eat these in preference to the
tougher basal portions of the plants. The plants were taken immediately
to the laboratory and were weighed in the bag. Then the bag was opened
and it and the contents placed in an incubator at 85 degrees Fahrenheit
for a period of at least 72 hours. About 48 hours were required to dry
the plants to a constant weight. The dried plants were weighed and their
percentages of moisture were determined. Plants lose some water upon
being placed in a closed bag; small drops of water appear immediately on
the inner surface of the bag. Therefore, the bag must be weighed at the
same time as the plants and the weight of the dried bag must be
subtracted later.

The three kinds of plants chosen were among the most widely distributed
species in the study area, and all three grow close to the ground,
within reach of mice. Stems and leaves of two of the plants, _Comandra
umbellata_ and _Penstemon linarioides_, were readily eaten by captive
animals. Mice also were observed to eat leaves of _Comandra_ after being
released from metal live traps. The third species, _Solidago
petradoria_, differs from the other two in having a short woody stem
that branches at ground level. The more succulent shoots arise from this
woody stem. The leaves of _Solidago_ are coarse and were not eaten by
captive mice. Nevertheless, this species was chosen because it is widely
distributed and has the growth form of several other species of plants
in the area.

The graph in Figure 20 shows that _Comandra_ contains the highest
percentage of water through most of the summer. Water content of both
_Penstemon_ and _Comandra_ was greatly reduced in the dry period that
occurred in early July. _Solidago_ maintained a relatively constant
percentage of moisture; perhaps its woody stem serves for water storage.
The rains of July and August increased the percentage of moisture in the
plants, but not to the extent expected. Neither _Solidago_ nor
_Comandra_ reached the levels of hydration of early June. All plants
were collected at or about 11 A. M. At night, when mice are active,
these plants would be expected to contain a higher percentage of water
than in the daytime.

The data in Figure 20 indicate that mice probably are not endangered by
water shortages in most years. The average percentage of moisture in the
plants studied was as follows: _Comandra umbellata_ 62.33 per cent;
_Solidago petradoria_ 53.0 per cent; _Penstemon linarioides_ 49.28 per
cent. If a mouse were to eat ten grams of plant material containing 50
per cent moisture, it would provide him with five grams of food and five
grams of water, both of which exceed the minimum daily needs for
non-pregnant adults of either species.

The data indicate that there are sufficient differences in water
consumption between _P. maniculatus_ and _P. truei_ to account for their
habitat preferences in Mesa Verde National Park. In years having average
precipitation, water present in the vegetation has the potential for
providing enough moisture for the needs of both species. Extended
drought would affect individuals of _P. truei_ more adversely than
individuals of _P. maniculatus_.

    [Illustration: FIG. 20: Graph showing percentages of moisture
       contained during the summer of 1964, by three abundant and
       widely-distributed species of plants in Mesa Verde National
       Park, Colorado.]


Ectoparasites were collected by placing specimens of _Peromyscus_ in
separate plastic bags soon after death, adding cotton saturated with
carbon tetrachloride, closing the bag for about five minutes, then
brushing the fur of the specimen above a sheet of white paper. The
ectoparasites were sorted and sent to specialists for identification.
Endoparasites were saved when stomach and intestinal contents were
examined. Larvae of botflies were collected from mice in the autumn of
1962, placed in sand in containers, and kept over winter until they
hatched. Eyelids of alcoholic specimens were inspected for mites by an
authority on these organisms.

In 1961, the incidence of parasitism by botflies was the highest for the
period 1960-1966. _P. maniculatus_ was more heavily infected with
warbles than was _P. truei_. In 84 individuals of _P. maniculatus_ taken
in September 1961, from Morfield Ridge, 32.1 per cent had warbles. The
average number of warbles per animal was 1.24, and it was not uncommon
to find two or three warbles per mouse. Sixty-nine per cent of the
warbles were in the third instar stage, and the rest were in the second
instar stage. Warble infestation was higher in the first half of
September (40 per cent of mice infected) than in the second half of the
month (30 per cent infected), but a larger percentage of the warbles
were found (69 per cent) in the second half of the month.

In October 1961, 12.9 per cent of 62 _P. truei_ were infected with
warbles. The average number of warbles per infected mouse was 1.37.
Seventy-three per cent of the warbles were in the third instar stage;
the rest were in the second instar stage. Warble infestation was higher
in the first half of October (16 per cent of the mice infected) than in
the second half of the month (5.5 per cent infected). These mice were
collected from several localities on Chapin Mesa, in pinyon-juniper

In Mesa Verde the greatest incidence of infestations is in late
September and early October. This agrees with the finding of other
investigators (Sealander, 1961:58).

Sealander (1961) investigated hematological values in deer mice infected
with botflies, and found that infected mice had significantly lower
concentrations of hemoglobin than non-infected mice. Myiasis, associated
with infection by _Cuterebra_, is likely to lead to a lowering of the
physiological resistance of a segment of the population, and perhaps to
a subsequent decline in the population (Sealander, 1961:60).

Mice infected by warbles were less agile than non-infected mice. Other
investigators also have reported awkwardness in locomotion in infected
mice (Scott and Snead, 1942:95; Sealander, 1961:58). Test and Test
(1943:507) noted that parasitized mice did not appear to be emaciated,
and this was also true of parasitized mice at Mesa Verde. Healed wounds,
where warbles had emerged, were apparent on a number of mice. The
warbles, and wounds, usually were found on the flanks and backs of the
mice. The large, third instar larvae weighed about one gram apiece;
there is little doubt that such large larvae induce trauma in their

The highest rate of infestation by botflies occurred in 1961, the year
in which the population density of _P. maniculatus_ was near its peak.
The population of this species was reduced considerably in 1962, and
remained low through 1964. In 1965, the density of _P. maniculatus_
appeared to be increasing. Other investigators have reported that
increased incidence of _Cuterebra_ infestation in deer mice coincides
with lower population densities and with a downward trend in the
population (Scott and Snead, 1942:95; Wilson, 1945). My data indicate
that this may not be the situation in Mesa Verde.

The intestines or stomachs of almost all individuals of _P. maniculatus_
contained parasites. Endoparasites were less abundant in individuals of
_P. truei_. This heavier infestation of _P. maniculatus_ by tapeworms,
roundworms, and spiny-headed worms probably reflects the larger
proportion of insects eaten by _P. maniculatus_ than by _P. truei_.

The most common endoparasite encountered was the nematode, _Mastophorus
numidica_ Seurat, 1914; it was found in the stomachs of many individuals
of both species of _Peromyscus_. This nematode has been reported from
_Felis ocreata_ in Algeria, _Bitis arietans_ in the Congo, and from the
following mammals in the United States: _Canis latrans_, _Peromyscus
crinitus_, _P. gossypinus_, _P. maniculatus_, _P. truei_, _Onychomys
leucogaster_, _Dipodomys ordii_, _Reithrodontomys megalotis_, and
_Eutamias minimus_.

Individuals of _P. maniculatus_ obtained on the northern end of
Wetherill Mesa in May and June of 1962 had numerous ectoparasites. At
this time, the population of _P. maniculatus_ was high, but on a
downward trend.

My data and observations lead me to conclude that individuals of _P.
maniculatus_ are more heavily parasitized by both botflies and
endoparasites than are individuals of _P. truei_. The reasons for this
unequal amount of parasitism in two species of mice occurring in the
same general area remain obscure.

The kinds of endoparasites and ectoparasites collected from _P.
maniculatus_ and from _P. truei_ are listed below (m = present in _P.
maniculatus_, t = present in _P. truei_).

ACARINA: Ixodidae: _Dermacentor andersoni_ mt, _Ixodes angustus_ mt,
_Ixodes spinipalpis_ m. Laelaptidae: _Androlaelaps glasgowi_ m.
Myobiidae: _Blarinobia_ sp. m. Trombiculidae: _Euschoengastia lanei_ mt,
_Euschoengastia criceticola_ m, _Euschoengastia dicipiens_ t,
_Euschoengastia peromysci_ m, _Leewenhoekia americana_ m, _Trombicula
loomisi_ m.

DIPTERA: Cuterebridae: _Cuterebra cyanella_ mt.

SIPHONAPTERA: _Callistopsyllus deuterus_ m, _Catallagia decipiens_ m,
_Epetedia stanfordi_ mt, _Malaraeus sinomus_ mt, _Malaraeus telchinum_
mt, _Megarthroglossus procus_ mt, _Monopsyllus wagneri wagneri_ mt,
_Orchopeas leucopus_ mt, _Peromyscopsylla hesperomys adelpha_ mt,
_Phalacropsylla allos_ t, _Rhadinopsylla sectilis goodi_ t,
_Stenistomera macrodactyla_ m, _Stenoponia_ (_ponera_ or _americana_)

CESTODA: _Choanotaenia_ sp. m, _Hymenolepis_ sp. t.

NEMATODA: _Mastophorus numidica_ mt, _Syphacia obvelata_ mt, _Trichuris
stansburyi_ t.

ACANTHOCEPHALA: _Moniliformis clarki_ mt.


In order to determine the relative numbers of each species of
_Peromyscus_ that were taken on a seasonal basis by predators, scats of
coyotes and foxes were collected from trails and roads at least twice
each month, from September 1963 through August 1964. Scats were
identified, labeled and dried; all bones and samples of hair were later
removed from each scat. Scats that were intermediate in size between the
droppings of foxes and coyotes, and that could not be identified readily
in the field, were not collected. Bones from the scats were identified
to species, and hair was identified to genus or species by comparing
color patterns or cuticular patterns with samples from known mammals.
More than 200 impression slides and whole mounts of guard hair and
underfur were prepared.

Seven individuals of _P. truei_ and three individuals of _P.
maniculatus_ were represented in 114 coyote scats (Table 13). Both
species of _Peromyscus_ comprised only 3.9 per cent of the 253 items of
food represented in the 114 scats. Rabbits, _Sylvilagus_ sp. and mule
deer, _Odocoileus hemionus_ were the major food items of coyotes. Mice
of the genus _Peromyscus_ apparently were preyed upon mostly in autumn
(September through November), when mouse populations were near their
yearly peaks.

Foxes also prey upon _Peromyscus_ in the park. One _P. truei_ was
represented in the 16 scats of foxes that were analyzed. This individual
was taken in the winter quarter (December through February).

The bobcat may be an important predator upon _Peromyscus_ in this
region, but few scats of this animal were found. Since these could not
be assigned to a specific month, they were not saved for analysis.
Anderson (1961:58) believed that bobcats and gray foxes were the most
abundant predators in the park. My observations over a period of two
years led me to conclude that coyotes were more abundant than foxes and
that foxes were, in turn, more abundant than bobcats.

    TABLE 13--Food Present in 114 Coyote Scats Collected at Mesa Verde
       National Park each Month from September 1963 through August 1964.

                              |    Number   | Percentage
           Food Item          |      of     |  of total
                              | occurrences |   items
  _Sylvilagus_ sp.            |     32      |   12.65
  _Spermophilus variegatus_   |      5      |    1.97
  _Eutamias_ sp.              |     12      |    4.74
  _Reithrodontomys megalotis_ |      4      |    1.58
  _Peromyscus boylei_         |      2      |    0.79
  _Peromyscus maniculatus_    |      3      |    1.18
  _Peromyscus truei_          |      7      |    2.76
  _Neotoma cinerea_           |      2      |    0.79
  _Neotoma mexicana_          |      9      |    3.56
  _Neotoma albigula_          |      5      |    1.97
  _Neotoma_ sp.               |      3      |    1.18
  _Microtus longicaudus_      |      1      |    0.39
  _Microtus mexicanus_        |     11      |    4.34
  _Microtus montanus_         |      1      |    0.39
  _Microtus_ sp.              |      1      |    0.39
  _Odocoileus hemionus_       |     59      |   23.32
  Grass                       |     34      |   13.44
  Juniper berries             |     23      |    9.09
  Pinyon needles              |     14      |    5.53
  Pinyon nuts                 |      1      |    0.39
  Arthropods                  |      7      |    2.76
  Juniper needles             |      3      |    1.18
  Rodent or Lagomorph bones   |      5      |    1.97
  _Sceloporus_ sp.            |      1      |    0.39
  Unidentified fruit          |      2      |    0.79
  Rocks                       |      3      |    1.18
  Paper                       |      4      |    1.58
  Soil                        |      3      |    1.18
  Feathers                    |      5      |    1.97
    Total                     |    253      |

Hawks, owls and eagles live in the park. Red-tailed hawks were seen
frequently in the burned area on the northern end of Wetherill Mesa.
Both hawks and owls probably prey upon _Peromyscus_ in Mesa Verde, for
they are well-known predators upon mice and small rodents in other
areas. I tried to find owl and hawk nests that were occupied, but
located only nests that were abandoned or impossible to reach.

Captive gopher snakes, _Pituophis melanoleucus_, ate adults of both
species of _Peromyscus_. Gopher snakes probably are the most abundant
snake in the park; they feed mostly on mice and other rodents. Fur of
_Peromyscus_ was found in the stomach of a striped whipsnake,
_Masticophis taeniatus_ (Douglas, 1966:734).


Five species of _Peromyscus_ inhabit Mesa Verde National Park (Anderson,
1961). Two of these species, _P. crinitus_ and _P. difficilis_ are rare,
and none was taken in more than 14,000 trap nights. Several individuals
of _P. boylei_ were taken in live traps, but this species could not be
regarded as common. The two remaining species, _P. truei_ and _P.
maniculatus_, are the most abundant species in the park. Comparison of
the habitats and life-cycles of these two forms and analyses of their
interrelationships have been the objectives of this study.

The distribution of _P. truei_ in the park is regulated by the presence
of living pinyon-juniper woodland where logs and hollow trees of
_Juniperus osteosperma_ provide nesting and hiding places, and where
seeds of juniper trees and nuts of pinyon trees provide food. Several
other investigators have reported _P. truei_ to be associated with
trees, but apparently these findings have not assumed the importance
they warrant in understanding the ecology of this species. Bailey
(1931:152) observed an individual of _P. truei_ nesting in a tree on
Conchas Creek, New Mexico, and thought that this species might be more
arboreal than was generally supposed. The type specimen of _P. t. truei_
was taken by Shufeldt from a "nest protruding from an opening in the
dead and hollow trunk of a small pinon, at least 2 feet above the
ground.... The nest, composed of the fine fibers of the inner bark of
the pinon, was soon pulled out, and its owner dislodged...." (Shufeldt,
1885:403). Individuals of _P. truei_ usually build nests in trees, or in
hollow logs, and are therefore more abundant in pinyon-juniper woodland
where there are many such nesting sites.

Rocks and stones are not necessary in the habitat of _P. truei_,
although this species was most abundant where there was stony soil. The
coincidence of rock or stones and a high density of _P. truei_ is
thought to be explainable in terms of vegetation. Stony soils support
mixed shrubs as well as pinyon and juniper trees; the additional cover
and source of food probably allow a greater abundance of _P. truei_ than
would be possible without the shrubs. Secondarily, the rock provides
nesting sites for more mice.

Stands of mixed shrubs, lacking a pinyon-juniper canopy, do not support
_P. truei_. Its absence was noteworthy on Navajo Hill and on the
northern end of Wetherill Mesa where only _P. maniculatus_ lived among
the mixed shrubs and grassland. On the Mesa Verde, pinyon and juniper
trees must be present in order for _P. truei_ to live in an area; and,
these trees must be alive. Dead pinyons and junipers still stand in the
burned part of Morfield Ridge, but no _P. truei_ were found there.

Although a few individuals of _P. truei_ were taken in stands of
sagebrush adjacent to pinyon-juniper woodlands, this species does not
ordinarily venture far from the forest.

_P. maniculatus_ lives almost everywhere in Mesa Verde; the preferred
habitats are open and grassy with an overstory of mixed shrubs.
Individuals of _P. maniculatus_ venture into ecotonal areas lying
between grasslands and pinyon-juniper forest, or between sagebrush and
pinyon-juniper forest. _P. maniculatus_ is found also in disturbed areas
and in stands of sagebrush that occur in clearings of the pinyon-juniper
woodland. In such areas, _P. maniculatus_ and _P. truei_ are sympatric;
their home ranges overlap and any inter-specific competition that might
occur would be expected in these places.

The ability of _P. maniculatus_ to live in many different habitats is
correlated in part with its ability to build nests in a variety of
sites. Whereas _P. truei_ usually builds nests only in dead branches or
logs, _P. maniculatus_ builds nests in such varied places as spaces
under rocks, at the bases of rotten trees, and in abandoned tunnels of
pocket gophers. This adaptability is advantageous for the dispersal of
young individuals and the movement of adults into new areas.

Nesting sites have important bearing on survival of the young. In Mesa
Verde the rainy season occurs in July and August, while both species of
_Peromyscus_ are reproducing. It is reasonable to assume that young
animals that remain dry survive better than those that become wet and
chilled. The nestling young of _P. truei_ are in a more favorable
position to remain dry and warm than are nestling young of _P.

Captives of each species differed in the amounts of water consumed per
gram of body weight. Individuals of _P. truei_ consumed more water per
gram of body weight than individuals of _P. maniculatus_. Animals may
drink more water than they require when allowed to drink _ad libitum_,
but Lindeborg (1952) has shown that species which consume less water
when it is not restricted also fare better on a reduced ration. _P.
maniculatus_ appears to be better adapted to aridity than _P. truei_.
The preferred habitats of each species are in accord with these

Within the trapping grid, the most moderate microenvironment, in terms
of temperature and humidity, was in the pinyon-juniper forest, where
_P. truei_ lives. The temperature extremes were wider in the
microenvironments of a thicket of oak brush and of two different stands
of sagebrush, where _P. maniculatus_ lives, than in the forest. _P.
maniculatus_ tends to live in the harsher, more arid parts of Mesa
Verde. Because of its propensity to build nests under things, or in the
ground, and because of its ability to use less water per gram of body
weight, _P. maniculatus_ is better adapted to withstand harsh
environments than is _P. truei_.

_P. truei_ may be restricted to the pinyon-juniper woodland because of
its need for more mesic conditions. Still, Mesa Verde is semi-arid and
there are few permanent sources of water available for animals. The
primary source of moisture for rodents must be their food. Analysis of
the percentages of moisture contained in the three most common plants in
the trapping grid showed that _P. truei_ could obtain the required
moisture by eating about ten grams of these plants daily; individuals of
_P. maniculatus_ would need to eat less in order to satisfy their water

Individuals of _P. truei_ died more frequently in warm live-traps than
did individuals of _P. maniculatus_. This indicates that _P. truei_ can
tolerate less desiccation, or a narrower range of temperatures, than can
_P. maniculatus_.

Both species of mice eat some of the same plants, but these plants occur
widely. _P. truei_ seems to rely more upon the nuts of pinyons and the
seeds of junipers than does _P. maniculatus_. Mounds of discarded
juniper seeds were associated with all nesting sites of _P. truei_.
Bailey (1931:153) also noticed the fondness of this species for pine
nuts and juniper seeds. Apparently, the availability of these foods is
one of the major factors affecting the distribution of _P. truei_.
However, this is not the only factor, as is shown by the presence of _P.
maniculatus_ but lack of _P. truei_ in a juniper-pinyon association with
an understory of bitterbrush. This habitat was seemingly too arid for
_P. truei_.

Factors Affecting Population Densities

The production of young, and success in rearing them, is essential to
continuity of any population. _P. maniculatus_ is favored in this
respect, because the females produce more young and wean them sooner
than do females of _P. truei_. In addition, lactating females of _P.
maniculatus_ require significantly less water than do females of _P.
truei_. Since young mice of both species require no more water per gram
of body weight than do adults, the young can disperse into any area that
is habitable by their species. _P. maniculatus_ probably is affected
less by prolonged drought than is _P. truei_. Since lactating females
require the most water of any animal in the population, they are the
weakest link in the system. Females of _Peromyscus_ are known to
reabsorb embryos when conditions are unfavorable for continued
pregnancy. If prolonged drought occurred in the reproductive season, and
desiccated the vegetation upon which the mice depend for moisture, the
populations should diminish the following year. Lactating females of _P.
truei_ would be affected more seriously by a shortage of water than
would lactating females of _P. maniculatus_.

Of two species, the one producing the more young probably would be
subjected to more parasitism and predation than the species producing
fewer young. A favorable season for botflies, _Cuterebra_ sp., revealed
that _P. maniculatus_ has a higher incidence of parasitism by these
flies than has _P. truei_; possibly the adult flies concentrate in the
open, grassy areas where _P. maniculatus_ is more abundant, rather than
in the woodlands where _P. truei_ lives. Perhaps the lower parasitism of
_P. truei_ by warbles is related to the physiology of this species of
mouse. Near Boulder, Colorado, the incidence of infection by warbles is
lower in _P. difficilis_, a species closely related to _P. truei_, than
in _P. maniculatus_ (V. Keen, personal communication).

Although predation by carnivores would be expected to be higher on _P.
maniculatus_, because this species does not climb, my data show that
more individuals of _P. truei_ were taken by coyotes. I lack confidence
in these findings, suspecting that another sample might indicate the
reverse. Birds of prey probably catch more individuals of _P.
maniculatus_, because this species lives in more open habitats. My data
do not warrant firm conclusions regarding predation.

The length of time females must care for their young influences the rate
at which individuals can be added to the population. Females of _P.
truei_ nurse their young longer and keep them in the nest longer than do
females of _P. maniculatus_. Although this may enhance the chances of
survival of young of _P. truei_, it also reduces the number of litters
that each female can have in each breeding season. Females of _P.
maniculatus_ can produce more young per litter, and each female probably
can produce more litters per year than females of _P. truei_.

Captives of _P. truei_ were tolerant of other individuals of the same
species, even when kept in close confinement. However, when there was
slight shortage of food or water they killed their litter mates, or
females killed their young. Only a short period of time was necessary
for one mouse to dispatch all others in the litter. The attacked mice
were bitten through the head before being eaten; the brains and viscera
were the first parts consumed. The population might be decimated rapidly
if drought forced this species to cannibalism. When the supply of food
or water was restored, the captive mice resumed their tolerant nature.

In captivity, _P. maniculatus_ is amazingly tolerant of close
confinement with members of the same species; individuals did not tend
to kill their litter mates, or their young, even during shortage of food
and water. This tolerance, especially under stressful conditions,
probably enables _P. maniculatus_ to persist in relatively unfavorable

Adaptations to Environment

Each of the two species of _Peromyscus_ illustrates one or more
adaptations to its environment. _P. truei_ is adapted to climbing by
possession of long toes, a long tail, and large hind feet. The tail is
used as a counterbalance when climbing (Horner, 1954). When frightened,
individuals of _P. truei_ often ran across the ground in a
semi-saltatorial fashion, bounding over clumps of grass that were as
much as 18 inches high. Such individuals usually ran to the nearest tree
and climbed to branches 10 to 20 feet above the ground.

Large eyes are characteristic of the _truei_ group of mice, and may be
an adaptation to a semi-arboreal mode of life. A similar adaptation is
shared by some other arboreal mammals, and of arboreal snakes. The large
eyes of _P. truei_ in comparison to those of _P. maniculatus_, probably
increase the field of vision, and permit the animal to look downward as
well as in other directions.

The above-mentioned adaptations of _P. truei_ permit these graceful mice
to use their environment effectively. By climbing, this species can nest
above-ground in the hollow branches of trees, and can rear its young in
a comparatively safe setting. The ability to climb also permits vertical
as well as horizontal use of a limited habitat. Because of the
three-dimensional nature of the home range of _truei_, its range is
actually larger than that of _maniculatus_ although the standard
trapping procedures makes the home range of the two appear to be about
the same size. Finally, trees may offer safety from predators, and a
source of food that probably is the winter staple of this species.

_Peromyscus maniculatus_ has adapted differently to its environment.
Small size of body and appendages permit this species to use a variety
of nesting sites and hiding places even though it is restricted, by its
anatomy, to life on the ground. The tail and hind feet are shorter than
in _P. truei_, and _P. maniculatus_ is an inefficient climber. I have
placed individuals in bushes, and found that many walk off into space
from a height of several feet. Perhaps the relative smallness of their
eyes accounts for their seeming lack of awareness of how high they are
above the ground.

When frightened, individuals of _P. maniculatus_ ran rapidly in a
zig-zag path and dove into the nearest cover. Mice, released from live
traps, often stuck their heads under leaves, leaving their bodies
exposed. This species tends to hide as rapidly as possible, and remain
motionless. This tactic would not be of much value as an escape from
carnivores, but it could be effective against birds of prey.

In Mesa Verde, _P. maniculatus_ inhabits the more arid, open areas. When
the population is dense, individuals of this species are found also in
pinyon-juniper woodland. Apparently _P. maniculatus_ prefers the grassy
areas and the thickets of oak brush. Although such habitats have harsh
climatic conditions, they offer innumerable hiding places, and thus have
great advantage for a species confined to the ground.

The low requirements of water per gram of body weight, the ability to
eat diversified foods, the use of varied habitats, the high fecundity,
and the ability to use any nook for retreat or nesting make _P.
maniculatus_ a successful inhabitant of most parts of Mesa Verde, and
indeed, of most of North America.



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       *       *       *       *       *

Transcriber's Notes

All obvious typographic errors corrected. The notation [=Y] in Table 10
represents the Mean Value for that column. The notation H_{2}O
represents the water molecule where the _{2} represents the subscripted
2. The notation 8-1/2 represents 8 and one half.

  Page  Correction
  ====  ==============
   429  nuaseosus => nauseosus
   430  Orthocarpos => Orthocarpus
   450  ludovociana => ludoviciana
   456  phrheliometer => pyrheliometer
   480  rudale => ruderale
   481  rates => rats
   482  bases => basis
   499  clumbs => clumps

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