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Title: A Population Study of the Prairie Vole (Microtus ochrogaster) in Northeastern Kansas
Author: Martin, Edwin P.
Language: English
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Copyright Status: Not copyrighted in the United States. If you live elsewhere check the laws of your country before downloading this ebook. See comments about copyright issues at end of book.

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  ==================================================================
                  UNIVERSITY OF KANSAS PUBLICATIONS
                      MUSEUM OF NATURAL HISTORY

           Volume 8, No. 6, pp. 361-416, 19 figures in text
  ----------------------    April 2, 1956    -----------------------


                          A Population Study
              of the Prairie Vole (Microtus ochrogaster)
                        in Northeastern Kansas

                                  BY

                            EDWIN P. MARTIN


                         UNIVERSITY OF KANSAS
                               LAWRENCE
                                 1956



     UNIVERSITY OF KANSAS PUBLICATIONS, MUSEUM OF NATURAL HISTORY

        Editors: E. Raymond Hall, Chairman, A. Byron Leonard,
                           Robert W. Wilson

           Volume 8, No. 6, pp. 361-416, 19 figures in text
                        Published April 2, 1956


                          UNIVERSITY OF KANSAS
                            Lawrence, Kansas


                               PRINTED BY
                    FERD VOILAND, JR., STATE PRINTER
                             TOPEKA, KANSAS
                                  1956

                                25-9225



CONTENTS


                                          PAGE
  INTRODUCTION                             363
  GENERAL METHODS                          364
  HABITAT                                  366
  POPULATION STRUCTURE                     373
  POPULATION DENSITY                       376
  HOME RANGE                               380
  LIFE HISTORY                             383
    Reproduction                           383
    Litter Size and Weight                 386
    Size, Growth Rates and Life Spans      388
    Food Habits                            397
    Runways and Nests                      398
    Activity                               400
  PREDATION                                401
  MAMMALIAN ASSOCIATES                     403
  SUMMARY AND CONCLUSIONS                  408
  LITERATURE CITED                         411



                          A POPULATION STUDY
              OF THE PRAIRIE VOLE (MICROTUS OCHROGASTER)
                        IN NORTHEASTERN KANSAS

                                  By

                            Edwin P. Martin



INTRODUCTION


Perhaps the most important species of mammal in the grasslands of Kansas
and neighboring states is the prairie vole, _Microtus ochrogaster_
(Wagner). Because of its abundance this vole exerts a profound influence
on the quantity and composition of the vegetation by feeding, trampling
and burrowing; also it is important in food chains which sustain many
other mammals, reptiles and birds. Although the closely related meadow
vole, _M. pennsylvanicus_, of the eastern United States, has been
studied both extensively and intensively, relatively little information
concerning _M. ochrogaster_ has been accumulated heretofore.

I acknowledge my indebtedness to Dr. Henry S. Fitch, resident
investigator on the University of Kansas Natural History Reservation. In
addition to supplying guidance and encouragement in both the planning
and execution of the investigation, Dr. Fitch made available for study
the data from his extensive field work. Interest in and understanding of
ecology were stimulated by his teaching and his example. Special debts
are also acknowledged to Mr. John Poole for the use of his field notes
and to Professor E. Raymond Hall, Chairman of the Department of Zoology,
for several courtesies. Dr. R. L. McGregor of the Department of Botany
at the University of Kansas assisted with the identification of some of
the plants. Drawings of skulls were made by Victor Hogg.

Of the numerous publications concerning _Microtus pennsylvanicus_, those
of Bailey (1924), Blair (1940; 1948) and Hamilton (1937a; 1937c; 1940;
1941) were especially useful in supplying background and suggesting
methods for the present study. Publications not concerned primarily with
voles, that were especially valuable to me in providing methods and
interpretations applicable to my study, were those of Blair (1941),
Hayne (1949a; 1949b), Mohr (1943; 1947), Stickel (1946; 1948) and
Summerhayes (1941). Faunal and ecological reports dealing with _M.
ochrogaster_ and containing useful information on habits and habitat
included those of Black (1937:200-202), Brumwell (1951:193-200; 213),
Dice (1922:46) and Johnson (1926). Lantz (1907) discussed the economic
relationships of _M. ochrogaster_; the section of his report concerning
the effects of voles on vegetation was especially useful to me.

Fisher (1945) studied the voles of central Missouri and obtained
information concerning food habits and nesting behavior. Jameson (1947)
studied _M. ochrogaster_ on and near the campus of the University of
Kansas. His report is especially valuable in its treatment of the
ectoparasites of voles. In my investigation I have concentrated on those
aspects of the ecology of voles not treated at all by Fisher and
Jameson, or mentioned but not adequately explored by them. Also I have
attempted to obtain larger samples.

The University of Kansas Natural History Reservation, where almost all
of the field work was done, is an area of 590 acres, comprising the
northeastern-most part of Douglas County, Kansas. Situated in the broad
ecotone between the deciduous forest and grassland, the reservation
provides a variety of habitat types (Fitch, 1952). Before 1948, much of
the area had been severely overgrazed and the original grassland
vegetation had been largely replaced by weeds. Since 1948 there has been
no grazing or cultivation. The grasses have partially recovered and, in
the summer of 1952, some grasses of the prairie climax were present even
on the parts of the Reservation which had been most heavily overgrazed.
Illustrative of the changes on the Reservation were those observed in
House Field by Henry S. Fitch (1953: _in litt._). He recalled that in
July, 1948, the field supported a closely grazed, grassy vegetation
providing insufficient cover for _Microtus_, with such coarse weeds as
_Vernonia_, _Verbena_ and _Solanum_ constituting a large part of the
plant cover. By 1950, the same area supported a lush stand of grass,
principally _Bromus inermis_, and supported many woody plants. Similar
changes occurred in the other study areas on the Reservation. Although
insufficient time has elapsed to permit analyses of successional
changes, it seems that trees and shrubs are gradually encroaching on the
grassland throughout the Reservation.

The vole population has changed radically since the Reservation was
established. In September and October of 1948, when Fitch began his
field work, he maintained lines of traps totaling more than 1000 trap
nights near the future vole study plots without capturing a single vole.
In November and December, 1948, he caught several voles near a small
pond on the Reservation and found abundant sign in the same area. Late
in 1949 he began to capture voles over the rest of the Reservation, but
not until 1950 were voles present in sufficient numbers for convenient
study.

I first visited the Reservation and searched there for sign of voles in
the summer of 1949. I found hardly any sign. In the area around the pond
mentioned above, however, several systems of runways were discovered.
This area had been protected from grazing for several years prior to the
reservation of the larger area. In House Field, where my main study plot
was to be established, there was no sign of voles. Slightly more than a
year later, in October, 1950, I began trapping and found _Microtus_ to
be abundant on House Field and present in smaller numbers throughout
grassland areas of the Reservation.



GENERAL METHODS


The present study was based chiefly on live-trapping as a means of
sampling a population of voles and tracing individual histories without
eliminating the animals. Live-trapping disturbs the biota less than
snap-trapping and gives a more reliable picture of the mammalian
community (Blair, 1948:396; Cockrum, 1947; Stickel, 1946:158; 1948:161).
The live-traps used were modeled after the trap described by Fitch
(1950). Other types of traps were tested from time to time but this
model proved superior in being easy to set, in not springing without a
catch, in protecting the captured animal and in permitting easy removal
of the animal from the trap. A wooden box was placed inside the metal
shelter attached to each trap and, in winter, cotton batting or woolen
scraps were placed inside the boxes for nesting material. With this
insulation against the cold, voles could survive the night unharmed and
could even deliver their litters successfully. In summer the nesting
material was removed but the wooden box was retained as insulation
against heat.

Bait used in live-traps was a mixture of cracked corn, milo and wheat,
purchased at a local feed store. The importance of proper baiting,
especially in winter, has been emphasized by Howard (1951) and Llewellyn
(1950) who found an adequate supply of energy-laden food, such as corn,
necessary in winter to enable small rodents to maintain body temperature
during the hours of captivity. The rare instances of death of voles in
traps in winter were associated with wet nesting material, as these
animals can survive much lower temperatures when they are dry. Their
susceptibility to wet and cold was especially evident in rainy weather
in February and March.

Preventing mortality in traps was more difficult in summer than in
winter. The traps were set in any available shade of tall grass or
weeds; or when such shade was inadequate, vegetation was pulled and
piled over the nest boxes. The traps usually were faced north so that
the attached number-ten cans, which served as shelters, cast shadows
over the hardware cloth runways during midday. Even these measures were
inadequate when the temperature reached 90°F. or above. Such high
temperatures rarely occurred early in the day, however, so that removal
of the animals from traps between eight and ten a. m. almost eliminated
mortality. Those individuals captured in the night were not yet harmed,
but it was already hot enough to reduce the activity of the voles and
prevent further captures until late afternoon. When it was necessary to
run trap lines earlier, the traps were closed in the morning and reset
in late afternoon.

Reactions of small mammals to live-traps and the effects of prebaiting
were described by Chitty and Kempson (1949). In general, the results of
my trapping program fit their conclusions. Each of my trapping periods,
consisting of seven to ten consecutive days, showed a gradual increase
in the number of captures per day for the first three days, with a
tendency for the number of captures to level off during the remainder of
the period. Leaving the traps baited and locked open for a day or two
before a trapping period tended to increase the catch during the first
few days of the period without any corresponding increase during the
latter part of the period. Initial reluctance of the voles to enter the
traps decreased as the traps became familiar parts of their environment.

At the beginning of the study the traps were set in a grid with
intervals of 20 feet. The interval was increased to 30 feet after three
months because a larger area could thus be covered and no loss in
trapping efficiency was apparent. The traps were set within a three foot
radius of the numbered stations, and were locked and left in position
between trapping periods.

Each individual that was captured was weighed and sexed. The resulting
data were recorded in a field notebook together with the location of the
capture and other pertinent information. Newly captured voles were
marked by toe-clipping as described by Fitch (1952:32). Information was
transferred from the field notebook to a file which contained a separate
card for each individual trapped.

In the course of the program of live-trapping, many marked voles were
recaptured one or more times. Most frequently captured among the females
were number 8 (33 captures in seven months) and number 73 (30 captures
in eight months). Among the males, number 37 (21 captures in six months)
and number 62 (21 captures in eight months) were most frequently taken.
The mean number of captures per individual was 3.6. For females, the
mean number of captures per individual was 3.8 and for males it was 3.4.
Females seemingly acquired the habit of entering traps more readily than
did males. No correlation between any seasonally variable factor and the
number of captures per individual was apparent. To a large degree, the
formation of trap habits by voles was an individual peculiarity.

In order to study the extent of utilization of various habitats by
_Microtus_, a number of areas were sampled with Museum Special
snap-traps. These traps were set in linear series approximately 25 feet
apart. The number of traps used varied with the size of the area sampled
and ranged from 20 to 75. The lines were maintained for three nights.
The catch was assumed to indicate the relative abundance of _Microtus_
and certain other small mammals but no attempt to estimate actual
population densities from snap-trapping data was made. In August, 1952,
when the live-trapping program was concluded, the study areas were
trapped out. The efficiency of the live-trapping procedure was
emphasized by the absence of unmarked individuals among the 45 voles
caught at that time.

Further details of the methods and procedures used are described in the
appropriate sections which follow.



HABITAT


Although other species of the genus _Microtus_, especially _M.
pennsylvanicus_, have been studied intensively in regard to habitat
preference (Blair, 1940:149; 1948:404-405; Bole, 1939:69; Eadie, 1953;
Gunderson, 1950:32-37; Hamilton, 1940:425-426; Hatt, 1930:521-526;
Townsend, 1935:96-101) little has been reported concerning the habitat
preferences of _M. ochrogaster_. Black (1937:200) reported that, in
Kansas, _Microtus_ (mostly _M. ochrogaster_) preferred damp situations.
_M. ochrogaster_ was studied in western Kansas by Brown (1946:453) and
Wooster (1935:352; 1936:396) and found to be almost restricted to the
little-bluestem association of the mixed prairie (Albertson, 1937:522).
Brumwell (1951:213), in a survey of the Fort Leavenworth Military
Reservation, found that _M. ochrogaster_ preferred sedge and bluegrass
meadows but occurred also in a sedge-willow association. Dice (1922:46)
concluded that the presence of green herbage, roots or tubers for use as
a water source throughout the year was a necessity for _M. ochrogaster_.
Goodpastor and Hoffmeister (1952:370) found _M. ochrogaster_ to be
abundant in a damp meadow of a lake margin in Tennessee. In a study made
on and near the campus of the University of Kansas, within a few miles
of the area concerned in the present report, Jameson (1947:132) found
that voles used grassy areas in spring and summer, but that in the
autumn, when the grass began to dry, they moved to clumps of Japanese
honeysuckle (_Lonicera japonica_) and stayed among the shrubbery
throughout the winter. Johnson (1926:267, 270) found _M. ochrogaster_
only in uncultivated areas where long grass furnished adequate cover. He
stated that the entire biotic association, rather than any single
factor, was the key to the distribution of the voles. None of these
reports described an intensive study of the habitat of voles, but the
data presented indicate that voles are characteristic of grassland and
that _M. ochrogaster_ can occupy drier areas than those used by _M.
pennsylvanicus_. Otherwise, the preferred habitats of the two species
seem to be much the same.

In the investigation described here I attempted to evaluate various
types of habitats on the basis of their carrying capacity at different
stages of the annual cycle and in different years. The habitats were
studied and described in terms of yield, cover and species composition.
The areas upon which live-trapping was done were studied most
intensively.

These two areas, herein designated as House Field and Quarry Field, were
both occupied by voles throughout the period of study. Population
density varied considerably, however (Fig. 5). Both of these areas were
dominated by _Bromus inermis_, and, in clipped samples taken in June,
1951, this grass constituted 67 per cent of the vegetation on House
Field and 54 per cent of the vegetation on Quarry Field. Estimates made
at other times in 1950, 1951 and 1952 always confirmed the dominance of
smooth brome and approximated the above percentages. Parts of House
Field had nearly pure stands of this grass. Those traps set in spots
where there was little vegetation other than the dominant grass caught
fewer voles than traps set in spots with a more varied cover. _Poa
pratensis_ formed an understory over most of the area studied,
especially on House Field, and attained local dominance in shaded spots
on both fields. The higher basal cover provided by the _Poa_ understory
seemed to support a vole population larger than those that occurred in
areas lacking the bluegrass. Disturbed situations, such as roadsides,
were characterized by the dominance of _Bromus japonicus_. This grass
occurred also in low densities over much of the study area among _B.
inermis_. Other grasses present included _Triodia flava_, common in
House Field, but with only spotty distribution in Quarry Field; _Elymus
canadensis_, distributed over both areas in spotty fashion and almost
always showing evidence of use by voles and other small mammals;
_Aristida oligantha_ and _Bouteloua curtipendula_, both more common on
the higher and drier Quarry Field; _Panicum virgatum_, _Setaria_ spp.,
especially on disturbed areas; and three bluestems, _Andropogon
gerardi_, _A. virginicus_ and _A. scoparius_. The bluestems increased
noticeably during the study period (even though grasses in general were
being replaced by woody plants) and they furnished a preferred habitat
for voles because of their high yield of edible foliage and relatively
heavy debris which provided shelter.

On House Field the most common forbs were _Vernonia baldwini_, _Verbena
stricta_ and _Solanum carolinense_. On Quarry Field, _Solidago_ spp. and
_Asclepias_ spp. were also abundant. All of them seemed to be used by
the voles for food during the early stages of growth, when they were
tender and succulent. The fruits of the horse nettle (_Solanum
carolinense_) were also eaten. The forbs themselves did not provide
cover dense enough to constitute good vole habitat. Mixed in a grass
dominated association they nevertheless raised the carrying capacity
above that of a pure stand of grass. Other forbs noted often enough to
be considered common on both House Field and Quarry Field included
_Carex gravida_, observed frequently in House Field and less often in
Quarry Field; _Amorpha canescens_, more common in Quarry Field;
_Tradescantia bracteata_, _Capsella bursapastoris_, _Oxalis violacea_,
_Euphorbia marginata_, _Convolvulus arvensis_, _Lithospermum arvense_,
_Teucrium canadense_, _Physalis longifolia_, _Phytolacca americana_,
_Plantago major_, _Ambrosia trifida_, _A. artemisiifolia_, _Helianthus
annuus_, _Cirsium altissimum_ and _Taraxacum erythrospermum_. Both areas
were being invaded from one side by forest-edge vegetation; the woody
plants noted included _Prunus americana_, _Rubus argutus_, _Rosa
setigera_, _Cornus drummondi_, _Symphoricarpus orbiculatus_, _Populus
deltoides_ and _Gleditsia triacanthos_.

In House Field the herbaceous vegetation was much more lush than in
Quarry Field and woody plants and weeds were more abundant. A graveled
and heavily used road along one edge of House Field, leading to the
Reservation Headquarters, was a barrier which voles rarely crossed. A
little-used dirt road crossing the trapping plot in Quarry Field
constituted a less effective barrier. The disturbed areas bordering the
roads were likewise little used and tended to reinforce the effects of
the roads as barriers. There were almost pure stands of _Bromus
japonicus_ along both roads. No mammal of any kind was taken in traps
set where this grass was dominant.

Because seasonal changes in vole density followed the curve for rate of
growth of the complex of grasses on the Reservation, and because years
in which there was a sparse growth of plants due to dry weather showed a
decrease in the density of voles, the relationships between productivity
of plants and vole population levels on the two study areas were
investigated. In both fields the composition of the plant cover was
similar, and the differences were chiefly quantitative. In June, 1951,
ten square-meter quadrats were clipped on each of the areas to be
studied. The clippings from each were dried in the sun and weighed. From
Quarry Field the mean yield amounted to 1513 ± 302 lbs. per acre; while
from House Field the yield was 2351 ± 190 lbs. per acre (Table 1). Using
experience gained in making these samples, I periodically estimated the
relative productivity of the two areas. House Field was from 1.5 to 3
times as productive as Quarry Field during the growing seasons of 1951
and 1952. Although House Field, being more productive, usually supported
a larger population of voles than Quarry Field the reverse was true at
the time of the clipping (Fig. 5).

  TABLE 1. RELATIONSHIP BETWEEN YIELD AND VARIOUS POPULATION DATA

  ======================================================================
                                             House Field  Quarry Field
  ----------------------------------------------------------------------
  Yield in June, 1951, lbs./acre               2351 ± 190    1513 ± 302
  _Microtus_, June, 1951, gms./acre              3867          5275
  Per cent immature _Microtus_, June, 1951         29.85         38.02
  Ratio _Microtus_, June/March                      0.73          2.63
  _Sigmodon_, June, 1951, gms./acre              1376           746
  Per cent immature _Sigmodon_, June, 1951         35.72         44.44
  Ratio _Sigmodon_, June/March                      1.40          2.25
  _Microtus-Sigmodon_, June, 1951, gms./acre     5243          6021
  _Microtus_ mean, gms./acre/month               2922          1831
  _Sigmodon_ mean, gms./acre/month                802           335
  _Sigmodon-Microtus_, gms./acre/month           3728          2166
  ----------------------------------------------------------------------

Although no explanation was discovered which accounted fully for the
seeming aberration, two sets of observations were made that may bear on
the problem. In June, 1951, the population of voles and cotton rats on
Quarry Field was increasing rapidly whereas in House Field that trend
was reversed. The trends were reflected by the percentages of immature
individuals in the two populations and by the ratios of the June, 1951,
densities to the March, 1951, densities (Table 1). Perhaps the density
curve was determined in part by factors inherent in the population and,
to that extent, was fluctuating independently of the environment
(Errington, 1946:153).

The flood in 1951 reduced the population of voles and obscured the
normal seasonal trends. Although House Field produced a heavier crop of
vegetation, Quarry Field produced a larger crop of rodents, chiefly
_Microtus_ and _Sigmodon_. In House Field, however, the ratio of
_Sigmodon_ to _Microtus_ was notably higher. Presumably the cotton rats
competed with the voles and exerted a depressing effect on their
numbers. The intensity of the effect seemed to depend on the abundance
of both species. That this depressing effect involved more than direct
competition for plant food was suggested by the fact that in House
Field, with a heavy crop of vegetation and a seemingly high carrying
capacity for both herbivorous rodents, the biomass of voles, and of all
rodents combined, were lower than in Quarry Field which had less
vegetation and fewer cotton rats. The relationships between voles and
cotton rats are discussed further later in this report.

When the centers of activity (Hayne, 1949b) of individual voles were
plotted it was seen that there was a shift in the places of high density
of voles on the trapping areas. This shift seemed to be related to the
advance of the forest edge with such woody plants as _Rhus_ and
_Symphoricarpos_ and young trees invading the area. These shifts were
clearly shown when the distribution of activity centers on both areas in
June, 1951, was compared with the distribution in June, 1952 (Fig. 1).
The shift was gradual and the more or less steady progress could be
observed by comparing the monthly trapping records. It was perhaps
significant that during the summers the centers of activity were less
concentrated than during the winter. The shift of voles away from the
woods was more nearly evident in winter when the voles were driven into
areas of denser ground cover, which provided better shelter.

[Illustration: FIG. 1. Progressive encroachment of woody vegetation onto
study areas, and the accompanying shift of the centers of populations of
voles. Activity centers of individuals were calculated as described by
Hayne (1949b) and are indicated by dots. The cross-hatched areas show
places where the vegetation was influenced by the shade of woody
plants.]

From 1948 to 1950 and again in 1952 and 1953 I trapped in various
habitat types in a mixed prairie near Hays, Kansas. Before the great
drought of the thirties, _Microtus ochrogaster_ was the most common
species of small mammal in that area. Since 1948, at least, it has been
taken only rarely and from a few habitats. No voles have been taken from
grazed sites. In a relict area, voles were trapped in a lowland
association dominated by big bluestem. Since 1948 only one vole has been
trapped in the more extensive hillside association characterized by a
mixture of big bluestem, little bluestem and side-oats grama. None was
taken in the upland parts of the relict area where buffalo grass and
blue grama dominated the association.

In the pastured areas there are nine livestock exclosures established by
the Department of Botany of Ft. Hays Kansas State College. These
exclosures included many types of habitat found in the mixed prairie.
All of these exclosures were trapped and voles were taken in only two of
them. An exclosure situated near a pond, on low ground producing a
luxuriant growth of big bluestem and western wheat grass, has supported
voles in 1948, 1949, 1952 and 1953. An upland exclosure containing only
short grasses also supported a few voles in 1953.

An examination of the nature of the various plant associations of the
mixed prairie indicates that yield of grasses, amount of debris and
basal cover may be critical factors in the distribution of voles. The
association to which the voles seemed to belong was the lowland
association. Hopkins _et al_ (1952:401; 409) reported the yield of
grasses from the lowland to be approximately twice as great as from the
hillside and upland in most years. Probably equally important to the
voles was the fact that debris accumulation in the lowland was
approximately five times as great as in the upland and approximately
2.5 times as great as on the hillside (Hopkins, unpublished data). The
unexpected presence of voles in the short grass exclosure was probably
due to two factors. In ungrazed short grass, basal cover may reach 90
per cent (Albertson, 1937:545), thus providing excellent cover for
voles. Also, the ungrazed exclosure had greater yield and a thicker mat
of debris than the grazed short grass surrounding it and was thus a
relatively good habitat, although it did not compare favorably with the
lowland type.

Samples of the populations of various areas, obtained by snap-trapping,
gave further information regarding the types of vegetation preferred by
voles. Voles were taken in all ungrazed and unmown grasslands trapped in
eastern Kansas, although some of the areas were not used at all seasons
of the year nor in years having a low population of _Microtus_. Reithro
Field, similar to Quarry Field in its general aspect, had a heavy
population of voles in the spring and summer of 1951, a time when voles
were generally abundant. On the same area the population of small
mammals was sampled in the summer of 1949 and, though occasional sign of
voles was seen, not one vole was trapped. Later trapping, in the spring
and summer of 1952, also failed to catch any voles and Fitch (1953, _in
litt._) caught none in several trapping attempts in 1953. These later
times were characterized by a general scarcity of voles. Reithro Field
was drier, with less dense vegetation, than the two main study areas and
had larger percentages of little bluestem (_Andropogon scoparius_) and
side-oats grama (_Bouteloua curtipendula_) and smaller percentages of
_Vernonia_, _Verbena_, _Solanum_ and _Solidago_.

Various species of foxtail (_Setaria_) dominated most roadsides in the
vicinity of the Reservation. Voles almost always used these strips of
grass but never were abundant in them. Voles were taken near the margin
of a weedy field, fallow since 1948, but there was none in the middle of
the field. Most individuals were confined to the grassy areas around the
field and made only occasional forays away from the edge. The dam of a
small pond on the Reservation and low ground near the water were used by
_Microtus_ at all times. In the summer of 1949 no voles were taken
anywhere on the Reservation but their runways were more abundant around
the pond than in the other places examined. Of all the areas studied in
the summer of 1949, only the pond area had been protected from grazing
in previous years. _Polygonum coccineum_ was the most prominent plant in
the pond edge association. A few voles were trapped in large openings in
the woods, where a prairie vegetation remained and where voles seemingly
lived in nearly isolated groups.

Voles were rarely taken in grazed or mown grassland or in fields of
alfalfa, stubble or row crops. The critical factor in these cases seemed
to be the absence of debris or other ground cover under which runways
and nests could be concealed satisfactorily. Woods, rocky outcroppings
and bare ground were not used regularly by voles. Fitch (1953, _in
litt._) has taken several _Microtus_ in reptile traps set along a rocky
ledge in woods but most of these voles were subadult males and seemed to
be transients. Fields in the early stages of succession also failed to
support a population of voles. Such areas on the Reservation were
characterized by giant ragweed, horse weed, thistles and other coarse
weeds. Basal cover was low and debris scanty. Not until an understory of
grasses was established did a population of voles appear on such areas.
The coarse weeds seemed to provide neither food nor cover adequate for
the needs of the voles.

An analysis of trapping success at each station in House Field further
clarified habitat preferences. The tendency of voles to avoid woody
vegetation was again demonstrated. Not only was the population
concentrated on that part of the study plot farthest from the forest
edge but, as a general rule, voles tended to avoid single trees or
clumps of shrubby plants wherever these occurred on the area. As an
example, trap number 18 never caught more than one per cent of the
monthly catch and in many trapping periods caught nothing. This trap was
under a wild plum tree. Adjacent traps often were entered; the general
area was the most heavily populated part of the study plot. Only under
the plum tree was there a relatively unused portion.

Traps number 29 and 30, in the shade of a large honey locust tree, also
caught but few voles. Trap number 30 was only six feet from the base of
the tree and caught but one vole throughout the study period. These two
traps caught more _Peromyscus leucopus_ than any other pair, however,
and both of them also caught pine voles (_M. pinetorum_). The area
shaded by this tree permitted an extension of parts of the forest edge
fauna into the grassland.

In spite of the marked general tendency to avoid woody plants, some
voles made their runways around the roots of blackberry bushes, sumac
and wild plum trees. Some nests were found under larger roots, as if
placed there for protection. More vegetation was found under the woody
plants which the voles chose to use for shelter than under those which
they avoided. It seemed probable that the actual condition avoided by
voles was the bareness of the ground (a result of the shade cast by the
woody plants) rather than the woody plants themselves.

Running diagonally across the eastern half of the trapping plot in House
Field there was a terracelike ridge of soil. On each side of this ridge
there was a slight depression. Observations of the study plot in the
growing season showed this strip to produce the most luxuriant
vegetation of any part of the plot. Clip-quadrat studies confirmed this
observation and showed the bluegrass understory to be especially heavy.
This strip included the areas trapped by traps numbered 4, 5, 17, 18,
22, 23 and 37. With the exception of trap number 18, discussed above,
these traps consistently made more captures than traps set in other
parts of the plot. In winter, these traps also caught more harvest mice
(_Reithrodontomys megalotis_) than any other comparable group of traps.

Although the amount of growing tissue of plants probably is at least as
important to voles as the total amount of vegetation, some correlation
seemed to exist between the density of grassy vegetation and the density
of populations of voles. A mixed stand of grasses, with an obvious weedy
component, can support a larger population of voles than can either a
nearly pure stand of grass or the typical early seral stages dominated
by weeds. Probably the more or less continual supply of young plants
provided preferred food easily available to voles. A more homogeneous
vegetation would tend to pass through the young and tender stage as a
unit, thus causing a feast to be followed by a relative famine.



POPULATION STRUCTURE


During the period of study the percentage of males in most of my samples
was less than 50 per cent (Fig. 2). Only once, in June, 1952, did the
mean percentage of males in samples from three areas (House Field,
Quarry Field, Fitch traps) exceed that level and then it was only 50.1
per cent. On several occasions, however, the percentage of males in a
sample from a single area was slightly above 50 per cent. The highest
percentage of males recorded was 56.69 per cent, in a sample taken from
the Quarry Field population in June, 1952. In the samples taken in
April, 1952, the mean percentage of males was 39.67 per cent, the lowest
mean recorded. The low point for one sample was 28.02 per cent in
August, 1952, from Quarry Field. The mean percentage of males in all
samples taken was 45.02 ± 2.72 per cent. Percentages observed would
occur in random samples taken from a population with 50 per cent males
less than one per cent of the time. Exactly 50 per cent of the young in
the 65 litters examined were classified as males but the sample was
small and the sexing of newborn individuals was difficult.

[Illustration: FIG. 2. Graphs of population structure showing the
monthly changes in the mean percentages of juveniles, subadults, adults
and males in samples from the three study areas.]

The extent to which sex ratios in samples were affected by trapping
procedure was not determined. A possibility considered was that the
greater wandering tendency of males (Blair, 1940:154; Hamilton,
1937c:261; Townsend, 1935:98) impaired the formation of trap habits
(Chitty and Kempson, 1949:536) on their part and thus unbalanced the sex
ratios of the samples. If this were the explanation, the apparent sex
ratio on larger areas would more nearly approximate the true ratio, and
the frequency of capture of females would exceed that of males. The
evidence is somewhat equivocal. In the populations described here the
mean number of captures per individual per month was 2.31 for females,
which was significantly greater (at the one per cent level) than the
2.20 captures per individual per month which was the mean number for
males. This difference supports the idea that differences in habits
between the sexes result in distorted sex ratios in samples obtained by
live-trapping. Mean percentages of males did not, however, differ
significantly between the House Field-Quarry Field samples and the
samples from the Fitch trapping area, nearly five times as large.

Three age classes, juvenal, subadult and adult, were separated on the
basis of condition of pelage. The percentage of adults in populations
varied seasonally (Fig. 2). January, February and March were the months
when the adult fraction of the population was highest and October and
November were low points, with May and June showing percentages almost
as low. The only marked variation in this seasonal pattern occurred in
July and August, 1952, when the percentage of adults rose sharply. This
was due to a depression in the reproductive rate during the dry summer
of 1952, which is discussed later in this report. Juveniles made up only
a small fraction of the population from December through March and a
relatively large fraction in the October-November and May-June periods
(Fig. 2). Again, July and August of 1952 were exceptions to the pattern
as the percentages of juveniles in these months fell to midwinter
levels. As expected, the curve of the percentages of subadults in the
population followed that of the juveniles and preceded that of the
adults. The mean percentages for the thirty month period for which data
were available were: adults, 77.72 ± 4.48 per cent; subadults, 14.06 ±
3.14 per cent; and juveniles, 8.22 ± 2.62 per cent. Seasonal and yearly
changes in the population structure occurred, with notable variation in
the ratio of breeding females to the entire population, as discussed in
this report under the heading of reproduction.

Since some of the juveniles did not move enough to be readily trapped,
the real percentage of juveniles in the population was probably far
greater than that shown by trapping data. I tried, therefore, to
estimate the number of juveniles on the study plot each month by
multiplying the number of lactating females by the mean litter size. As
expected, the results were consistently higher than the estimate based
on trapping data. The discrepancy was largest in April, May, June and
October. During the winter there was no important difference between the
two estimates. Even when the discrepancy was greatest, the estimated
weight of the juveniles missed by trapping was not large enough to
modify the picture of habitat utilization in any important way. I chose,
therefore, to count only those juveniles actually trapped. Although
probably consistently too low, such a figure seemed more reliable than
an estimate made on any other basis.

[Illustration: FIG. 3. Percentages of individuals captured each month
surviving in subsequent months. The graph shows differential survival
according to time of birth. Individuals born in autumn seem to have a
longer life expectancy. The numbers on the lines refer to months of
first capture.]

A study of the age groups in each month's population revealed a
differential survival based on the season of birth. Blair (1948:405)
found that chances of survival in _Microtus pennsylvanicus_ were
approximately equal throughout the year. In the present populations of
_M. ochrogaster_, however, voles born in October, November, December and
January tended to live longer than those born in other months (Fig. 3).
Presumably these animals, born in autumn and early winter, were more
vigorous than their older competitors and were therefore better able to
survive the shrinking habitat of winter. Their continued survival after
large numbers of younger voles had been added to the population probably
was permitted by the expanding habitat of spring and summer. The
percentage of the population surviving the winter of 1951-1952 was
approximately double the percentage surviving the winter of 1950-1951.
This difference seemed to be due to the smaller population entering the
winter of 1951-1952 rather than any major difference in the
environmental resistance.

As a consequence of the differential survival, most of the breeding
population in the spring was made up of animals born the previous
October and November. Fig. 4 shows that in February, when the percentage
of breeding females ordinarily began to rise, 51.6 per cent of the
population was born in the previous October and November. Voles born in
these two months continued to form a large part of the population
through March (45.1 per cent), April (38.5 per cent), May (23.9 per
cent), June (18.7 per cent) and July (16.2 per cent) (Fig. 4). These
percentages suggest that the habitat conditions in October and November
were probably important in determining the population level for at least
the first half of the next year.

[Illustration: FIG. 4. Differential survival of voles according to month
when first caught. Each column represents the percentage of the monthly
sample first caught in each of the preceding months. Those voles caught
first in October and November survived longer than those first caught in
other months. Relatively few individuals remained in the population as
long as one year.]



POPULATION DENSITY


Population densities were ascertained on the study areas by means of the
live-trapping program. Blair (1948:396) stated that almost all small
mammals old enough to leave the nest (except shrews and moles) are
captured by live-trapping. My experience, and that of other workers on
the Reservation, requires modification of such a statement. The distance
between traps is an important factor in determining the efficiency of
live-trapping. As mentioned earlier, when House Field and Quarry Field
were trapped out at the conclusion of the live-trapping program no
unmarked voles were taken. This showed that the 30 foot interval between
traps was short enough to cover the area as far as _Microtus_ was
concerned. The fact that unmarked adults were caught almost entirely in
marginal traps is additional evidence. On the other hand, the Fitch
traps were 50 feet apart and voles seemed to have lived within the grid
for several months before being captured. Fitch (1954:39) has shown that
some kinds of small mammals are missed in a live-trapping program
because of variation in bait acceptance, both seasonal and specific.

A few individuals, missed in a trapping period, were captured again in
subsequent months. These voles were assumed to have been present during
the month in which they were not caught. The area actually trapped each
month was estimated by a modification of the method proposed by Stickel
(1946:153). The average maximum move was calculated each month and a
strip one half the average maximum move in width was added to each side
of the study area actually covered by traps. The study plots were
bounded in part by gravel roads and forest edge acting as barriers, and
for these parts no marginal strip was added. Trap lines on the opposite
sides of these roads rarely caught marked voles that had crossed in
either direction. It is perhaps advisable to say here that the size of
House Field and Quarry Field study plots (0.56 acres) was too small for
best results in estimating population levels (Blair, 1941:149). In the
computations of population levels the data for males and females were
combined, because no significant difference between the average maximum
move of the sexes was apparent.

Fluctuations of the populations were graphed in terms of individuals per
acre (Fig. 5). The variation was great in the 30 month period for which
data were available, and was both chronological and topographical. The
lowest density recorded was 25.2 individuals per acre and the highest
density was 145.8 individuals per acre. The weight varied from a low of
847 grams per acre to a high of 5275 grams per acre.

[Illustration: FIG. 5. Variations in density of voles from three
populations, as shown by live-trapping, and the mean density of these
populations. Juveniles are not represented in their true numbers since
many voles were caught first as subadults. The samples from the Fitch
trap line were incomplete due to the wide spacing of the traps.]

There are few records of density of _M. ochrogaster_ in the literature.
Brumwell (1951:213) found nine individuals per acre in a prairie on the
Fort Leavenworth Military Reservation and Wooster (1939:515) reported
38.5 individuals per acre for _M. o. haydeni_ in a mixed prairie in
west-central Kansas. High densities for _M. pennsylvanicus_ reported in
the literature include 29.8 individuals per acre (Blair, 1948:404), 118
individuals per acre (Bole, 1939:69), 160-230 individuals per acre
(Hamilton, 1937b:781) and 67 individuals per acre (Townsend, 1935:97).

Because the study period included one period of unusually high rainfall
and one year of unusually low rainfall, the normal pattern of seasonal
variation of population density was obscured. An examination of the data
suggested, however, that the greatest densities were reached in October
and November with a second high point in the April-May-June period.
These high points generally followed the periods of high levels of
breeding activity (Fig. 8). The autumn rise in population may have been
due, in part, to the addition of spring and early summer litters to the
breeding population, but the rise occurred too late in the year to be
explained by that alone. Another factor may have been the spurt in
growth of grasses occurring in Kansas in early autumn, in September and
October. There was a seeming correlation between high rainfall with
rapid growth of grasses and reproductive activity, and, secondarily with
high population densities of voles. These relationships are discussed in
connection with reproduction. Lowest annual densities were found to
occur in January when there is but little breeding activity and when
rainfall is low and plant growth has ceased.

Marked deviation from the usual seasonal trends accompanied flood and
drought. In the flood of July, 1951, although the study areas were not
inundated, the ground was saturated to the extent that every footprint
at once became a puddle. Immediately after the floods, on all three
areas studied, populations were found to have been drastically reduced.
The effect was most severe on the population of House Field, the lowest
area studied, and the recovery of the population there was much slower
than that of those on the other study areas (Fig. 5). Newborn voles were
killed by the saturated condition of the ground in which they lay. The
more precocious young of _Sigmodon hispidus_ survived wetting better.
They thus acquired an advantage in the competitive relationship between
cotton rats and voles. These relationships are discussed more fully in
the section on mammalian associates of _Microtus_.

Adverse effects of heavy rainfall on populations of small mammals have
been reported by Blair (1939) and others. Goodpastor and Hoffmeister
(1952:370) reported that inundation sharply reduced populations of _M.
ochrogaster_ for a year after flooding but that the area was then
reoccupied by a large population of voles. Such a reoccupation may have
begun on the areas of this study in the spring of 1952 when the upward
trend of the population was abruptly reversed by drought. While cotton
rats were abundant their competition may have been an important factor
in depressing population levels of voles. The population of voles began
to rise only after the population of cotton rats had decreased (Fig.
19).

In the unusually dry summer of 1952, there was a marked decline of
population levels beginning in June and continuing to August when my
field work was terminated. Dr. Fitch (1953, _in litt._) informed me that
the decline continued through the winter of 1952-53 and into the summer
of 1953, until daily catches of _Microtus_ on the Reservation were
reduced to 2-10 per cent of the number caught on the same trap lines in
the summer of 1951. The drought seemed to affect population levels by
inhibiting reproduction, as described elsewhere in this report. A
similar sensitivity to drought was reported by Wooster (1935:352) who
found _M. o. haydeni_ decreased more than any other species of small
mammal after the great drought of the thirties.

No evidence of cycles in _M. ochrogaster_ was observed in this
investigation. All of the fluctuations noted were adequately explained
as resulting from the direct effects of weather or from its indirect
effect in determining the kinds and amounts of vegetation available as
food and shelter.

The differences in densities supported by the various habitats were
discussed earlier in connection with the analysis of habitats.



HOME RANGE


Home ranges were calculated for individual voles according to the method
described by Blair (1940:149-150). The term, home range, is used as
defined by Burt (1943:350-351). Only those voles captured at least four
times were used for the home range studies. Individuals which included
the edge of the trap grid in their range were excluded unless a barrier
existed (see description of habitat) confining the seeming range to the
study area.

The validity of home range calculations has been challenged (Hayne,
1950:39) and special methods of determining home range have been
advocated by a number of authors. The ranges calculated in this study
are assumed to approximate the actual areas used by individuals and are
considered useful for comparison with other ranges calculated by similar
methods, but no claim to exactness is intended. It is obvious, for
instance, that many plotted ranges contain so-called blank areas which,
at times, are not actually used by any vole (Elton, 1949:8; Mohr,
1943:553). Studies of the movements of mammals on a more detailed scale,
perhaps by live-traps set at shorter intervals and moved frequently, are
needed to increase our understanding of home range.

In order to test the reliability of the range calculated, an examination
of the relationship between the size of the seeming range and the number
of captures was made. For the first three months, trapping on House
Field was done with a 20 foot grid and throughout the remainder of the
study a 30 foot grid was used. The effect of these different spacings on
the size of the seeming home range was also investigated. Hayne
(1950:38) found that an increase in the distance between traps caused an
increase in the size of the seeming home range, but in my study the
increased interval between traps was not accompanied by any change in
the sizes of the calculated ranges.

The number of captures, above the minimum of four, did not seem to be a
factor in determining the size of the calculated monthly range. A
seeming relationship was observed between the number of times an
individual was trapped and the total area used during the entire time
the vole was trapped. Closer examination revealed that the most
important factor was the length of time over which the vole's captures
extended. Table 2 shows the progressive increase in sizes of the mean
range of animals taken over periods of time from one month to ten
months.

  TABLE 2. RELATIONSHIP BETWEEN HOME RANGE SIZE AND LENGTH OF TIME ON THE
  STUDY AREA

  ======================================================================
  No. months on area      1    2    3    4    5    6    7    8    9   10
  Mean range in acres   .09  .09  .10  .14  .13  .17  .22  .22  .26  .24
  ----------------------------------------------------------------------

Nothing concerning the home range of _Microtus ochrogaster_ was found in
the literature. Several workers, including Blair (1940) and Hamilton
(1937c), have studied the home range of _M. pennsylvanicus_. Blair
(1940:153) reported a larger range for males than for females in all
habitats and in all seasons represented in his sample. In _M.
ochrogaster_, however, I found that the mean monthly range for both
sexes was 0.09 of an acre. Blair (_loc. cit._) reported no individuals
with a range so small as that mean, but Hamilton (_op. cit._:261)
mentioned two voles with ranges of less than 1200 square feet. The mean
total range used by an individual during the entire time it was being
trapped showed a slight difference between the sexes. Males used an
average of 0.14 of an acre whereas females used an average of but 0.12
of an acre. This suggested that, as in _M. pennsylvanicus_ (Hamilton,
_loc. cit._), males tended to wander more than females and to shift
their home range more often.

The largest monthly range recorded was 0.28 of an acre used by a female
in March, 1951, and calculated on the basis of four captures. The
largest monthly range of a male was 0.25 of an acre for a vole caught
eight times in November, 1950. The smallest monthly range was 0.02 of an
acre; several individuals of both sexes were restricted to areas of this
size. Juveniles, not included in the home range study, were usually
restricted to 0.01 or, at most, 0.02 of an acre. Seasonal differences in
the sizes of home ranges were not significant. However, the voles caught
in the winter often enough to be used for home range studies were too
few for a thorough study of seasonal variation in the size of home
ranges.

One female was captured 22 times in the seven-month period of October,
1950, to April, 1951. She used an area of 0.83 of an acre, but this
actually comprised two separate ranges. From October, 1950, through
December, 1950, she was taken 17 times within an area of 0.12 of an
acre; and from January, 1951, to April, 1951, she was taken five times
within an area of 0.15 of an acre. The largest area assumed to represent
one range of a female was 0.38 of an acre, recorded on the basis of six
captures in three months. The largest area encompassed by the record of
an individual male was 0.41 of an acre. He, too, shifted his range,
being taken five times on an area of 0.07 of an acre and twice, two
months later, on an area of 0.09 of an acre. Presumably, the remainder
of his calculated total range was used but little, or not at all. The
largest single range of a male was 0.36 of an acre, calculated on the
basis of 18 captures in seven months. The smallest total range for both
sexes was 0.02 of an acre.

Many voles shifted their home range and a few did so abruptly. The large
range of a female vole, described above and plotted in Fig. 6, indicated
an abrupt shift from one home range to another. More common is a gradual
shift as indicated by the range of the male shown in Fig. 7. Large parts
of each monthly range of this vole overlapped the area used in other
months but his center of activity shifted from month to month.

[Illustration: FIG. 6. Map with cross-hatched areas showing the range of
vole #20 (female). Dots show actual points of capture at permanent trap
stations 30 feet apart. Vertical lines mark area in which vole was taken
17 times in October and November, 1950. Horizontal lines mark area in
which vole was taken five times in March and April, 1951. This vole was
not captured in December and January.]

[Illustration: FIG. 7. Map showing range of vole #52 (male) with seeming
shifts in its center of activity. Dots show actual points of capture at
permanent trap stations 30 feet apart. Solid line encloses points of six
captures in October and November, 1950. Broken line encloses points of
five captures in February and March, 1951. Dotted line encloses points
of nine captures in April, May and June, 1951.]

That home ranges overlapped was demonstrated by frequent capture of two
or more individuals together in the same trap. No territoriality has
been reported in any species of _Microtus_, to my knowledge, and my
voles showed no objection to sharing their range. Voles taken from the
field into the laboratory lived together in pairs or larger groups
without much friction.

Definable systems of runways and home ranges were not coextensive.
Runway systems tended to merge, as described later in this report, and
relationships between them and home range were not apparent. Home ranges
had no characteristic shape.



LIFE HISTORY


Reproduction

Reproductive activity might have been measured in a number of ways.
Three indicators were tested: the percentage of females gravid or
lactating, the percentage of juveniles in the month following the
sampling period, and the percentage of females with a vaginal orifice in
the sampling period. The condition of vagina proved to be most useful.
Whether or not there is a vaginal cycle in _Microtus_ is uncertain.
Bodenheimer and Sulman (1946:255-256) found no evidence of such a cycle,
nor did I in my work with laboratory animals at Lawrence. How much the
artificial environment of the laboratory affected these findings is
unknown. The presence of an orifice seemed to indicate sexual activity
(Hamilton, 1941:9). The percentage of gravid females in the population
could not be determined accurately by a live-trapping study and was not
useful in this investigation. The percentage of juveniles trapped in the
month following the sampling period tended to follow the curve of the
percentage of adult females with a vaginal orifice. The ratio of trapped
juveniles to adults trapped was a poor indicator of reproductive
activity. Juveniles were caught in relatively small numbers because of
their restricted movements, and no way to determine prenatal and juvenal
mortality was available.

Reproductive activity continues throughout the year. Within the
thirty-month period for which data were obtained, December and January
showed the lowest percentages of females with vaginal orifices (Fig. 8).
The other months all showed higher levels of reproductive activity with
a slight peak in the August-September-October period in both 1950 and
1951. In the species of _Microtus_ that are found in the United States,
such summer peaks of breeding seem to be the rule (Blair, 1940:151;
Gunderson, 1950:17; Hamilton, 1937b:785). Jameson (1947:147) worked in
the same county where my field study was made and found that the high
point of reproduction was in March, although his samples were too small
to be reliable. The peak of reproductive activity slightly preceded the
highest level of population density in each year (Fig. 8).

[Illustration: FIG. 8. Variations in density and reproductive rate of
voles, with variation in monthly precipitation. Abnormally low rainfall
in 1952 caused a decrease in breeding activity and eventually in the
numbers of voles. The solid line indicates the number of voles per acre,
the broken line the percentage of females with a vaginal orifice and the
dotted line the inches of rainfall.]

A marked reduction in the percentage of females having vaginal orifices
was observed in the unusually dry summer of 1952. The rate of
reproduction was found to be positively correlated with rainfall (Fig.
9). Correlation coefficients were higher in each case when the amount of
rainfall in the month preceding each sampling period was used instead of
that in the month of the sample. This suggested that the rainfall
exerted its influence indirectly through its effect on plant growth.
Bailey (1924:530) reported that a reduction in either the quantity or
quality of food had a depressing effect on reproduction. Drought, such
as occurred in 1952, would certainly have a depressing effect on both.
The critical factor seems to be the supply of new, actively growing
shoots available to the voles for food rather than the total amount of
vegetation. As far as could be determined from the small sample of males
examined, their fecundity was not affected by rainfall. Some decrease in
the percentage of males that were fecund was noted in the winter and was
reported also by Jameson (1947:145) but most of the males in any sample
were fecund. Thus any depression in the reproductive rate was due to
loss of fecundity by females. This was in agreement with reports in the
literature on the subject (Baker and Ransom, 1932a:320; 1932b:43).

The correlation coefficient between rainfall and the percentage of adult
females with a vaginal orifice was 0.53. This was considered to be
surprisingly high in view of the expected effects on the breeding rate
of temperature, seasonal diet variations and whatever rhythms were
inherent in the voles. When only the summer months were considered the
correlation coefficient between rainfall and the percentage of adult
females with a vaginal orifice was 0.84. This indicated that, during the
season when breeding was at its height, rainfall was a factor in
determining the rate of reproduction and when rainfall was scarce, as in
the summer of 1952, it seemed to be a limiting factor (Fig. 9).

[Illustration: FIG. 9. Comparison between monthly rainfall and
reproductive rate of voles in summer. The dry summer of 1952 caused a
notable decrease in reproductive activity. The correlation coefficient
between rainfall and the percentage of females with a vaginal orifice
was 0.84.]

Of the total captures 20.6 per cent involved more than one individual.
When the distribution of these multiple captures was graphed for the
period of study, a high correlation between the percentage of captures
that were multiple and the percentage of females with a vaginal orifice
(r = 0.70) was found. An even higher correlation (r = 0.76) was observed
between the percentage of captures that were multiple and the population
density. The higher percentage of multiple captures may have been
largely a result of fewer available traps per individual on the area and
thus only indirectly related to the rate of reproduction.

Of the multiple captures, 66 per cent involved both sexes. The
correlation coefficient between the percentage of captures involving
both sexes and the level of reproductive activity was 0.58. Among those
pairs of individuals caught together more than once, 61 per cent were
composed of both sexes. Among those pairs taken together three or more
times 76 per cent were male and female and among those pairs taken
together four or more times 80 per cent were male and female. When adult
voles stayed together any length of time their relationship usually
appeared to be connected with sex. Family groups were also noted, as
pairs were often trapped which seemed to be mother and offspring. A
lactating female would sometimes enter a trap even after it had been
sprung by a juvenile, presumably her offspring, or a juvenal vole would
enter a trap after its mother had been captured. Such family groups
persisted only until the young voles had been weaned.

The youngest female known to be gravid was 26 days old and weighed 28
grams. During summer most of the females were gravid before they were
six weeks old, although females born in October and after were often
more than 15 weeks old before they became gravid. The youngest male
known to be fecund was approximately six weeks old. Male fecundity was
determined as described by Jameson (1950). Difference in the age of
attainment of sexual maturity serves to reduce the mating of litter
mates (Hamilton, 1941:7) and has been noticed in various species of the
genus _Microtus_ by several authors (Bailey, 1924:529; Hatfield,
1935:264; Hamilton, _loc. cit._; Leslie and Ransom, 1940:32).

For 35 females, each of which was caught at least once each month for
ten consecutive months or longer, the mean number of litters per year
was 4.07. Certain of the more productive members of the group produced
11 litters in 16 months. _M. ochrogaster_ seems to be less prolific than
_M. pennsylvanicus_. Bailey (1924:528) reported that one female meadow
vole delivered 17 litters in 12 months. Hamilton (1941:14) considered 17
litters per year to be the maximum and stated that in years when the
vole population was low the females produced an average of five to six
litters per year. In "mouse years" the average rose to eight to ten
litters per year. During this study several females delivered two or
more litters in rapid succession. This was noted more frequently in
spring and early summer than in other parts of the year. Those females
which produced two or three litters in rapid succession in spring and
early summer often did not litter again until fall. Post-parous
copulation has been observed in _M. pennsylvanicus_ by Bailey (1924:528)
and Hamilton (1940:429; 1949:259) and probably occurs also in _M.
ochrogaster_.

The gestation period was approximately 21 days, the same as reported for
_M. pennsylvanicus_ (Bailey, _loc. cit._; Hamilton, 1941:13) and _M.
californicus_ (Hatfield, 1935:264). A more precise study of the breeding
habits of _M. ochrogaster_ failed to materialize when the voles refused
to breed in captivity. Fisher (1945:437) also reported that _M.
ochrogaster_ failed to breed in captivity although _M. pennsylvanicus_
(Bailey, 1924) and _M. californicus_ (Hatfield, 1935) reproduced readily
in the laboratory.


Litter Size and Weight

In the course of this study 65 litters were observed. The mean number of
young per litter was 3.18 ± 0.24 and the median was three (Fig. 10).
Three litters contained but one individual and the largest litter
contained six individuals. Other investigators have reported the number
of young per litter in _M. ochrogaster_ as three or four (Lantz,
1907:18) and 3.4 (1-7) (Jameson, 1947:146). _M. pennsylvanicus_ seems to
have larger litters. Although Poiley (1949:317) found the mean size of
416 litters to be only 3.72 ± 0.18, both Bailey (1924:528) and Hamilton
(1941:15) found five to be the commonest number of young per litter in
that species. Leslie and Ransom (1940:29) reported the average number of
live births per litter to be 3.61 in the British vole, _M. agrestis_.
Selle (1928:96) reported the average size of five litters of _M.
californicus_ to be 4.8. Hatfield (1935:265), working with the same
species, found that litter size varied directly with the age of the
female producing the litter. He reported litters of young females as two
to four young per litter and of older females as five to seven young per
litter. In the litters of _M. ochrogaster_ that I examined, young
females did not have more than three young and usually had but two.
However, older females had litters of one, two and three often enough so
that no relationship, as described above, was indicated clearly.

[Illustration: FIG. 10. Distribution of litter size among 65 litters of
voles.]

No seasonal variation in litter size was noted. The mean size of the
litters in 1950, 2.68 ± 0.30, was significantly lower than that found in
1951 (3.76 ± 0.20) but neither differed significantly from the mean size
of litters in 1952 (3.35 ± 0.66). The lower mean size of litters was in
part coincidental with a high population level and the higher mean of
the two later years was in part coincidental with a low population
level. Since a sharp break in the curve for population density occurred
after the flood in July, 1951, the litters were arranged in pre-flood
and post-flood categories for study. Pre-flood litters averaged 3.07 ±
0.28 young per litter whereas post-flood litters averaged 3.34 ± 0.48.
This difference was not significant. Increase in litter size, if it had
actually occurred, might have been a response to the increasing food
supply and lower population density after the flood.

A difference in the mean number of young per litter was noted for those
litters delivered in traps as compared with those delivered in captivity
and the numbers of embryos examined in the uterus. The mean number of
embryos per female was higher than the mean number of young per litter
delivered in captivity and the mean number of young per litter delivered
in traps was lower than in those delivered in captivity. The differences
were not statistically significant. In some instances females that
delivered young voles in traps may have delivered others prior to
entering the trap or the mother or her trapmates may have eaten some of
the newborn voles before they were discovered.

The mean weight of 16 newborn (less than one day old) individuals was
2.8 ± 0.36 grams. No other data on the weight of newborn _M.
ochrogaster_ were found in the literature but this mean was close to the
3.0 grams (Bailey, 1924:530) and 2.07 grams (Hamilton, 1937a:504;
1941:10) reported for _M. pennsylvanicus_ and to the 2.7 grams (Selle,
1928:97) and 2.8 grams (Hatfield, 1935:268) reported for _M.
californicus_. No correlation between the weight of the individual
newborn vole and the number of voles per litter was observed.

Although the ratio of the average weight of newborn voles to the average
weight of an adult female was approximately equal for _M.
pennsylvanicus_ and _M. ochrogaster_, the ratio of the weight of a
litter to the average weight of an adult female was larger in the
eastern meadow vole because the mean litter size was larger. Perhaps
this is related to the more productive habitat in which the eastern
meadow vole is ordinarily found.


Size, Growth Rates and Life Spans

The mean weight of adult voles during the period of study was 43.78
grams. The females averaged slightly heavier than the males but the
overlapping of weights was so extensive that sexual difference in weight
could not be affirmed. The difference observed was less in December and
January when gravid females were rare, suggesting that the difference
was due, at least in part, to pregnancy. Jameson (1947:128) found, for a
sample of 50 voles, a mean weight of 44 grams and a range of 38 to 58
grams. The range in the adult voles I studied was much greater, from 25
to 73 grams. In part, this increase in the range of adult weights was
due to a much larger sample.

[Illustration: FIG. 11. Relationship between rainfall and the mean
weight of adult males in summer. The abnormally low rainfall in the
summer of 1952 was accompanied by a decrease in mean weight. The solid
line represents mean weight and the broken line rainfall. The
correlation coefficient between the two was 0.68.]

During the unusually dry summer of 1952, a notable reduction in the mean
weight of adults was recorded (Fig. 11). The correlation coefficient
between the mean weight of adults and the amount of rainfall for the
summer months was 0.68. It seems reasonable to attribute the drop in
mean weight to an alteration of plant growth due to decreased rainfall.
Some of the reduction in mean weight was due to the loss of weight in
older individuals but most of it was due to the failure of voles born in
the spring to continue growing.

No data on the growth rate of _M. ochrogaster_ were found in the
literature. According to the somewhat scanty data from my study, secured
from observations of individuals born in the laboratory, young voles
gained approximately 0.6 of a gram per day for the first ten days,
approximately one gram per day up to an age of one month, and
approximately 0.5 of a gram per day from an age of one month until
growth ceases. This growth rate was especially variable after the voles
reached an age of thirty days. The growth rate approximates those
described for _M. pennsylvanicus_ (Hamilton, 1941:12) and for _M.
californicus_ (Hatfield, 1935:269; Selle, 1928:97). Although the data
were inadequate for a definite statement, I gained the impression that
there was no difference between the sexes in growth rate. In general,
young voles grow most rapidly in the April-May-June period and least
rapidly in mid-winter. Several voles, born in late autumn, stopped
growing while still far short of adult size and lived through the winter
without gaining weight, then gained as much as 30 per cent after spring
arrived (Fig. 12).

[Illustration: FIG. 12. Growth rates of two voles selected to show
typical growth pattern of voles born late in the year. Growth nearly
stops in winter and is resumed in spring.]

The recorded life spans of most voles studied were less than one year.
No accurate mean life span could be determined. Leslie and Ransom
(1940:46), Hamilton (1937a:506) and Fisher (1945:436) also found that
most voles lived less than one year. Leslie and Ransom (_op. cit._: 47)
reported a mean life span of 237.59 ± 10.884 days in voles of a
laboratory population. In the present study one female was trapped 624
days after first being captured; another female was trapped 617 days
after first being captured; and a male was trapped 611 days after first
being captured. The two females were subadults when first captured. The
male was already an adult when first captured; consequently its life
span must have exceeded 650 days. No evidence of any decrease in vigor
or fertility was observed to accompany old age.

Of the 45 marked voles snap-trapped in August of 1952, 21 had been
captured first as juveniles. The ages of these voles could be estimated
within a few days, and the series presented a unique opportunity for
studying individual and age variation. Only individuals weighing less
than 18 grams when first captured were used, and their ages were
estimated according to the growth rate described above. Howell (1924)
reported an analysis of individual and age variation in a series of
specimens of _Microtus montanus_, and Hall (1926) studied the changes
due to growth in skulls of _Otospermophilus grammarus beecheyi_. The
series of specimens described here differs from those of Hall and
Howell, and from any other collection known to me, in the fact that the
specimens are of approximately known age and drawn from a wild
population.

Unfortunately, this sample was small, and the distribution of the
specimens among age groups left much to be desired. No specimens less
than one and one-half months old were taken and only a few individuals
older than four and one-half months. Table 3 shows the age distribution.
The small size of the sample and the absence of juveniles were due,
partly, to the unusually dry weather in the summer of 1952. The
reduction in the rate of reproduction, caused by drought (as described
elsewhere in this paper), reduced the populations and the percentage of
juveniles to low levels.

  TABLE 3. DISTRIBUTION AMONG AGE GROUPS OF 21 VOLES USED IN THE STUDY OF
  VARIATION DUE TO AGE

  ======================================================================
  Age in months       1-1/2   2   2-1/2   3   3-1/2   4   4-1/2   6   12
  ----------------------------------------------------------------------
  No. of individuals   1      4    5      1    3      2    3      1    1
  ----------------------------------------------------------------------

In the series of voles studied, ten individuals were in the process of
molting from subadult to adult pelage. Jameson (1947:131) reported the
molt to occur between eight and 12 weeks of age and selected 38 grams as
the lower limit of weight of adults. I also found all voles molting to
be between eight and 12 weeks old but found none so large as 38 grams
without full adult pelage. This may have been, in part, due to the dry
weather delaying or inhibiting growth. Because of the small size of the
sample and the influence of the unusual weather conditions, no
conclusions concerning normal molting were drawn from the data described
below. They are presented only as a description of a small sample drawn
from a single population at one time. Table 4 summarizes these data.

  TABLE 4. MEAN SIZES AND AGES OF VOLES MOLTING FROM SUBADULT TO ADULT
  PELAGE

  =====================================================================
                              Body length   Condylo-basilar
                  Weight       minus tail      length         Age
  ---------------------------------------------------------------------
  Six males      32.67 gms.   106.16 mm.     23.78 mm.        9.67 wks.
                  (30-36)      (96-116)       (23.2-24.4)    (8-12)
  Four females    29.0 gms.   100.25 mm.     23.45 mm.        10.5 wks.
                  (28-30)      (98-102)       (23.5-23.8)    (8-12)
  Ten voles       31.2 gms.   103.8 mm.      23.73 mm.        10.0 wks.
                  (28-36)      (96-116)       (23.2-24.4)    (8-12)
  ---------------------------------------------------------------------

The mean age of the ten voles molting was ten weeks (8-12). Six males
averaged 9.67 weeks, almost a week younger than four females, who
averaged 10.5 weeks. The difference in age at time of molting between
the sexes was not significant. Differences between the sexes in other
characteristics to be described also lacked significance. Mean weights
at the time of molting were: males, 32.67 gms. (30-36); females, 29.0
gms. (28-30); and all individuals, 31.2 gms. (28-36). Because a piece of
the tail of each vole had been removed in marking, the total length of
the voles could not be determined. Body length, excluding tail, was
used. Howell (1924:986) found this measurement subject to less
individual variation than total length and thought body length was
probably a better indicator of age. Mean body length at the time of
molting was 103.8 mm. (96-116). Males averaged longer than females and
were also more variable. The mean body length of males was 106.16 mm.
(96-116) and that of females was 100.25 mm. (98-102).

Of the subadults showing no signs of molting, none was above the mean
age of molting. Twenty-five per cent of them were longer and heavier
than the mean length and weight of those that were molting. Of the 20
adults in the series, one was below the mean weight of molting and one
was shorter than the mean length of molting.

When Howell (_op. cit._:1014) studied skulls of _Microtus montanus_ he
found that the condylobasilar length was the most satisfactory means for
arranging his series of specimens according to their age. When the
skulls of my series were arranged according to their age (as determined
from trapping records) the graph of the condylobasilar lengths showed a
clear, though not perfect, relationship to age (Fig. 13). No separation
of sexes was made because the sample did not permit it. In Fig. 13
graphs of weight, as determined in the field, and of length (excluding
tail) also were included because they are the most easily measured
characters of live voles. The graphs indicate individual variation in
these characters which limits their usefulness in determining age.

[Illustration: FIG. 13. Graphs of the condylobasilar lengths, body
lengths and weights of a series of voles of known age. Within each age
group, the youngest vole is on the left in the graphs.]

When other cranial measurements, and ratios of pairs of measurements,
were plotted in the same order, individual variation obscured some of
the variation due to age and the curves resembled those of weight and
length of body rather than that of condylobasilar length. When the
cranial measurements were averaged for the age groups the curves showed
a relationship to age but the relationship of mean measurements is of
little use in determining the age of individual specimens. The data
described above indicated that a study of the relationship of the
condylobasilar length and age in a large sample might provide useful
information.

Anyone who has examined mammalian skulls knows of many other characters
which vary with age but which are difficult to measure and describe with
precision. Figs 14 and 15 are drawings of skulls of voles of known age.
The most obvious change, related to aging, evident in the dorsal view of
the skulls (Fig. 14) is the increasing prominence and closer
approximation of the temporal ridges in older specimens. The lambdoidal
ridge is also more prominent in older voles, and their skulls have a
generally rougher and more angular appearance. The individual variation
evident in these ridges is probably due to variations in the development
of the muscles operating the jaws (Howell, 1924:1003). There is an
increased flattening of the roof of the skull of older voles.

[Illustration: 1-1/2 months 2-1/2 months 3 months 3-1/2 months

4 months 4-1/2 months 6 months 12 months

All × 3.

FIG. 14. Dorsal views of skulls of voles of known age.]


[Illustration: 1-1/2 months 2-1/2 months 3 months 3-1/2 months

4 months 4-1/2 months 6 months 12 months

All × 3.

FIG. 15. Palatal views of skulls of voles of known age.]

From a palatal view (Fig. 15) the skulls of voles also showed age
variation which was apparent but not easily correlated with precise age.
The median ridge on the basioccipital bone increases in prominence in
older voles. The shape of the posterior margin of the palatine bones
changes from a V-shape to a U-shape. On the skull of the oldest (12
months) vole the pterygoid processes are firmly fused to the bullae, a
condition not found in any of the other specimens. The anterior spine of
the palatine approaches the posterior projection of the premaxillae more
closely as age increases and, in the oldest vole is firmly attached and
forms a complete partition separating the incisive foramina.

Tooth wear during the life of a vole causes a considerable variation in
the enamel patterns, especially of the third upper molar. Howell
(1924:1012) considered such variation to be independent of age, but
Hinton (1926:103) related the changes to age and interpreted them as a
recapitulation of the evolution of microtine molars. In my series, an
indentation on the medial margin of the posterior loop of the third
upper molar seemed to be related to age. This indentation was absent in
the youngest vole (one and one-half months), absent or indefinite in
those voles less than 3-1/2 months of age, and progressively more marked
in the older voles.


Food Habits

The prairie vole, like other members of the genus _Microtus_, feeds
mostly on growing grass in spring and summer. Piles of cuttings in the
runways are characteristic sign of the presence of voles. The voles cut
successive sections from the bases of grasses until the young and tender
growing tips are within reach. The quantity of grass destroyed is
greater than that actually eaten, a fact which will have to be
considered in any attempt to evaluate the effects of voles upon a range.

In all piles of cut plants that were examined, _Bromus inermis_ was the
most common grass, and _Poa pratensis_ was the grass second in
abundance. These were, by far, the most common grasses present on the
areas studied; in most places, _B. inermis_ was dominant. Other grasses
present on the areas were occasionally found in the piles of cuttings.
Jameson (1947:133-136) found no utilization of _B. inermis_ by voles but
that grass was present in a relative abundance of only one per cent in
the areas studied by him. The voles that he studied ate alfalfa in large
amounts and alfalfa was, perhaps, the most common plant on the
particular areas where his voles were caught. Seemingly, the diet of
voles is determined mostly by the species composition of the habitat.

Other summer foods included pokeberries, blackberries and a few forbs
and insects. Forbs most commonly found in the piles of cuttings were the
leaves of the giant ragweed (younger plants only) and dandelion. Insect
remains were found in the stomachs of voles killed in summer and
occurred most frequently in those killed in August and September. At no
time did insects seem to be a major part of the diet but they were
present in most vole stomachs examined in late summer. Laboratory
experiments with summer foods gave inconclusive results but suggested
that the voles chose grasses on the basis of their growth stage rather
than according to their species. Young and tender grasses were chosen,
regardless of species, when various combinations of _Triodia flava_,
_Bromus inermis_ and _Poa pratensis_ were offered to the voles. At
times the voles showed a marked preference for dandelion greens, perhaps
because of their high moisture content; the voles' water needs were
satisfied mostly by eating such succulent vegetation.

Winter foods consisted of stored hay and fruits and of underground plant
parts. _Bromus inermis_ made up nearly all of the hay and was stored in
lengths of up to ten inches in underground chambers specially
constructed for storage. Underground parts of plants were reached by
tunnelling and were an especially important part of the voles' diet in
January and February. The fruit of _Solanum carolinense_ was eaten
throughout the winter and one underground chamber, opened in February,
1952, was packed full of these seemingly unsavory fruits. Fisher
(1945:436), in Missouri, found this fruit to be an important part of the
winter diet of voles. An occasional pod of the honey locust tree was
found partly eaten in a runway. Fitch (1953, _in litt._) often observed
girdling of honey locust and crab apple (_Pyrus ioensis_) root crowns on
the Reservation but I saw no evidence of bark eating, perhaps because my
study plots were mostly grassland. On two occasions when two voles were
in the same trap one of them was eaten. In both traps, all of the bait
had been eaten and the captured voles probably were approaching
starvation. Because the trapping procedure offered abundant opportunity
for cannibalism, the low frequency of its occurrence suggested that it
was not an important factor in satisfying food requirements under normal
conditions.


Runways and Nests

Perhaps the most characteristic sign of the presence of _Microtus
ochrogaster_ were their surface runways and underground tunnels. Only
rarely was a vole observed to expose itself to full view. When a trapped
vole was released it immediately dove out of sight into a runway. Once
in a runway, the vole showed no further evidence of alarm and was
usually in no hurry to get away. The runways seemed to provide a sense
of security and the voles were familiar with their range only through
runway travel. The urge to seek a runway immediately when exposed has
obvious survival value.

Surface runways were usually under a mat of debris. In areas where
debris was scanty or lacking, runways were usually absent. Jameson
(1947:136) reported that in alfalfa and clover fields the voles did not
make runways as they did in grassland, even in fields where trapping
records showed voles to be abundant. Typical surface runways are
approximately 50 mm. wide, only slightly cut into the ground and bare of
vegetation while in use. Usually they could be distinguished from the
runways of the pine vole, which were cut more deeply into the ground,
and those of the cotton rat which were wider and not so well cleared of
vegetation. Some runways ended in surface chambers and some of these
were lined with grass. Their size varied from a diameter of 90 mm. to
250 mm. and they seemed to be used primarily for resting places.

A runway system usually consisted of a long, crooked runway and several
branches. Two typical systems are illustrated in Fig. 16. The runway
systems often were not clearly limited; they merged with other systems
more or less completely. One map showed a runway system extending across
140 square meters and including 12 underground burrows. All of these
runways seemed to be part of a single runway system but the system
probably was used by more than one vole or family group of voles.
Sixteen of the 22 maps that were made extended across areas between 50
and 90 square meters. One map, mentioned above, was larger and the
remaining five smaller. The smallest extended across only 20 square
meters. Of course, the area encompassed by a set of runways changed
almost daily, as the voles extended some runways, added some and
abandoned others in the course of their daily travels.

[Illustration: FIG. 16. Maps of runway systems of the prairie vole. The
runways follow an irregular course and are frequently changed. The solid
lines represent surface runways and the dotted lines underground
passages.]

Each runway system contained underground nests. These were in chambers
from 70 mm. to 200 mm. below the surface and were up to 200 mm. in
diameter. Most systems that were mapped had from two to six of these
burrows. Most of these were lined with dried grass and seemed to be used
for delivering and nursing litters. Each burrow was connected to a
surface runway by a tunnel. Often the tunnel was short and the hole
opened almost directly into the burrow from the surface runway. Others
had tunnels several meters long. Jameson (1947:137) reported every
burrow to have two connections with the surface. In the present study,
however, I found three arrangements in approximately equal frequency of
occurrence: (1) one hole to one tunnel leading to a burrow; (2) two
holes to two short tunnels which joined a long tunnel leading to a
burrow; and (3) two separate tunnels from the surface to a burrow. The
size, depth and number of underground burrows in the systems that I
studied varied and so did those reported in the literature. Jameson
(_loc. cit._) found burrows in eastern Kansas as deep as 18 inches, far
deeper than any found in my study. Fisher (1945:435) reported none
deeper than five inches in central Missouri. The soil data in my study,
as well as in the two reports cited immediately above, were not adequate
to permit conclusions, but the type and condition of the soil probably
determine the extent of burrowing by the voles of any given locality.

The number of voles using a runway system at one time was difficult to
ascertain. In one system, however, four adult individuals were trapped
in a ten day period. In August, 1952, at the conclusion of the
live-trapping program, a runway system was mapped which had included two
trapping stations. In the preceding ten days, four adult voles (three
males and one female) had been taken in both traps. During that time,
therefore, the runway system was shared by at least four voles. The
voles used an area that was considerably larger than that encompassed by
any one runway system, a fact obvious when the sizes of home ranges as
computed from trapping data were compared with the sizes of the runway
systems mapped. A runway system seemed not to be a complete unit, but
was only a part of the network of runways used by a single individual.


Activity

Although no special investigation of activity was made, some conclusions
concerning it were apparent in the data gathered. There have been a few
laboratory studies of the activity pattern of _Microtus_ by various
methods. Calhoun (1945:256) reported _M. ochrogaster_ to be mainly
nocturnal with activity reaching a peak between dark and midnight and
again just before dawn. Davis (1933:235), working with _M. agrestis_,
and Hatfield (1935:263), working with _M. californicus_, both found
voles to be more nocturnal than diurnal. In a field study of _M.
pennsylvanicus_, Hatt (1930:534) found the species to be chiefly
nocturnal, although some activity was reported throughout the day.
Hamilton (1937c:256-259), however, reported the same species to be more
active in the daytime. Agreement on the activity patterns of these
species of _Microtus_ has not yet been attained.

From occasional changes in the time of tending a trap line, and from
running lines of traps at night a few times in the summer of 1951, I
gained the impression that these voles were primarily diurnal.
Relatively few of them were caught in the hours of darkness. In summer,
however, their activity was mostly limited to the periods between dawn
and approximately eight o'clock and between sunset and dark. In colder
weather, there was increased activity on sunny days.



PREDATION


Although voles were a common item of prey for many species of predators
on the Reservation, no marked effect on the density of the population of
this vole could be attributed to predation pressure. Only when densities
reached a point that caused many voles to expose themselves abnormally
could they be heavily preyed upon. Their normally secretive habits,
keeping them more or less out of sight, suggest that they are an
especially obvious illustration of the concept that predation is an
expression of population vulnerability, rising to high levels only when
a population is ecologically insecure, rather than a major factor
regulating population levels (Errington, 1935; 1936; 1943; Errington _et
al_, 1940).

Scats from predatory mammals and reptiles and pellets from raptorial
birds were examined. Most of these materials were collected by Dr. Henry
S. Fitch, who kindly granted permission to use them. The results of the
study of the scats and pellets are summarized in Table 5. Remains of
voles were identified in 28 per cent of the scats of the copperhead
snake (_Ancistrodon contortix_) examined. Copperheads were moderately
common on the Reservation (Fitch, 1952:24) and were probably important
as predators on voles in some habitats. Uhler _et al_ (1939:611), in
Virginia, reported voles to be the most important prey item for
copperheads. A vole was taken from the stomach of a rattlesnake
(_Crotalus horridus_) found dead on a county road adjoining the
Reservation. Rattlesnakes were present in small numbers on the
Reservation but were usually found along rocky ledges rather than in
areas where voles were common (Fitch, _loc. cit._). The rattlesnakes
probably were less important as predators on voles than on other small
mammals more common in the usual habitat of these snakes. The blue racer
(_Coluber constrictor_) was common in grassland situations on the
Reservation (Fitch, 1952:24) and twice was observed in the role of a
predator on voles; one small blue racer entered a live-trap in pursuit
of a vole and another blue racer was observed holding a captured vole in
its mouth. The blue racer seems well adapted to hunt voles and probably
preys on them extensively. The pilot black snake (_Elaphe obsoleta_) has
been reported as a predator on _M. ochrogaster_ in the neighboring state
of Missouri (Korschgen, 1952:60) and was moderately common on the
Reservation (Fitch, _loc. cit._). _M. pennsylvanicus_, with habits
similar to those of _M. ochrogaster_, has been reported as a prey for
all of the above snakes (Uhler, _et al_, 1939).

  TABLE 5. FREQUENCY OF REMAINS OF VOLES IN SCATS AND PELLETS

  =========================================================================
                      No. of scats or       No. containing
  Predator            pellets examined     remains of voles      Percentage
  -------------------------------------------------------------------------
  Copperhead                 25                    7              28
  Red-tailed hawk            25                    3              12
  Long-eared owl             25                   18              72
  Great horned owl           32                    6              19
  Crow                       25                    4              16
  Coyote                     25                    3              12
  -------------------------------------------------------------------------

The red-tailed hawk (_Buteo jamaicensis_), the long-eared owl (_Asio
otus_), the great horned owl (_Bubo virginianus_) and the crow (_Corvus
brachyrhynchos_) fed on _Microtus_. All four birds were fairly common
permanent residents on the Reservation (Fitch, 1952:25). The low density
and the strict territoriality of the red-tailed hawk (Fitch, _et al_,
1946:207) prevented it from exerting any important influence on the
population of voles, even though individual red-tailed hawks ate many
voles. Predation by the long-eared owl was especially heavy; remains of
voles were identified in 72 per cent of its pellets examined. Korschgen
(1952:39) found remains of voles in 70 per cent of 704 pellets of the
long-eared owl. The reason for the heavy diet of _Microtus_ seems to be
that both the owl and the vole are especially active at dusk. A group of
long-eared owls, living near the edge of Quarry Field, probably exerted
an influence on the density of the local population of voles because of
the high ratio of predator to prey animals. The crows ate some, and
perhaps most, of their voles after the animals had died from other
causes. Other birds, mostly raptors, occurring in northeastern Kansas
and reported to prey on voles include the sharp-shinned hawk (_Accipiter
striatus_), Cooper's hawk (_A. cooperi_), red-shouldered hawk (_Buteo
lineatus_), broad-winged hawk (_B. platypterus_), American rough-legged
hawk (_B. lagopus_), ferruginous rough-legged hawk (_B. regalis_), marsh
hawk (_Circus cyaneus_), barn owl (_Tyto alba_), screech owl (_Otus
asio_), barred owl (_Strix varia_) and shrike (_Lanius excubitor_)
(Korschgen, 1952:26; 28; 34; 35; 37; McAtee, 1935:9-27; Wooster,
1936:396).

Coyotes, house cats and raccoons were identified as predators on voles
in the study areas. Remains of voles were present in 12 per cent of the
scats of the coyote (_Canis latrans_) examined. In Missouri, Korschgen
(1952:40-43) reported remains of voles in slightly more than 20 per cent
of the coyote stomachs that he examined. Fitch (1948:74), Hatt
(1930:559) and others have reported other species of _Microtus_ as eaten
by the coyote. Although coyotes were rarely seen on the Reservation,
coyote sign was abundant (Fitch, 1952:29) and coyotes probably ate large
numbers of voles. House cats (_Felis domesticus_), seemingly feral, were
observed to tour the trap lines on several occasions and were noted by
Fitch (_loc. cit._) as important predators on small vertebrates. Four
cats were killed in the course of the study and remains of voles were
found in the stomachs of all of them. On several occasions, raccoon
tracks were noted following the trap line when the traps had been
overturned and broken open, suggesting that raccoons are not averse to
eating voles although no further evidence of predation on voles by
raccoons was obtained. Fitch (_loc. cit._) reported raccoons (_Procyon
lotor_) to be moderately common on the Reservation. Reports of predation
by raccoons on voles are numerous (Hatt, 1930:554; Lantz, 1907:41). The
opossum (_Didelphis marsupialis_), common on the Reservation,
occasionally eats voles (Sandidge, 1953:99-101). Other mammals which are
probably important predators on voles on the Reservation, though no
specific information is available, are the striped skunk (_Mephitis
mephitis_), spotted skunk (_Spilogale putorius_), weasel (_Mustela
frenata_) and the red fox (_Vulpes fulva_). Eadie (1944; 1948; 1952),
Shapiro (1950:360) and others have reported that the short-tailed shrew
(_Blarina brevicauda_) was an important predator on _Microtus_. Shrews
were present on the Reservation but were not trapped often enough to
permit study.

The variety of vertebrates preying on voles suggests that they occupy a
position of importance in many food chains. Errington (1935:199) and
McAtee (1935:4) refer to voles as staple items of prey for all classes
of predatory vertebrates. An attempt to evaluate prey species was made
by Wooster (1939). He proposed a formula which involved multiplying the
density of a species, its mean individual weight, the fraction of the
day it was active and the fraction of the year it was active to give a
numerical index of prey value. Although his methods of determining
population densities would now be considered questionable, the purpose
of his investigation merits further consideration. He reported _M.
ochrogaster_ to be second only to the jack-rabbit (_Lepus californicus_)
as a prey species in west-central Kansas.



MAMMALIAN ASSOCIATES


In the course of live-trapping operations several species of small
mammals other than _Microtus ochrogaster_ were taken in the traps. Also,
from time to time, direct observations of certain mammals were made and
various types of sign of larger mammals were noted. These records gave a
picture of the mammalian community of which the voles were a part. The
three associated species which were most commonly trapped were _Sigmodon
hispidus_, _Reithrodontomys megalotis_ and _Peromyscus leucopus_. These
three species have been commonly found associated with _Microtus_ in
this part of the country (Fisher, 1945:435; Jameson, 1947:137).

The Texas cotton rat, _Sigmodon hispidus_, was the most commonly trapped
associate of the voles between November, 1950, and February, 1952.
Although a greater number of individuals of the harvest mouse were taken
in a few months, the cotton rat had a greater ecological importance
because of its larger size (Figs. 17, 18, 19). The cotton rat was an
especially noteworthy member of the community for two reasons. It has
arrived in northern Kansas only recently and its progressive range
extension northward and westward has attracted the attention of many
mammalogists (Bailey, 1902:107; Cockrum, 1948; 1952:183-187; Rinker,
1942b). Secondly, _Sigmodon_ has long been considered to be almost the
ecological equivalent of _Microtus_ and to replace the vole in the
southern United States (Calhoun, 1945:251; Svihla, 1929:353). Since the
two species are now found together over large parts of Kansas their
relationships in the state need careful study.

[Illustration: FIG. 17. Variations in density and mass of three common
rodents on House Field. The upper graph shows the sum of the biomass of
the three rodents. In the two lower graphs the solid line represents
_Microtus_, the broken line _Sigmodon_, and the dotted line
_Reithrodontomys_.]

[Illustration: FIG. 18. Variations in density and biomass of three
common rodents on Quarry Field. For key, see legend of Fig. 17.]

[Illustration: FIG. 19. Changing biomass ratios of three common rodents
on House Field and Quarry Field. In late 1951 and early 1952 the cotton
rats attained relatively high levels and seemingly caused compensatory
decreases in the numbers of voles. The solid line represents _Microtus_,
the broken line _Sigmodon_, and the dotted line _Reithrodontomys_.]

Both this study and the literature (Black, 1937:197; Calhoun, _loc.
cit._; Meyer and Meyer, 1944:108; Phillips, 1936:678; Rinker, 1942a:377;
Strecker, 1929:216-218; Svihla, 1929:352-353) showed that, in general,
the habitat needs of _Microtus_ and _Sigmodon_ were similar. Studies on
the Natural History Reservation, both in connection with my problem and
otherwise, suggested, however, that _Sigmodon_ occurred in only the more
productive habitat types used by voles, where the vegetation was
relatively high and rank. On the Reservation the cotton rat was found
mostly in the lower meadows; they were more moist and had a more
luxuriant vegetation than the higher fields. Although a few cotton rats
were taken in Quarry Field and still fewer in Reithro Field, the
population of those hilltop areas did not approach, at any time, the
levels reached on House Field, which produced a more luxuriant cover.
Only when the levels of population were exceptionally high did the
cotton rats spread into less productive habitats. At all times, there
were areas on the Reservation used by _Microtus_ which could not support
a population of _Sigmodon_.

The cotton rats reacted differently to the floods of July, 1951, than
did the voles. Although the population of the cotton rat decreased
slightly immediately after the wet period, this decrease was
insignificant when compared with the drop in population level of other
species of small mammals on the same area. During the autumn of 1951 and
until March, 1952, the cotton rat became the most important mammal on
the House Field study area in terms of grams per acre (Fig. 17),
although the number of cotton rats per acre never matched the density of
the voles. A similar, though less pronounced, trend was observed on the
Quarry Field study area (Fig. 18). One factor in the success of the
cotton rat at this time seemed to be the greater resistance to wetting
shown by very young individuals. Few adults (of any species) marked
before the heavy rains of July, 1951, were trapped in September, 1951,
when trapping was resumed after a lapse of one month. Several subadults
and some juvenal cotton rats did survive, however, and provided a
breeding population from which the area was repopulated. Cotton rats are
born fully furred and able to move well, and are often weaned at ten
days (Meyer and Meyer, 1944:123-124). Voles, on the other hand, are born
naked and helpless and are often not weaned for three weeks. It seems,
therefore, that extremely wet soil would harm the voles more than it
would the cotton rats.

Several instances of cotton rats eating voles, caught in the same
live-trap, were noted. There is reason to believe that young voles,
unable to leave the nest, are subject to predation by cotton rats. This
would accentuate any competitive advantage gained otherwise by the
cotton rats.

The population of _Sigmodon_ retained its high level, relative to
_Microtus_, until February, 1952. In March only one individual was
captured and after that none was trapped until August, 1952, when a
single subadult male was captured. Early in March, 1952, before the
trapping period for the month had begun, the area suffered three
successive days of unusually low temperature, with snow, which lay more
than six inches deep in places. As suggested by Cockrum (1952:185), such
conditions proved detrimental to the cotton rats and, at least to the
end of the study period in August, 1952, the population of cotton rats
had failed to recover. Perhaps the extremely dry weather which followed
the heavy winter mortality delayed the recovery of the population.

These limited data seem to indicate competition between _Sigmodon_ and
_Microtus_ in Kansas. Extremely wet conditions seem to give _Sigmodon_ a
competitive advantage whereas _Microtus_ is better able to survive dry
summers and severe winters. However, these relationships need further
clarification by an intensive study of the life history of _Sigmodon_ in
Kansas (especially the more arid western part), including its coactions
with the communities it has invaded successfully recently.

The harvest mouse (_Reithrodontomys megalotis_) also was a common
inhabitant of the study plots, but this small rodent seemed not to be a
serious competitor of the voles, as its food consists almost entirely of
seeds (Cockrum, _op. cit._:165) not usually used by voles. In this
study, at least, no conflict over space was apparent. Harvest mice
frequently were taken in the runways of voles and even in the same trap
with voles. Reithro Field, the part of the Reservation having the
heaviest population of the harvest mouse, differed from the habitats
that were better for voles in being higher, drier and less densely
covered with vegetation. However, during the summer of 1951 when the
voles were most abundant, Reithro Field supported a large population of
voles. Estimates of population of the harvest mouse were of doubtful
validity in summer because it was readily trapped only in winter and
early spring. Many individuals marked in late spring were not trapped
again until late autumn although presumably they remained on the area.
This seasonal variation in trapping success seemed to be a matter of
acceptance and refusal of bait (Fitch, 1954:45).

The presence of the wood mouse (_Peromyscus leucopus_) on the study
plots indicated an overlapping of habitats. Both House and Quarry Fields
were on the ecotone between forest and meadow and a mixture of mammals
from both types of habitat occurred. No sign of the homes of the wood
mouse was found on the study plots, and on the larger trap line,
operated by Fitch, wood mice were captured only near the edge of the
woods.

Only six deer mice (_Peromyscus maniculatus_) were taken on the study
plots. This small number probably provided an inaccurate index of the
association of the deer mouse and the prairie vole, because samples from
snap-traps and the data of other workers on the Reservation showed a
more common occurrence of the two species together. The deer mice seemed
to prefer a sparser vegetation and did not approach so closely to the
forest edge as did the voles. It may have been, in part, the presence of
_P. leucopus_ in the ecotonal region which made it unsuitable for _P.
maniculatus_.

Other mammals noted on the study areas were the following: _Didelphis
marsupialis_, _Blarina brevicauda_, _Scalopus aquaticus_, _Canis
familiaris_, _Canis latrans_, _Procyon lotor_, _Felis domesticus_,
_Sylvilagus floridanus_, _Microtus pinetorum_, _Mus musculus_ and _Zapus
hudsonius_.



SUMMARY AND CONCLUSIONS


In the 23-month period from October, 1950, to August, 1952, the ecology
of the prairie vole, _Microtus ochrogaster_, was investigated on the
Natural History Reservation of the University of Kansas. In all, 817
voles were captured 2941 times in 13,880 "live-trap days." For some
aspects of this study, Dr. Henry S. Fitch, resident investigator on the
Reservation, permitted the use of his trapping records. He had captured
1416 voles 5098 times. The total number of live voles used in the study
was thus 2233, and they were captured 8039 times. In addition to the
voles, I caught 96 cotton rats, 108 harvest mice, 29 wood mice, 2 pine
voles and 6 deer mice in live traps. When Fitch's records were used, the
live-trapping data covered a thirty-month period and general field data
were available from July, 1949, to August, 1952.

Hall and Cockrum (1953:406) stated that probably all microtine rodents
fluctuate markedly in numbers. Certainly the populations I studied did
so, but the fluctuations were not regularly recurring for _M.
ochrogaster_ as they seem to be for some species of the genus in more
northern life zones. The changes in the density of populations described
in this paper can be explained without recourse to cycles of long
time-span and literature dealing specifically with _M. ochrogaster_
makes no references to such cycles. There is, however, an annual cycle
of abundance: greatest density of population occurs in autumn, and the
least density in January.

This annual pattern is often, perhaps usually, obscured because of the
extreme sensitivity of voles to a variety of changes in their
environment. These changes are reflected as variations in reproductive
success. In this study, some of these changes were accentuated by the
great range in annual precipitation. Annual rainfall was approximately
average in 1950 (36.32 inches, 0.92 inches above normal), notably high
in 1951 (50.68 inches, 15.28 inches above normal) and notably low in
1952 (23.80 inches, 11.60 inches below normal).

Among the types of environmental modification to which the populations
of voles reacted were plant succession, an increase in competition with
_Sigmodon_, abnormal rainfall and concentration of predators. In the
overgrazed disclimax existing in 1948 when the study areas were
reserved, no voles were found because cover was insufficient. After the
area was protected a succession of good growing years hastened the
recovery of the grasses and the populations of voles reached high
levels. In areas where the vegetation approached the climax community,
the densities of voles decreased from the levels supported by the
immediately preceding seral stages. The higher carrying capacity of
these earlier seral stages was probably due to the greater variety of
herbaceous vegetation which tended to maintain a more constant supply of
young and growing parts of plants which were the preferred food of
voles. Later in the period of study the succession from grasses to woody
plants on parts of the study areas also affected the population of
voles. Not only did the voles withdraw from the advancing edge of the
forest, but their density decreased in the meadows as the number of
shrubs and other woody plants increased. These influences of the
succession of plants on the population density of voles were exerted
through changes in cover and in the quality, as well as the quantity, of
the food supply.

Whenever voles were in competition with cotton rats, there was a
depression in the population levels of voles. Primarily, the competition
between the two species is the result of an extensive coincidence of
food habits, but competition for space, cover and nesting material is
also present. There was one direct coaction between these two species
observed. Cotton rats, at least occasionally, ate voles, especially
young individuals. In extremely wet weather, as in the summer of 1951,
the high survival rate of newborn cotton rats resulted in an increase in
their detrimental effect on the population of voles. However, cotton
rats proved to be less well adapted to severe cold or drought than were
voles.

Heavy rainfall reduced the densities of populations of voles by killing
a large percentage of juveniles. During the summer of 1951 the
competition of cotton rats further depressed the population level of the
voles, but the relative importance of competition with cotton rats and
superabundant moisture in effecting the observed reduction in population
density is difficult to judge. Perhaps most of the decrease in
population which followed the heavy rains was due to competition rather
than to weather. Subnormal rainfall, as in 1952, reduced the population
of voles by inhibiting reproduction. Presumably because of an altered
food supply, reproduction almost ceased during the drought. Utilization
of the habitat was further reduced in the summer of 1952 because the
voles did not grow so large as they otherwise did.

Predation, as a general rule, does not significantly affect densities of
populations, but large numbers of predators concentrating on small areas
may rapidly reduce the numbers of prey animals. In the course of my
study, such a situation occurred but once, when a group of long-eared
owls roosted in the woods adjacent to Quarry Field. The population of
voles in that area was probably reduced somewhat as a result of
predation by owls.

Population trends in either direction may be reversed suddenly by
changes in the factors discussed above. In the fall of 1951, a downward
trend in the density of the voles was evident. At this time, populations
of cotton rats were increasing rapidly and competition between cotton
rats and voles was intensified. In February, 1952, the population of
cotton rats was decimated suddenly by a short period of unusually cold
weather. The voles were suddenly freed from the stress of competition
and the population immediately began to rise. The upward trend began
prior to the annual spring increase and was subsequently reinforced by
it. In the last part of May, 1952, the upward trend of the population
was reversed, as the drought became severe, and the density of the
population decreased rapidly. This drop was too sudden and too extreme
to be only the normal summer slump. The relatively rapid response of
voles to a heavy rain after a dry period, first by increased breeding
and later by increases in density, is one more example of abrupt changes
in population trends caused by altered environmental conditions.

In the population changes that I observed, no evident "die-off" of
adults accompanied even the most drastic reductions in population
density. The causative factor directly influences the population either
by inhibiting reproduction or by increasing infant and prenatal
mortality. The net reduction is due to an inadequate replacement of
those voles lost by normal attrition.

Most voles, under natural conditions, live less than one year. Those
individuals born in the autumn live longer, as a group, than those born
at any other time. Since the heaviest mortality is in young voles,
adults which become established in an area may live more than 18 months
and, if they are females, may produce more than a dozen litters. No
decrease in vigor and fertility was found to accompany aging. A
relationship between the condylobasilar length of the skull and the age
of a vole was discovered and, with further study, may yield a method of
aging voles more accurately than has been possible heretofore. Other
characteristics, varying with age, were described. The most reliable
indicator of age seemed to be the prominence of the temporal ridges.

Runway systems and burrows are used by groups of voles rather than by
individuals. Most of the activity of voles is confined to these runways
and an exposed individual is seldom seen. A home range may include
several runway systems, and the ranges of individuals overlap
extensively. Both home ranges and patterns of runway systems change
constantly. Runways seem to be primarily feeding trails, and are
extended or abandoned as the voles change their feeding habits. Groups
of adult voles using a system of runways seem to have no special
relationship. Juveniles tend to stay near their mothers, but as they
mature, they shift their ranges and are replaced by other individuals.
Males wander more than females, and shift their ranges more often. No
intolerance of other voles exists and, in laboratory cages, groups of
voles lived together peaceably from the time they are placed together.
Crowding does not seem to be harmful directly, therefore, and high
densities will develop if food and cover resources permit.

As a prey item, the prairie vole proved to be an important part of the
biota of the Reservation. It was eaten frequently by almost all of the
larger vertebrate predators on the Reservation and was, seemingly, the
most important food item of the long-eared owl. The ability of the
prairie vole to maintain high levels of population over relatively broad
areas enhances its value as a prey species.



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     Mgmt., 14:84-85.

MCATEE, W. L.

     1935. Food habits of common hawks. USDA Circ., 370:1-36.

MEYER, B. J., and R. K. MEYER.

     1944. Growth and reproduction of the cotton rat, _Sigmodon hispidus
     hispidus_, under laboratory conditions. Jour. Mamm., 25:107-129.

MOHR, C. O.

     1943. A comparison of North American small mammal censuses. Amer.
     Midland Nat., 29:545-587.

     1947. Table of equivalent populations of North American small
     mammals. Amer. Midland Nat., 37:223-249.

PHILLIPS, P.

     1936. The distribution of rodents in overgrazed and normal
     grassland in central Oklahoma. Ecol., 17:673-679.

POILEY, S. M.

     1949. Raising captive meadow voles (_Microtus p. pennsylvanicus_).
     Jour. Mamm., 30:317.

RINKER, G. C.

     1942a. Litter records of some mammals of Meade County, Kansas.
     Trans. Kansas Acad. Sci., 45:376-378.

     1942b. An extension of the range of the Texas cotton rat in Kansas.
     Jour. Mamm., 23:439.

SANDIDGE, L. L.

     1953. Food and dens of the opossum (_Didelphis virginiana_) in
     northeastern Kansas. Trans. Kansas Acad. Sci., 56:97-106.

SELLE, R. M.

     1928. _Microtus californicus_ in captivity. Jour. Mamm., 9:93-98.

SHAPIRO, J.

     1950. Notes on the population dynamics of _Microtus_ and _Blarina_
     with a record of albinism in _Blarina_. Jour. Wildlife Mgmt,
     14:359-360.

STICKEL, L. F.

     1946. Experimental analysis of methods of measuring small mammal
     populations. Jour. Wildlife Mgmt., 10:150-159.

     1948. The trap line as a measure of small mammal populations. Jour.
     Wildlife Mgmt., 12:153-161.

STRECKER, J. K.

     1929. Notes on the Texas cotton and Atwater wood rats. Jour. Mamm.,
     10:216-220.

SUMMERHAYES, V. S.

     1941. The effects of voles (_Microtus agrestis_) on vegetation.
     Jour. Ecol., 29:14-48.

SVIHLA, A.

     1929. Life history notes on _Sigmodon hispidus hispidus_. Jour.
     Mamm., 10:352-353.

TOWNSEND, M. T.

     1935. Studies on some small mammals of central New York. Roosevelt
     Wildlife Annals, 4:1-120.

UHLER, F. M., C. COTTAM, and T. E. CLARKE.

     1939. Food of the snakes of George Washington National Forest,
     Virginia. Trans. 4th N. A. Wildlife Conf., 605-622.

WOOSTER, L. D.

     1935. Notes on the effects of drought on animal populations in
     western Kansas. Trans. Kansas Acad. Sci., 38:351-352.

     1936. The contents of owl pellets as indicators of habitat
     preferences of small mammals. Trans. Kansas Acad. Sci., 39:395-397.

     1939. An ecological evaluation of predatees on a mixed prairie area
     in western Kansas. Trans. Kansas Acad. Sci., 42:515-517.


     _Transmitted May 19, 1955._



UNIVERSITY OF KANSAS PUBLICATIONS, MUSEUM OF NATURAL HISTORY


Institutional libraries interested in publications exchange may obtain
this series by addressing the Exchange Librarian, University of Kansas
Library, Lawrence, Kansas. Copies for individuals, persons working in a
particular field of study, may be obtained by addressing instead the
Museum of Natural History, University of Kansas, Lawrence, Kansas. There
is no provision for sale of this series by the University Library which
meets institutional requests, or by the Museum of Natural History which
meets the requests of individuals. However, when individuals request
copies from the Museum, 25 cents should be included, for each separate
number that is 100 pages or more in length, for the purpose of defraying
the costs of wrapping and mailing.

* An asterisk designates those numbers of which the Museum's supply (not
the Library's supply) is exhausted. Numbers published to date, in this
series, are as follows:

Vol. 1.

     Nos. 1-26 and index. Pp. 1-638, 1946-1950.

     Index. Pp. 605-638.

*Vol. 2.

     (Complete) Mammals of Washington. By Walter W. Dalquest. Pp. 1-444,
     140 figures in text. April 9, 1948.

Vol. 3.

     *1. The avifauna of Micronesia, its origin, evolution, and
     distribution. By Rollin H. Baker. Pp. 1-359, 16 figures in text.
     June 12, 1951.

     *2. A quantitative study of the nocturnal migration of birds. By
     George H. Lowery, Jr. Pp. 361-472, 47 figures in text. June 29,
     1951.

     3. Phylogeny of the waxwings and allied birds. By M. Dale Arvey.
     Pp. 473-530, 49 figures in text, 13 tables. October 10, 1951.

     4. Birds from the state of Veracruz, Mexico. By George H. Lowery,
     Jr., and Walter W. Dalquest. Pp. 531-649, 7 figures in text, 2
     tables. October 10, 1951.

     Index. Pp. 651-681.

*Vol. 4.

     (Complete) American weasels. By E. Raymond Hall. Pp. 1-466, 41
     plates, 31 figures in text. December 27, 1951.

Vol. 5.

     1. Preliminary survey of a Paleocene faunule from the Angels Peak
     area, New Mexico. By Robert W. Wilson. Pp. 1-11, 1 figure in text.
     February 24, 1951.

     2. Two new moles (Genus Scalopus) from Mexico and Texas. By Rollin
     H. Baker. Pp. 17-24. February 28, 1951.

     3. Two new pocket gophers from Wyoming and Colorado. By E. Raymond
     Hall and H. Gordon Montague. Pp. 25-32. February 28, 1951.

     4. Mammals obtained by Dr. Curt von Wedel from the barrier beach of
     Tamaulipas, Mexico. By E. Raymond Hall. Pp. 33-47, 1 figure in
     text. October 1, 1951.

     5. Comments on the taxonomy and geographic distribution of some
     North American rabbits. By E. Raymond Hall and Keith R. Kelson. Pp.
     49-58. October 1, 1951.

     6. Two new subspecies of Thomomys bottae from New Mexico and
     Colorado. By Keith R. Kelson. Pp. 59-71, 1 figure in text. October
     1, 1951.

     7. A new subspecies of Microtus montanus from Montana and comments
     on Microtus canicaudus Miller. By E. Raymond Hall and Keith R.
     Kelson. Pp. 73-79. October 1, 1951.

     8. A new pocket gopher (Genus Thomomys) from eastern Colorado. By
     E. Raymond Hall. Pp. 81-85. October 1, 1951.

     9. Mammals taken along the Alaskan Highway. By Rollin H. Baker. Pp.
     87-117, 1 figure in text. November 28, 1951.

     *10. A synopsis of the North American Lagomorpha. By E. Raymond
     Hall. Pp. 119-202, 68 figures in text. December 15, 1951.

     11. A new pocket mouse (Genus Perognathus) from Kansas. By E.
     Lendell Cockrum. Pp. 203-206. December 15, 1951.

     12. Mammals from Tamaulipas, Mexico. By Rollin H. Baker. Pp.
     207-218. December 15, 1951.

     13. A new pocket gopher (Genus Thomomys) from Wyoming and Colorado.
     By E. Raymond Hall. Pp. 219-222. December 15, 1951.

     14. A new name for the Mexican red bat. By E. Raymond Hall. Pp.
     223-226. December 15, 1951.

     15. Taxonomic notes on Mexican bats of the Genus Rhogeëssa. By E.
     Raymond Hall. Pp. 227-232. April 10, 1952.

     16. Comments on the taxonomy and geographic distribution of some
     North American woodrats (Genus Neotoma). By Keith R. Kelson. Pp.
     233-242. April 10, 1952.

     17. The subspecies of the Mexican red-bellied squirrel, Sciurus
     aureogaster. By Keith R. Kelson. Pp. 243-250, 1 figure in text.
     April 10, 1952.

     18. Geographic range of Peromyscus melanophrys, with description of
     new subspecies. By Rollin H. Baker. Pp. 251-258, 1 figure in text.
     May 10, 1952.

     19. A new chipmunk (Genus Eutamias) from the Black Hills. By John
     A. White. Pp. 259-262. April 10, 1952.

     20. A new piñon mouse (Peromyscus truei) from Durango, Mexico. By
     Robert B. Finley, Jr. Pp. 263-267. May 23, 1952.

     21. An annotated checklist of Nebraskan bats. By Olin L. Webb and
     J. Knox Jones, Jr. Pp. 269-279. May 31, 1952.

     22. Geographic variation in red-backed mice (Genus Clethrionomys)
     of the southern Rocky Mountain region. By E. Lendell Cockrum and
     Kenneth L. Fitch. Pp. 281-292, 1 figure in text. November 15, 1952.

     23. Comments on the taxonomy and geographic distribution of North
     American microtines. By E. Raymond Hall and E. Lendell Cockrum. Pp.
     293-312. November 17, 1952.

     24. The subspecific status of two Central American sloths. By E.
     Raymond Hall and Keith R. Kelson. Pp. 313-317. November 21, 1952.

     25. Comments on the taxonomy and geographic distribution of some
     North American marsupials, insectivores, and carnivores. By E.
     Raymond Hall and Keith R. Kelson. Pp. 319-341. December 5, 1952.

     26. Comments on the taxonomy and geographic distribution of some
     North American rodents. By E. Raymond Hall and Keith R. Kelson. Pp.
     343-371. December 15, 1952.

     27. A synopsis of the North American microtine rodents. By E.
     Raymond Hall and E. Lendell Cockrum. Pp. 373-498, 149 figures in
     text. January 15, 1953.

     28. The pocket gophers (Genus Thomomys) of Coahuila, Mexico. By
     Rollin H. Baker. Pp. 499-514, 1 figure in text. June 1, 1953.

     29. Geographic distribution of the pocket mouse, Perognathus
     fasciatus. By J. Knox Jones, Jr. Pp. 515-526, 7 figures in text.
     August 1, 1953.

     30. A new subspecies of wood rat (Neotoma mexicana) from Colorado.
     By Robert B. Finley, Jr. Pp. 527-534, 2 figures in text. August 15,
     1953.

     31. Four new pocket gophers of the genus Cratogeomys from Jalisco,
     Mexico. By Robert J. Russell. Pp. 535-542. October 15, 1953.

     32. Genera and subgenera of chipmunks. By John A. White. Pp.
     543-561, 12 figures in text. December 1, 1953.

     33. Taxonomy of the chipmunks, Eutamias quadrivittatus and Eutamias
     umbrinus. By John A. White. Pp. 563-582, 6 figures in text.
     December 1, 1953.

     34. Geographic distribution and taxonomy of the chipmunks of
     Wyoming. By John A. White. Pp. 584-610, 3 figures in text. December
     1, 1953.

     35. The baculum of the chipmunks of western North America. By John
     A. White. Pp. 611-631, 19 figures in text. December 1, 1953.

     36. Pleistocene Soricidae from San Josecito Cave, Nuevo Leon,
     Mexico. By James S. Findley. Pp. 633-639. December 1, 1953.

     37. Seventeen species of bats recorded from Barro Colorado Island,
     Panama Canal Zone. By E. Raymond Hall and William B. Jackson. Pp.
     641-646. December 1, 1953.

     Index. Pp. 647-676.

*Vol. 6.

     (Complete) Mammals of Utah, _taxonomy and distribution_. By Stephen
     D. Durrant. Pp. 1-549, 91 figures in text, 30 tables. August 10,
     1952.

Vol. 7.

     *1. Mammals of Kansas. By E. Lendell Cockrum. Pp. 1-303, 73 figures
     in text, 37 tables. August 25, 1952.

     2. Ecology of the opossum on a natural area in northeastern Kansas.
     By Henry S. Fitch and Lewis L. Sandidge. Pp. 305-338, 5 figures in
     text. August 24, 1953.

     3. The silky pocket mice (Perognathus flavus) of Mexico. By Rollin
     H. Baker. Pp. 339-347, 1 figure in text. February 15, 1954.

     4. North American jumping mice (Genus Zapus). By Philip H.
     Krutzsch. Pp. 349-472, 47 figures in text, 4 tables. April 21,
     1954.

     5. Mammals from Southeastern Alaska. By Rollin H. Baker and James
     S. Findley. Pp. 473-477. April 21, 1954.

     6. Distribution of some Nebraskan Mammals. By J. Knox Jones, Jr.
     Pp. 479-487. April 21, 1954.

     7. Subspeciation in the montane meadow mouse, Microtus montanus, in
     Wyoming and Colorado. By Sydney Anderson. Pp. 489-506, 2 figures in
     text. July 23, 1954.

     8. A new subspecies of bat (Myotis velifer) from southeastern
     California and Arizona. By Terry A. Vaughn. Pp. 507-512. July 23,
     1954.

     9. Mammals of the San Gabriel mountains of California. By Terry A.
     Vaughn. Pp. 513-582, 1 figure in text, 12 tables. November 15,
     1954.

     10. A new bat (Genus Pipistrellus) from northeastern Mexico. By
     Rollin H. Baker. Pp. 583-586. November 15, 1954.

     11. A new subspecies of pocket mouse from Kansas. By E. Raymond
     Hall. Pp. 587-590. November 15, 1954.

     12. Geographic variation in the pocket gopher, Cratogeomys
     castanops, in Coahuila, Mexico. By Robert J. Russell and Rollin H.
     Baker. Pp. 591-608. March 15, 1955.

     13. A new cottontail (Sylvilagus floridanus) from northeastern
     Mexico. By Rollin H. Baker. Pp. 609-612. April 8, 1955.

     14. Taxonomy and distribution of some American shrews. By James S.
     Findley. Pp. 613-618. June 10, 1955.

     15. Distribution and systematic position of the pigmy woodrat,
     Neotoma goldmani. By Dennis G. Rainey and Rollin H. Baker. Pp.
     619-624, 2 figs. in text. June 10, 1955.

     Index. Pp. 625-651.

Vol. 8.

     1. Life history and ecology of the five-lined skink, Eumeces
     fasciatus. By Henry S. Fitch. Pp. 1-156, 2 pls., 26 figs. in text,
     17 tables. September 1, 1954.

     2. Myology and serology of the Avian Family Fringillidae, a
     taxonomic study. By William B. Stallcup. Pp. 157-211, 23 figures in
     text, 4 tables. November 15, 1954.

     3. An ecological study of the collared lizard (Crotaphytus
     collaris). By Henry S. Fitch. Pp. 213-274, 10 figures in text.
     February 10, 1956.

     4. A field study of the Kansas ant-eating frog, Gastrophryne
     olivacea. By Henry S. Fitch. Pp. 275-306, 9 figures in text.
     February 10, 1956.

     5. Check-list of the birds of Kansas. By Harrison B. Tordoff. Pp.
     307-359, 1 figure in text. March 10, 1956.

     6. A population study of the prairie vole (Microtus ochrogaster) in
     Northeastern Kansas. By Edwin P. Martin. Pp. 361-416, 19 figures in
     text. April 2, 1956.

     More numbers will appear in volume 8.

Vol. 9.

     1. Speciation of the wandering shrew. By James S. Findley. Pp.
     1-68, 18 figures in text. December 10, 1955.

     2. Additional records and extensions of ranges of mammals from
     Utah. By Stephen D. Durrant, M. Raymond Lee, and Richard M. Hansen.
     Pp. 69-80. December 10, 1955.

     3. A new long-eared myotis (Myotis evotis) from northeastern
     Mexico. By Rollin H. Baker and Howard J. Stains. Pp. 81-84.
     December 10, 1955.

     More numbers will appear in volume 9.



       *       *       *       *       *



Transcriber's note:

A Table of Contents has been added to this ebook for the reader's
convenience.

Some words in this text are found in both hyphenated and non-hyphenated
form (for instance: Condylo-basilar/condylobasilar,
mid-winter/midwinter). These variations match the text of the original
document. A few obvious punctuation errors have been repaired. Spelling
has been retained as it appears in the original publication, except as
follows:

  p. 372, in "A more homogeneous vegetation would tend to pass" homogenous
  has been changed to homogeneous.

  p. 415, "1953. Foods, and dens of the opossum ..." has been changed to
  "1953. Food and dens of the opossum ..."

In Fig. 11 the bottommost y-axis label in the scale of gms. is probably
an error: 45 should be 35.

Some illustrations have been moved from their original locations to
paragraph breaks, so as to be nearer to their corresponding text, and
for ease of document navigation. References to scale in illustration
captions are those of the original publication, and therefore do not
correspond to the scale of the images in the HTML version of this ebook.

The list of University of Kansas Publications from the front of the
original document has been joined to its mate at the end of this text.

Because the cover of the original document contained text exactly
duplicated on the title page, this cover information has been omitted.





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