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Title: Life History of the Kangaroo Rat
Author: Vorhies, Charles Taylor, 1879-1949, Taylor, Walter P. (Walter Penn), 1888-1972
Language: English
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UNITED STATES DEPARTMENT OF AGRICULTURE
BULLETIN No. 1091
Also Technical Bulletin No. 1 of the Agricultural Experiment Station
University of Arizona
Washington, D. C. PROFESSIONAL PAPER September 13, 1922
LIFE HISTORY OF THE KANGAROO RAT
_Dipodomys spectabilis spectabilis_ Merriam
BY
CHARLES T. VORHIES, Entomologist
Agricultural Experiment Station, University of Arizona; and
WALTER P. TAYLOR, Assistant Biologist
Bureau of Biological Survey, U. S. Department
of Agriculture
CONTENTS
Importance of Rodent Groups 1
Identification 3
Description 5
Occurrence 7
Habits 9
Food and Storage 18
Burrow Systems, or Dens 28
Commensals and Enemies 33
Abundance 36
Economic Considerations 36
Summary 38
Bibliography 40
[Illustration]
WASHINGTON
GOVERNMENT PRINTING OFFICE
1922
[Illustration: PLATE I.--Banner-tailed Kangaroo Rat (Dipodomys spectabilis
spectabilis Merriam).
From _Dipodomys merriami_ Mearns and subspecies, which occur over much
of its range, this form is easily distinguished by its larger size and
the conspicuous white brush on the tail.]
UNITED STATES DEPARTMENT OF AGRICULTURE
BULLETIN No. 1091
Also Technical Bulletin No. 1 of the
Agricultural Experiment Station, University of Arizona
Washington, D. C. PROFESSIONAL PAPER September, 1922
LIFE HISTORY OF THE KANGAROO RAT,
_Dipodomys spectabilis spectabilis_ Merriam.
By CHARLES T. VORHIES, _Entomologist, Agricultural Experiment Station,
University of Arizona_; and WALTER P. TAYLOR, _Assistant Biologist,
Bureau of Biological Survey, U. S. Department of Agriculture_.
CONTENTS.
Page
Importance of rodent groups 1
Investigational methods 2
Identification 3
Description 5
General characters 5
Color 6
Oil gland 6
Measurements and weights 7
Occurrence 7
General distribution 7
Habitat 7
Habits 9
Evidence of presence 9
Mounds 9
Runways and tracks 10
Signals 11
Voice 12
Daily and seasonal activity 12
Pugnacity and sociability 13
Sense developments 14
Movements and attitudes 15
Storing habits 15
Breeding habits 16
Food and storage 18
Burrow systems, or dens 28
Commensals and enemies 33
Commensals 33
Natural checks 34
Parasites 35
Abundance 36
Economic considerations 36
Control 37
Summary 38
Bibliography 40
NOTE.--This bulletin, a joint contribution of the Bureau of Biological
Survey and the Arizona Agricultural Experiment Station, contains a
summary of the results of investigations of the relation of a subspecies
of kangaroo rat to the carrying capacity of the open ranges, being one
phase of a general study of the life histories of rodent groups as they
affect agriculture, forestry, and grazing.
IMPORTANCE OF RODENT GROUPS.
As the serious character of the depredations by harmful rodents is
recognized, State, Federal, and private expenditures for their control
increase year by year. These depredations include not only the attacks
by introduced rats and mice on food materials stored in granaries,
warehouses, commercial establishments, docks, and private houses, but
also, particularly in the Western States, the ravages of several groups
of native ground squirrels and other noxious rodents in grain and
certain other field crops. Nor is this all, for it has been found that
such rodents as prairie dogs, pocket gophers, marmots, ground squirrels,
and rabbits take appreciable and serious toll of the forage on the open
grazing range; in fact, that they reduce the carrying capacity of the
range to such an extent that expenditures for control measures are amply
justified. Current estimates place the loss of goods due to rats and
mice in warehouses and stores throughout the United States at no less
than $200,000,000 annually, and damage to the carrying capacity of the
open range and to cultivated crops generally by native rodents in the
Western States at $300,000,000 additional; added together, we have an
impressive total from depredations of rodents.
The distribution and life habits of rodents and the general
consideration of their relation to agriculture, forestry, and grazing,
with special reference to the carrying capacity of stock ranges, is a
subject that has received attention for many years from the Biological
Survey of the United States Department of Agriculture. As a result of
the investigations conducted much has been learned concerning the
economic status of most of the more important groups, and the knowledge
already gained forms the basis of the extensive rodent-control work
already in progress, and in which many States are cooperating with the
bureau. If the work is to be prosecuted intelligently and the fullest
measure of success achieved, it is essential that the consideration
largely of groups as a whole be supplemented by more exhaustive
treatment of the life histories of individual species and of their place
in the biological complex. The present report is based upon
investigations, chiefly in Arizona, of the life history, habits, and
economic status of the banner-tailed kangaroo rat, _Dipodomys
spectabilis spectabilis_ Merriam (Pl. I).
INVESTIGATIONAL METHODS.
Some 18 years ago (in 1903) a tract of land 49.2 square miles in area on
the Coronado National Forest near the Santa Rita Mountains, Pima County,
southern Arizona, was closed to grazing by arrangement between the
Forest Service and the Agricultural Experiment Station of the University
of Arizona. Since that time another small tract of nearly a section has
been inclosed (Griffiths, 1910, 7[1]). This total area of approximately
50 square miles is known as the United States Range Reserve, and is
being devoted to a study of grazing conditions in this section and to
working out the best methods of administering the range (Pl. II, Fig.
1).
For some years an intensive study of the forage and other vegetative
conditions of this area has been made, the permanent vegetation quadrat,
as proposed by Dr. F. E. Clements (1905, 161-175), being largely
utilized. During the autumn of 1917 representatives of the Carnegie
Institution and the Arizona Agricultural Experiment Station visited the
Reserve and were impressed with the evidence of rodent damage to the
grass cover. The most conspicuous appearance of damage was noted about
the habitations of the banner-tailed kangaroo rat (_Dipodomys
spectabilis spectabilis_ Merriam), although it was observed also that
jack rabbits of two species (_Lepus californicus eremicus_ Allen and _L.
alleni alleni_ Mearns), which were very abundant in some portions of the
reserve, were apparently affecting adversely the forage conditions in
particular localities. Accordingly, the Biological Survey, the
Agricultural Experiment Station of the University of Arizona, the
Carnegie Institution of Washington, and the U. S. Forest Service have
undertaken a study of the relation of the more important rodents to the
forage crop of the Range Reserve in Arizona.
The present paper is a first step in this larger investigation.[2] In
this work the authors have made no attempt to deal with the taxonomic
side of the kangaroo rat problem. It is not unlikely that intensive
studies will show that the form now known as _Dipodomys spectabilis
spectabilis_ is made up of a number of local variants, some of them
perhaps worthy of recognition as additional subspecies. But it is felt
that the conclusions here reached will be little, if at all, affected by
such developments.
Color descriptions are based on Ridgway's Color Standards and Color
Nomenclature published in 1912.
[Footnote 1: References in parentheses are to the Bibliography, p.
40 (the last figure being to the page of the publication).
References to authorities where no citation of literature is
appended relate for the most part to manuscript notes in the files
of the Biological Survey or the University of Arizona Agricultural
Experiment Station.]
[Footnote 2: In addition to assistance rendered by officials of the
Biological Survey and the University of Arizona, which is hereby
acknowledged, the authors are indebted to the following persons for
helpful suggestions and assistance: G. S. Miller and J. W. Gidley,
of the U. S. National Museum; Dr. Frederic E. Clements and Gorm
Loftfield, of the Carnegie Institution; Morgan Hebard, of the
Academy of Natural Sciences of Philadelphia; James T. Jardine and
R. L. Hensel, both formerly connected with the U. S. Forest Service;
and R. R. Hill, of the Forest Service. They are also indebted to
William Nicholson, of Continental, Ariz., for many courtesies
extended in connection with work on the Reserve.]
IDENTIFICATION.
There are only three groups of mammals in the Southwest having external
cheek pouches. These are (_a_) the pocket gophers (Geomyidæ), which have
strong fore feet, relatively weak hind feet, and short tail, as compared
with weak fore feet, relatively strong hind feet, and long tail in the
other two; (_b_) the pocket mice (_Perognathus_), which are considerably
smaller than the kangaroo rats and lack the conspicuous white hip
stripe possessed by all the latter; and (_c_) the kangaroo rats
(_Dipodomys_).
[Illustration: FIG. 1.--Range, east of the Colorado River, of _Dipodomys
spectabilis spectabilis_ compared with that of _Dipodomys merriami_.
Cross hatching indicates area of overlapping of the two forms. The range
of _Dipodomys deserti_, not shown on the map, is west of that of
_spectabilis_, and so far as known the two do not overlap.]
_Dipodomys spectabilis spectabilis_ Merriam requires comparison with
three other forms of kangaroo rats in the same general region, namely,
_D. deserti_ Stephens, of approximately the same size, and _D. merriami_
Mearns and _D. ordii_ Woodhouse, the last two of decidedly smaller size.
The range of _deserti_ lies principally to the west of that of
_spectabilis_, and the two do not, so far as known, overlap. On the
other hand, _merriami_ and _ordii_, and subspecies, occur over a large
part of the range of _spectabilis_, living in very close proximity to
its burrows; _merriami_ is even suspected of pillaging the stores of
_spectabilis_. The range of _merriami_, however, is much more extensive
than that of _spectabilis_ (Fig. 1), which argues against a definite
ecological dependence or relationship. Separation of the four forms
mentioned may be easily accomplished by the following key:
_Key to Species of_ Dipodomys _in Arizona._
_a^1_. Size much larger (hind foot and greatest length of skull more
than 42 millimeters); tail tipped with white.
_b^1_. Upper parts dark brownish buffy; tail dark brownish or
blackish with more sharply contrasted white tip; interparietal
broader, distinctly separating mastoids (range in Arizona mainly
southeastern part) =Dipodomys spectabilis.=
_b^2_. Upper parts light ochraceous-buffy; tail pale brownish with
less sharply contrasted white tip; interparietal narrower, reduced
to mere spicule between mastoids (range in Arizona mainly
southwestern part) =Dipodomys deserti.=
_a^2_. Size much smaller (hind foot and greatest length of skull less
than 42 millimeters); tail not tipped with white.
_b^1_. Hind foot with four toes =Dipodomys merriami.=
_b^2_. Hind foot with five toes =Dipodomys ordii.=
On account of the small size, _merriami_ and _ordii_ do not require
detailed color comparison with the other two. The general color of the
upperparts of _spectabilis_ is much darker than that of _deserti_;
whereas _spectabilis_ is ochraceous-buff or light ochraceous-buff
grizzled with blackish, _deserti_ is near pale ochraceous-buff and lacks
the blackish.
The color of the upperparts alone amply suffices to distinguish
_spectabilis_ and _deserti_; but the different coloration of the tail is
the most obvious diagnostic feature. The near black of the middle
portion of the tail, the conspicuous white side stripes, and the pure
white tip make the tail of _spectabilis_ stand in rather vivid contrast
to the pale-brown and whitish tail of _deserti_.
The dens of the two larger species of _Dipodomys_--_spectabilis_ and
_deserti_--can be distinguished at a glance from those of the two
smaller--_merriami_ and _ordii_--by the fact that the mounds of the
former are usually of considerable size and the burrow mouths are of
greater diameter. On the Range Reserve _merriami_ erects no mounds, but
excavates its burrows in the open or at the base of _Prosopis_,
_Lycium_, or other brush. The mounds of _spectabilis_ are higher than
those of _deserti_, the entrances are larger, and they are located in
harder soil (Pl. III, Fig. 1). The dens of _deserti_ are usually more
extensive in surface area than those of _spectabilis_, and have a
greater number of openings (Pl. III, Fig. 2).
[Illustration: PLATE II. FIG. 1.--WINTER VIEW OF AREA INHABITED BY
KANGAROO RATS.
A water-hole scene on the U. S. Range Reserve at the base of the Santa
Rita Mountains, Ariz., where cooperative investigations are being
conducted to ascertain the relation of rodents to forage. This is
typical of a large section of country occupied by _Dipodomys spectabilis
spectabilis_ and _Dipodomys merriami_. The brush is mesquite
(_Prosopis_), cat's-claw (_Acacia_), and paloverde (_Cercidium_).]
[Illustration: PLATE II. FIG. 2.--KANGAROO RAT COUNTRY FOLLOWING SUMMER
DROUGHT.
An area of the U. S. Range Reserve in the autumn of 1918, showing the
result of failure of summer rains. Such a condition is critical not only
for the stockmen but also for kangaroo rats and other desert rodents,
and results in competition between them as to which shall benefit by
what the range has to offer.]
[Illustration: PLATE III. FIG. 1.--KANGAROO RAT MOUND (DIPODOMYS S.
SPECTABILIS).
Typical _Dipodomys s. spectabilis_ mound on the Range Reserve, under
shelter of desert hackberry (_Celtis pallida_). Most dens on the reserve
are located in the shelter of brush plants, the more important being
mesquite (_Prosopis velutina_), cat's-claw (_Acacia_ spp.), and the
desert hackberry. (See also Pl. VIII Fig. 2.)]
[Illustration: PLATE III. FIG. 2.--KANGAROO RAT MOUND (DIPODOMYS DESERTI).
Den of _Dipodomys deserti deserti_, showing typical wide, low mound with
numerous entrance holes. This species excavates its den in soft, sandy
soil. The tree is a species of _Dalea_.]
DESCRIPTION.
GENERAL CHARACTERS.
Size large; ears moderate, ear from crown (taken in dry skin) 9 or 10
millimeters; eyes prominent; whiskers long and sensitive; fore feet
short and weak; hind feet long and powerful, provided with four
well-developed toes; tail very long, usually 30 to 40 per cent longer
than the body. Cranium triangular, the occiput forming the base and the
point of the nose the apex of the triangle, much flattened, auditory and
particularly mastoid bullae conspicuously inflated.
COLOR.
General color above, brownish buffy, varying in some specimens to
lighter buffy tints, grizzled with black; oblique hip stripes white;
tail with dark-brown or blackish stripes above and below, running into
blackish about halfway between base and tip, and with two lateral side
stripes of white to a point about halfway back; tail tipped with pure
white for about 40 millimeters (Pl. I). Underparts white, hairs white to
bases, with some plumbeous and buffy hairs about base of tail; fore legs
and fore feet white all around; hind legs like back, brown above, hairs
with gray bases, becoming blackish (fuscous-black or chætura-black)
about ankles, hairs on under side white to bases; hind feet white above,
dark-brown or blackish (near fuscous) below.
Color variations in a series of 12 specimens from the type locality and
points widely scattered through the range of _spectabilis_ consist in
minor modifications of the degree of coloration, length of white tip of
tail, and length of white lateral tail stripes. In general the color
pattern and characters are remarkably uniform. Young specimens, while
exhibiting the color pattern and general color of adults, are
conspicuously less brown, and more grayish.
There appears to be little variation in color with season. In the series
at hand, most specimens taken during the fall, winter, and spring are
very slightly browner than those of summer, suggesting that the fresh
pelage following the fall molt is a little brighter than is the pelage
after being worn all winter and into the following summer. But at most
the difference is slight.
OIL GLAND.
Upon separating the hairs of the middle region of the back about a third
of the distance between the ears and the rump, one uncovers a prominent
gland, elliptical in outline, with long axis longitudinal and about 9
millimeters in length. The gland presents a roughened and granular
appearance, and fewer hairs grow upon it than elsewhere on the back. The
hairs in the vicinity are frequently matted, as if with a secretion. In
worn stage of pelage the gland may be visible from above without
separating the hairs. Bailey has suggested that this functions as an oil
gland for dressing the fur, and our observations bear out this view.
Kangaroo rats kept in captivity without earth or sand soon come to have
a bedraggled appearance, as if the pelage were moist. When supplied with
fine, dusty sand, they soon recover their normal sleek appearance.
Apparently the former condition is due to an excess of oil, the latter
to the absorption of the excess in a dust bath. The oil is doubtless an
important adjunct to the preservation of the skin and hair amid the
dusty surroundings in which the animal lives.
MEASUREMENTS AND WEIGHTS.
External measurements include: _Total length_, from tip of nose to tip
of tail without hairs, measured before skinning; _tail vertebræ_, length
of tail from point in angle when tail is bent at right angles to body to
tip of tail without hairs; and _hind foot_, from heel to tip of longest
claw.
The following are measurements of a series from the U. S. Range Reserve:
[Transcriber note: Next line was corrected per erratum. The
original text was of the following paragraph (_Averages for 17
adult females: Total length, 326.4 millimeters_).]
Average measurements of 30 adult specimens of both sexes: Total length,
326.2 millimeters (349-310); tail vertebræ, 188.4 (208-180); hind foot,
49.5 (51-47); the average weight of 29 adult specimens of both sexes was
114.5 grams (131.9-98.0).
Averages for 17 adult females: Total length, 326.4 millimeters
(349-310); tail vertebræ, 188.8 (208-179); weight (16 individuals),
113.7 (131.9-98.0); excluding pregnant females, 13 individuals averaged
112.9 grams (131.9-98.0).
Averages for 13 adult males: Total length, 326 millimeters (345-311);
tail vertebræ, 187.8 (202-168); weight, 116.8 grams (129-100).
There appears to be no significant difference in the measurements and
weights of males and females, with the possible exception of the
comparison of adult males and adult nonpregnant females.
OCCURRENCE.
GENERAL DISTRIBUTION.
_Dipodomys spectabilis spectabilis_ is found in southeastern Arizona, in
northwestern, central, and southern New Mexico, in extreme western
Texas, in northern Sonora, and in northern and central Chihuahua (Fig.
1). A subspecies, _D. s. cratodon_ Merriam, has been described from
Chicalote, Aguas Calientes, Mexico, the geographic range of which lies
in central Mexico in portions of the States of Zacatecas, San Luis
Potosi, and Aguas Calientes.
HABITAT.
In the Tucson region _spectabilis_ is typically a resident of the Lower
Sonoran Zone. This is perhaps the principal zone inhabited over its
entire range, but the animal is often found in the Upper Sonoran also,
and in the Gallina Mountains of New Mexico Hollister found it invading
the yellow pine Transition where the soil was dry and sandy and the pine
woods of open character. The same observer found it common in grassy and
weed-grown parks among the large junipers, pinyons, and scattering
yellow pines of the Bear Spring Mountains, N. Mex. Bailey calls
attention to the fact that the animal apparently does not inhabit the
lower half of the Lower Sonoran Zone, as it extends neither into the Rio
Grande Valley of Texas nor the Gila Valley of Arizona. In extreme
western Texas it is common at the upper edge of the arid Lower Sonoran
Zone, and in this region does not enter the Upper Sonoran to any extent.
In July, 1914, Goldman found this kangaroo rat common on the plain at
4,600 feet altitude, near Bonita, Graham County, Ariz., and noted a few
as high as 5,000 feet altitude on the warm southwestern slopes of the
Graham Mountains, near Fort Grant. Apparently _spectabilis_ reaches its
upper altitude limit in the Burro Mountains, N. Mex., where Bailey has
found it sparingly on warm slopes up to 5,700 feet, and at the western
base of the Sandia Mountains, east of Albuquerque, N. Mex., where dens
occur at approximately 6,000 feet.
About Tucson it is undoubtedly more common in the somewhat higher
portions of the Lower Sonoran Zone, above the _Covillea_ association,
than elsewhere (Pl. IV, Figs. 1 and 2). A few scattered dens are to be
seen in the _Covillea_ belt, but as one rises to altitudes of 3,500 to
4,000 feet, and the _Covillea_ is replaced by the cat's-claws (_Acacia_
sp. and _Mimosa_ sp.) and scattered mesquite (_Prosopis_), with the
_Opuntia_ becoming less abundant, kangaroo rat mounds come more and more
in evidence. Here is to be found the principal grass growth supporting
the grazing industry, and the presence of a more luxuriant grass flora
is probably an important factor in the greater abundance of kangaroo
rats, both _spectabilis_ and _merriami_. In this generally preferred
environment the desert hackberry (_Celtis pallida_) is one of the most
conspicuous shrubs; clumps of this species are commonly accompanied by
kangaroo rat mounds.
In order to ascertain whether the banner-tailed kangaroo rat has any
marked preference for building its mounds under _Celtis_ or some other
particular plant, all the observable mounds were counted in a strip
about 20 rods wide and approximately 4 miles long, an area of
approximately 160 acres, particular note being taken of the kind of
shrub under which each mound was located. Of 300 mounds in this area, 96
were under _Prosopis_, 95 under _Acacia_, 65 under _Celtis_, 11 under
_Lycium_, 31 in the open, 1 about a "cholla" cactus (_Opuntia
spinosior_), and 1 about a prickly pear (_Opuntia_ sp.). There is
apparently no strongly marked preference for any single species of
plant. While both desert hackberry and the cat's-claws afford a
better protection than mesquite--since cattle more often seek shade
under the latter, and in so doing frequently trample the mounds
severely--it appears that the general protection of a tree or shrub of
some sort is sought by kangaroo rats, rather than the specific
protection of the thickest or thorniest species.
The following records indicate particular habitat preferences of
_spectabilis_ as noted at different points in its range:
Occurs on open bare knolls exposed to winds, also on gravelly
places at lower edge of foothills (Franklin Mountains, Tex., Gaut);
here and there over the barest and hardest of the gravelly mesas
(Bailey, Tex., 1905, 147); on open creosote-bush and giant-cactus
desert (Tucson, Ariz., Vorhies and Taylor); on firm, gravelly, or
even rocky soil on the grassy bajada land along the northwest base
of the mountains, either in the open or under _Celtis_, _Prosopis_,
_Lycium_, _Acacia greggii_, or other brush (Santa Rita Mountains,
Ariz., Vorhies and Taylor); mounds usually thrown up around a bunch
of cactus or mesquite brush (Magdalena, Sonora, Bailey); in heavy
soil (Ajo, Ariz., A. B. Howell); loamy soil (Gunsight, Ariz., A. B.
Howell); in mesa where not too stony (Magdalena, Sonora, Bailey);
grassy plain (Gallego, Chihuahua, Nelson); in open valley and high
open plains (Santa Rosa, N. Mex., Bailey); in grassy and weed-grown
parks among the larger junipers, pinyons, and scattering yellow
pines (Bear Spring Mountains, N. Mex., Hollister); on sand-dune
strip (east side of Pecos River, 15 miles northeast of Roswell, N.
Mex., Bailey); among _Ephedra_ patches (San Juan Valley, N. Mex.,
Birdseye); in open sandy soil along dry wash (Rio Alamosa, N. Mex.,
Goldman); on sides and crests of bare, stony hills (Mesa Jumanes,
N. Mex., Gaut); in open, arid part of the valley and stony mesas
(Carlsbad and Pecos Valley, N. Mex., Bailey); about the edges of
the plains of San Augustine and the foothills of the Datil and
Gallina Mountains, and in the Transition Zone yellow-pine forest of
the Gallina Mountains (Datil region, N. Mex., Hollister); on hard
limy ridges (Monahans, Tex., Cary).
A. Brazier Howell notes that _spectabilis_ occurs in harder soil than
does _deserti_. This observation is confirmed by others, and seems to
afford a conspicuous habitat difference between the two, for _deserti_
is typically an animal of the shifting aeolian sands.
Usually, as on the Range Reserve, the rodents are widely distributed
over a considerable area. Occasionally, as in the vicinity of Rio
Alamosa, N. Mex., as reported by Goldman, they occur only in small
colonies.
[Illustration: PLATE IV. FIG. 1.--RANGE CONDITIONS FAVORING KANGAROO RATS.
View on higher portion of Range Reserve, showing type of country where
_Dipodomys s. spectabilis_ is most abundant. Good growth of grama and
needle grasses in October, following summer growth and before grazing
off by cattle and rodents.]
[Illustration: PLATE IV. FIG. 2.--RANGE CONDITIONS LESS FAVORABLE TO
KANGAROO RATS.
View on lower portion of Range Reserve, where _Dipodomys s. spectabilis_
is less abundant. Vegetation consists principally of _Lycium_, mesquite,
rabbit brush, and cactus, there being very little grass.]
HABITS.
EVIDENCE OF PRESENCE.
MOUNDS.
One traveling over territory thickly occupied by the banner-tailed
kangaroo rat is certain to note the numerous and conspicuous mounds so
characteristic of the species, particularly if the region is of the
savannah type, grassy rather than brushy. These low, rounded mounds
occupy an area of several feet in diameter, and rise to varying heights
above the general surface of the surrounding soil, the height depending
rather more upon the character of the soil and the location of the mound
as to exposure or protection than upon the area occupied by the burrow
system which lies within and is the reason for the mound.
A den in sandy soil in the open may be of maximum size in area occupied
and yet scarcely present the appearance of a mound in any sense, due
probably both to the fact that the sandy soil will not heap up to such a
height over a honeycomb of tunnels as will a firmer or rocky soil, and
also to its greater exposure to the leveling action of rains and the
trampling of animals. These mounds are in themselves large enough to
attract some attention, but their conspicuousness is enhanced by the
fact that they are more or less completely denuded of vegetation and are
the centers of cleared areas often as much as 30 feet in diameter (Pl.
V, Fig. 1); and further that from 3 to 12 large dark openings loom up in
every mound. The larger openings are of such size as to suggest the
presence of a much larger animal than actually inhabits the mound. Add
to the above the fact that the traveler by day never sees the mound
builder, and we have the chief reasons why curiosity is so often aroused
by these habitations.
On the Range Reserve the mounds are usually rendered conspicuous by the
absence of small vegetation, but Nelson writes that in the vicinity of
Gallego, Chihuahua, they can be readily distinguished at a distance
because of a growth of weeds and small bushes over their summits, which
overtop the grass. In the vicinity of Albuquerque, N. Mex., Bailey
reports (and this was recently confirmed by Vorhies) that the mounds
about the holes of _spectabilis_ are often hardly noticeable. Hollister
writes that in the yellow-pine forests of the Gallina Mountains the
burrows are usually under the trunk of some fallen pine, both sides of
it in some cases being taken up with holes, there being some eight or
ten entrances along each side, the burrows extending into the ground
beneath the log. In the vicinity of Blanco, N. Mex., Birdseye says that
occasionally _spectabilis_ makes typical dens but more often lives in
old prairie-dog holes (_Cynomys_), or in holes which look more like
those of _D. ordii_.
RUNWAYS AND TRACKS.
Still other features add to the interest in the dwelling places of
_spectabilis_. Radiating in various directions from some of the openings
of the mounds well-used runways are to be seen, some of them fading out
in the surrounding vegetation, but others extending 30, 40, or even 50
or more yards to neighboring burrows or mounds (Pl. V, Fig. 2; Pl. VI,
Fig. 1). These runways and the entrances to the mounds are well worn,
showing that the inhabitants are at home and are at some time of day
very active. The worn paths become most conspicuous in the autumnal
harvest season, when they stand out in strong contrast to surrounding
grass. One usually finds not far distant from the main habitation one or
more smaller burrows, each with from one to three typical openings,
connected by the trail or runway system with the central den, and these
we have called "subsidiary burrows" (Pl. VI, Fig. 2). These will be
again referred to in discussing the detailed plan of the entire shelter
system.
Examination of the runways and of the denuded area about a mound
discloses an abundance of almost indecipherable tracks. The dust or sand
is ordinarily much too dry and shifting to record clear footprints, and
there are no opportunities to see footprints of this species recorded in
good impressionable soil. Very characteristic traces of kangaroo rats
may be readily observed in the dust about the mounds, however, and these
are long, narrow, sometimes curving, furrows made by the long tails as
the animals whisk about their work or play.
[Illustration: PLATE V. FIG. 1.--CLEARING ABOUT A MOUND.
A typical clearing about a mound of _Dipodomys s. spectabilis_, showing
the autumnal denudation of the mound and surrounding areas. In this
instance about 30 feet in diameter.]
[Illustration: PLATE V. FIG. 2.--MOUND AND RUNWAYS.
A small mound of _Dipodomys s. spectabilis_ in early autumn, showing
runways radiating from the den. Evidences of activity may be noted in
and about the surface of the mound.]
[Illustration: PLATE VI. FIG. 1.--RUNWAY OF DIPODOMYS S. SPECTABILIS.
Well-traveled path leading from the main den, in the foreground, to a
subsidiary burrow (see Fig. 2, below), about 30 feet distant, at
apparent end of runway.]
[Illustration: PLATE VI. FIG. 2.--SUBSIDIARY BURROW OF DIPODOMYS S.
SPECTABILIS.
Located at the end of the 30-foot runway shown in Figure 1, above. This
has three openings, two in the foreground and the third a little to the
rear and indicated by an arrow.]
SIGNALS.
If a scratching or tapping sound be made at the mouth of a burrow, even
in the daytime, one is likely to hear a muffled tapping in response, and
this may at times be heard while one is engaged in excavating a mound.
It has a chirring or fluttering quality, described by Fisher as
resembling the noise of a quail flying. Bailey (1905, 148) is of the
opinion that it is used as a signal of alarm, call note, or challenge, a
view which the present authors believe to be correct. During the winter
of 1920-21, however, both Bailey and Vorhies discovered that this sound,
or a very similar one, is made by the rapid action of the forefeet in
digging. On one occasion in the laboratory the sound was given by one of
a pair and was responded to at once by the other, the two being in
separate but contiguous cages. This observation, however, could not be
repeated. (Vorhies MS.)
One evening, while working in the vicinity of the Burro Mountains, N.
Mex., Goldman heard a kangaroo rat near camp making this thumping noise.
Taking a lantern, he approached the den, very cautiously, until within
10 feet. The kangaroo rat was just outside the entrance of one of its
burrows, and though moving about more or less restlessly at first showed
little fear, and kept up the thumping or drumming at intervals. When
making the noise the animal was standing with the forefeet on the ground
and the tail lying extended. The noise seemed to be made with the hind
feet only, and the vibration of the feet could be seen. The tapping was
kept up for a second or two at a time, the sounds coming close together
and being repeated rhythmically after a very short interval, suggesting
the distant galloping of a horse. After continuing in this way for a
short time, the animal turned quickly about, with its head in the
opposite direction, and began tapping. It appeared to pay little
attention to the light, but finally gave a sudden bound and entered one
of its holes about 4 feet from the one in front of which it had been
standing.
Vorhies has repeatedly noted when watching for the appearance of a
kangaroo rat at night that this sound invariably precedes the rodent's
first emergence into the open, and often its appearance after an alarm,
though when the storage season has begun and the kangaroo rat is
carrying loads of grass heads or other material into its den, it
regularly comes out without preliminary signaling. Vorhies has also
observed it making the sound while on top of the mound, and certainly
not digging, but was unable to see how it was made.
VOICE.
No data concerning any call notes or sounds other than those described
above are at hand, with the following exception: Price (in Allen, 1895,
213), who studied the habits of the animal in the moonlight, at Willcox,
Ariz., says that a low chuckle was uttered at intervals; and Vorhies has
had one captive female that would repeatedly utter a similar chuckle in
a peevish manner when disturbed by day, and one captive male which, when
teased into a state of anger and excitement, would squeal much like a
cornered house rat. Vorhies has spent many moonlight hours observing
kangaroo rats, but without ever hearing a vocal sound uttered by free
individuals.
DAILY AND SEASONAL ACTIVITY.
The kangaroo rat is strictly nocturnal. An observer watching patiently
by a den in the evening for the animal's first appearance is not
rewarded until darkness has fallen completely, and unless the moon is
shining the animal can hardly be seen. Were it not for the white
tail-brush of _spectabilis_ and its white belly when upright on the hind
legs and tail, one could not as a rule see the animal at all when it
makes its first evening appearance. With the first streak of dawn
activity usually ceases completely and much more abruptly than it began
with the coming of darkness, but on a recent occasion Vorhies observed
that a kangaroo rat which did not appear until near morning remained
above ground until quite light, but not fully daylight. On removal of
the plug from the mouth of a kangaroo rat burrow, one may sometimes see
a fresh mass of earth and refuse shoved into the opening from within. As
often as not, however, even this unwelcome attention does not elicit any
response by day, the great majority of the burrow openings of this
species, as observed by the authors, remaining permanently open.
The ordinary activities of the kangaroo rat in southern Arizona can
scarcely be said to show any true seasonal variation. The animals are
active all the year in this region, there being neither hibernation nor
estivation, both perhaps being rendered unnecessary by the storage
habit, to be discussed in full later (pp. 15-16), and by the mildness of
the winter climate. On any particular night that the weather is rainy,
or the ground too wet and cold, activity is confined to the interior of
the burrow system, and for this reason one has no opportunity to see a
perfect imprint of the foot in freshly wet soil or in snow. On two or
three of the comparatively rare occasions on which there was a light
fall of snow on the Range Reserve a search was made for tracks in the
snow. At these times, however, as on rainy nights, the only signs of
activity were the pushing or throwing out of fresh earth and food refuse
from within the burrow. This is so common a sight as to be complete
evidence that the animals are active within their dens during stormy
weather but do not venture outside. Trapping has again and again proved
to be useless on rainy nights, unless the rain is scant and a part of
the night favorable, in which case occasional individuals are taken.
These statements apply to the Range Reserve particularly; the facts may
be quite different where the animals experience more winter, as at
Albuquerque, N. Mex., although in November, 1921, Vorhies noted no
indications of lessened activity in that region.
PUGNACITY AND SOCIABILITY.
So far as their reactions toward man are concerned, kangaroo rats are
gentle and make confiding and interesting pets; this is especially the
case with _merriami_. This characteristic is the more surprising in view
of the fact that they will fight each other so readily and so viciously,
and yet probably it is explained in part by their method of fighting.
They do not appear to use their teeth toward each other, but fight by
leaping in the air and striking with the powerful hind feet, reminding
one most forcibly of a pair of game cocks, facing each other and
guarding in the same manner. Sometimes they carry on a sparring match
with their fore feet. Biting, if done at all, is only a secondary means
of combat. When taken in hand, even for the first time, they will use
their teeth only in the event that they are wounded. The jaws are not
powerful, and though the animals may lay hold of a bare finger, with the
apparent intention of biting, usually they do not succeed in drawing
blood. As Bailey says (1905, 148), they are gentle and timid, and, like
rabbits, depend upon flight and their burrows for protection.
The well-traveled trails elsewhere described (p. 10) indicate a degree
of sociability difficult to explain in connection with their pugnacity
toward each other. While three or four individuals may sometimes be
trapped at a single mound, more than two are seldom so caught, and most
often only one in one night. Trapping on successive nights at one mound
often yields the larger number, yet in some cases the number is
explained by the fact that two or three nearly mature young are taken,
and the capture of several individuals at a single mound can not be
taken to indicate that all are from the one den. Our investigations tend
strongly to the conclusion that only one adult occupies a mound, except
during the period when the young are in the parental (or maternal) den.
In the gassing and excavating of 25 or more mounds we have never found
more than one animal in a den, except in one instance, and then the two
present were obviously young animals.
SENSE DEVELOPMENTS.
Without making special investigations through a study of behavior or
other special methods, one can speak in only general terms regarding
what appear to be the special sense developments of kangaroo rats. The
eyes are large, as is very often the case in nocturnal animals, and when
brought out into the bright light of day the rats perhaps do not see
well. Yet, if an animal leaves a den which is in process of excavation,
and follows one runway, even in bright sunlight, it makes excellent
speed to the next opening, often a distance of several yards. Whether
this is accomplished chiefly by the aid of sight or in large measure by
a maze-following ability, such as experiments have shown some rodents to
have, can not be stated without precise experimentation. Marked ability
to follow a maze would not be at all surprising in view of the
labyrinthine character of the underground passages which make up the
normal habitation.
When watching beside a mound by moonlight one is impressed with the fact
that the rats possess either a very keen sense of hearing or of sight,
probably both. The very slightest movement or noise on the part of the
observer results, with a timid individual, in an instantaneous leap for
safety, a disappearance into the burrow so sudden as to be almost
startling. All attempts to obtain flashlight photographs at the mounds
were failures, the animal either having gotten completely out of the
field before the light flashed following the pull of the trigger, or
leaving merely an indistinguishable blur on the plate as it went, and
this in spite of carefully hiding the trigger chain behind a screen. A
slight noise accompanying the trigger action gave the alarm in one case,
and in another the length of time of the flash was sufficient for the
get-away. The marvelous quickness of the animal clearly indicates a
remarkably short reaction time. Occasionally a bold individual is
found, as in the case of one which came out repeatedly, even after being
flashed twice in the same night.
Certain peculiar physical characteristics suggest a relationship to
sense reactions. On these, however, the authors are not prepared to do
more than offer suggestions for future work. The extremely large
mastoids found in kangaroo rats suggest a connection in some way with
special developments of the sense of hearing or of balance. It may be
noted that an intermediate condition between the kangaroo rats and the
majority of rodents in respect to this character is to be found in the
pocket mice (_Perognathus_), which belong to the same family. Herein
lies a field for some interesting experimentation and discovery.
The small, pointed nose might suggest a not overkeen sense of smell, and
there appears no reason to believe that this sense is particularly well
developed. However, the turbinals are very complex. The vibrissæ are
long and sensitive, and may indicate a special development of the sense
of touch as an adaptation to nocturnal habits and to life in an
underground labyrinth. The long, well-haired tail doubtless serves as an
important tactile organ as well as a balance.
MOVEMENTS AND ATTITUDES.
Movements and attitudes are characteristic. As a kangaroo rat emerges
from the burrow a reason for the relatively large size of the opening is
seen in the fact that, kangaroolike, the animal maintains a partially
upright position. Its ordinary mode of progression is hopping along on
the large hind legs, or, when in the open and going at speed, leaping.
When moving slowly about over the mound, as if searching for food, it
uses the fore legs in a kind of creeping movement. It appears to be
creeping when pocketing grain strewn about, but close observation shows
that the fore feet are then used for sweeping material into the pockets,
reminding one somewhat of a vacuum cleaner. When it assumes a partially
upright position the fore limbs are usually drawn up so closely that
they can be seen only by looking upward from a somewhat lower level than
that occupied by the animal. The slower movements of searching or
playing about the mound are occasionally interrupted by a sudden leap
directly upward to a height of 1-1/2 to 2 feet, often with no apparent
reason other than play. This is, however, a fighting or guarding
movement, though indulged in for play. The play instinct seems to be
well developed, and in evidence on any moonlight night when actual
harvesting operations are not going on.
STORING HABITS.
Probably no instinct is of greater importance to the kangaroo rat than
that of storing food supplies. When a crop of desirable seeds is
maturing the animal's activities appear to be concentrated on this work.
During September, 1919, when a good crop of grass seed was ripening
following the summer rains, a kangaroo rat under observation made
repeated round trips to the harvest field of grass heads. Each outward
trip occupied from 1 to 1-1/2 minutes, while the unloading trip into the
burrow took only 15 to 20 seconds.
One individual in a laboratory cage, which had not yet been given a nest
box, busied itself in broad daylight in carrying its grain supply into
the darkest corner of the cage. When a nest box is supplied the
individual will retreat into its dark shelter, and will only come forth
after darkness has fallen unless forcibly ejected, but will store the
food supplied.
In another case an animal escaped while being handled, and sought refuge
behind a built-in laboratory table, where it could not be recovered
without tearing out the table. For four days and nights it had the run
of the laboratory. On the first night of its freedom it found and
entered a burlap bag of grass seed that had been taken from a mound. A
trail of seed and chaff next morning showed that it had been busily
engaged in making its new quarters comfortable with bedding and food.
After four nights of freedom it was captured alive in a trap, and later
it was found that it had moved from the corner behind the table to the
space beneath a near-by drawer, where it had stored about 2 quarts of
the grass seed and a handful of the oatmeal used for trap bait.
BREEDING HABITS.
Observations on breeding habits have consisted mainly in taking records
from the females trapped at all seasons of the year throughout the
course of the investigation, and from examinations made during poisoning
operations, and yet from this source the number of pregnant females
taken or of young discovered is disappointingly small. The records
indicate a breeding period of considerable length, extending from
January to August, inclusive. It is possible that the length of the
period may be increased by a second litter from the earliest breeding
females in summer, but the large percentage of nonpregnant or
nonbreeding animals which occurs throughout the season would indicate a
wide variation in the time of breeding of different individuals.
Trapping in February and March for the purpose of securing greater
numbers of female specimens, begun with the idea that these months were
most likely to be the breeding months, has invariably yielded an
unsatisfactory number of nonbreeding specimens and males. Unfortunately,
the numbers of females secured in some months were not sufficient to be
significant if worked out in percentages of breeding and nonbreeding
individuals, and this, coupled with the fact that the importance of
recording carefully all nonbreeders was not at first recognized, makes
it impossible to tabulate such information reliably. The total of
females taken in April, for example, is only 3, of which 1 was breeding;
while in June, during the course of poisoning operations, 45 females
were examined, of which 21 were breeding.
Five breeding females were taken in January, all during the last three
days of the month. One of these was a suckling female, the young of
which were secured alive and were probably at least a week old when
taken. This must have been exceptionally early for young, since of a
number of adult kangaroo rats taken during the first week of January
none have been found to be breeding. Two records from Vernon Bailey are
as follows: May 19-June 8, 1903, young specimen in nest (Santa Rosa, N.
Mex.); June 12, 1889, one female, two embryos (Oracle, Ariz.).
The considerable proportion (which we believe to be more than 50 per
cent) of nonbreeding females taken during all those months in which
breeding has been found to occur may also indicate an extended period of
breeding, with a small percentage breeding at any one time. This period
also furnishes ample time for the rearing of two litters a year by some
females, but we have no evidence as to the occurrence of two litters.
Young of the year, practically grown, are taken during and after the
month of April.
The mammae are arranged in three pairs, pectoral, 1/1; inguinal, 2/2.
Kangaroo rats are among those rodents in which the vagina becomes
plugged with a rather solid material, translucent, and of the
consistency of a stiff gelatine, after copulation. This must occur very
soon after coitus, since in those individuals taken in this condition no
definite evidence of the beginning of development of embryos could be
detected by examination.
The length of the gestation period of _spectabilis_ is unknown. The
young are born naked, a fact inferred by failure to find any fetus
showing noticeable hair development, and from the conditions observed in
such young as have been seen. A suckling female was taken by Vorhies,
January 31, 1920, and her den immediately excavated in the hope of
securing the young. Two juveniles were found in a special nest chamber
(see p. 30). These were estimated to be perhaps two weeks old. A serious
effort was made to raise the little animals by feeding milk with a
pipette and keeping them warm with a hot water bottle, but they survived
only 10 days, without the eyes having opened. The uneven temperature as
well as the character of the food was probably responsible for their
deaths. On February 3 they were measured and weighed, with the following
results:
---------------------------------------------------
| | Measurements (in millimetres).
| Weight |-------------------------------
| (in | Total | Tail | Hind
| grams). | length. | vertebrae. | foot.
---------|---------|---------|------------|--------
No. 1 | 13.3 | 90 | 38 | 24
No. 2 | 12.6 | 93 | 38 | 24
---------------------------------------------------
At this stage the young were partially clothed with a coat of fine
velvety fur, more especially on the bodies, the tails being still nearly
naked. The body color was dark plumbeous, just the color of the dark
underfur of the adult, or a shade darker, while the characteristic white
markings of the adult stood out sharply as pinkish-white areas against
the dark background (see Pl. IX, Fig. 2, at p. 32). The proportions were
much as in the adult, except that the tails were relatively much shorter
and the feet relatively longer.
Only one other record of young is at hand, that by Bailey, who secured
the young after capture of a suckling female at Santa Rosa, N. Mex. In
this case the litter contained only one. This was squeaking when found,
but was not large enough to crawl away. Its eyes and ears were closed,
and its soft, naked skin was distinctly marked with the pattern of the
adult, the colors being as given for the other two. This juvenile lived
only a week. Young less than half grown were not trapped or noted in our
poisoning operations outside the dens.
Kangaroo rats, if _spectabilis_ be representative, reproduce at a slow
rate as compared with many other small rodents. We have records of 67
females with embryos or scars showing the number produced, and of the
two litters of young described above. Of the 69 females thus recorded,
15, or 21.7 per cent, had but one offspring each; 52, or 75.3 per cent,
but two each; while only 2 individuals, or 2.9 per cent, had three.
Three young is the maximum litter recorded. This, taken in connection
with the protracted breeding season and lack of sure evidence of the
production of two broods a year, gives a surprisingly low rate of
reproduction, indicating relative freedom from inimical factors.
Our breeding records for _merriami_ are fewer than for _spectabilis_,
but are very similar in every way so far as they go, both as to the time
of year and number of young.
FOOD AND STORAGE.
_Dipodomys s. spectabilis_ does not hibernate, so must prepare for
unfavorable seasons by extensive storage of food materials. There are
two seasons of the year, in southeastern Arizona at least, when storage
of food takes place, namely, in spring, during April or May, and in
fall, from September to November, the latter being the more important.
For the periods between, the animal must rely largely on stored
materials. Not infrequently a season of severe drought precludes the
possibility of any storage. The summer and fall of 1918 was such a
season on the Range Reserve (Pl. II, Fig. 2). If food stores are
inadequate at such a time the kangaroo rats must perish in considerable
numbers. Fisher found many deserted mounds in the vicinity of Dos
Cabezos, Ariz., in June, 1894, which may be accounted for in this way.
In 1921 Vorhies found all mounds within 4 or 5 miles of Albuquerque, N.
Mex., deserted by _spectabilis_, resulting probably from overgrazing by
sheep and goats during a succession of dry years. In the arid Southwest
natural selection probably favors the animals with the largest food
stores, and it is not surprising that the storing habit has been
developed to a remarkable degree.
Some stored material is likely to be found at any time of year in any
mound examined, the largest quantity usually in fall and winter, the
smallest in July or August (Table 1, dens 1, 2, 14, and 24). Amounts
found by different observers vary from a few ounces to several quarts or
pecks, and stored materials taken from 22 mounds on the Range Reserve
vary in weight from 5 to 4,127 grams (more than 9 pounds). This is
exceeded by one lot from New Mexico, which totaled 5,750 grams (12.67
pounds). It is fairly evident that in seasons of scanty forage for stock
the appropriation of such quantities of grass seeds and crowns and other
grazing materials by numerous kangaroo rats may appreciably reduce the
carrying capacity of the range. Studies of cheek-pouch contents and food
stores taken from dens show that the natural food of _spectabilis_
consists principally of various seeds and fruits, particularly the seeds
of certain grasses. The study of burrow contents has been especially
illuminating and valuable.
All of the stored material from 22 dens on the Range Reserve and from 2
near Albuquerque, N. Mex., has been saved and analyzed as to species as
carefully as the conditions of storage would permit. Within the mound
the food stored is usually more or less segregated by plant species,
though the stores of material of any one kind may be found in several
places through the mound, and often the material is mixed. In the latter
case the quantities of the various species can only be estimated, but in
the former the species may be kept separate by the use of several bags
for collecting the seeds, and a fairly accurate laboratory weighing can
be made later. Very frequently, the explanation of this separation of
species lies in the different seasons of ripening, but sometimes where
two species are ripe at the same time near the mound, one is worked upon
for a time to the exclusion of the other. The one kind is often packed
in tightly against the other, but with a very abrupt change in the
character of the material.
A number of the more interesting and representative results of the
weighing and analyses of burrow contents are presented herewith in
tabular form. The data for each den, or lot, shows in grams the quantity
of stored material removed and the best estimate it was possible to make
of the percentages or weights of the various species. When the weight
was less than 5 grams, the mere trace of the species frequently is
indicated in the following tables by the abbreviation "Tr."
TABLE 1.--_Analyses of plants stored by _Dipodomys spectabilis
spectabilis_ Merriam, obtained from examination of representative dens
(all except Den 24 from U. S. Range Reserve, near the Santa Rita
Mountains, Ariz.)._
DEN 1.
February 7, 1918. Burrow typical, located on bank of wash in partially
denuded grass-land, _Bouteloua rothrockii_ and weed type; soil sandy;
burrow photographed in section (Pl. VII, Fig. 1).
Species stored. Grams.
Bouteloua rothrockii 2,205
Bouteloua aristidoides (B. eriopoda and B. rothrockii, Tr.) 1,445
Plantago ignota 442
Eriogonum polycladon 35
-----
Total 4,127
Four species of plants represented in burrow contents (Pl. VII, Fig. 2).
Maximum quantity for single burrow in series of 22 from Range Reserve.
DEN 2.
March 9, 1918. Surroundings overgrazed and partially restored by
complete protection. Red soil, with much coarse rough gravel and stone.
Species stored. Grams.
Bouteloua rothrockii (nearly pure) 1,460
Bouteloua rothrockii (mixed with Aristida spp.) 945
Boerhaavia wrightii 660
Bouteloua rothrockii }
Bouteloua aristidoides } 525
Aristida divaricata }
Aristida bromoides }
Kallstroemia laetevirens Tr.
Heterotheca subaxillaris Tr.
Plantago ignota 15
Fleshy fungi 10
-----
Total 3,615
Eight species of plants represented by seeds. One species of fleshy
fungus in addition.
DEN 4.
September 20, 1918. In _Calliandra_ type. Stony or gravelly soil, red,
nearly denuded of grass.
Species stored. Grams.
Prosopis velutina 190
Mollugo verticillata (pure) 90
Anisolotus trispermus (mixed, but mostly of this genus) 50
Solanum elaeagnifolium (12 fruits) 2
Per cent.
Mollugo verticillata (inseparable) 50 }
Bouteloua rothrockii 1 }
Bouteloua aristidoides 10 }
Lepidium lasiocarpum Tr. }
Polygala puberula Tr. }
Ayenia microphylla 2 }
Portulaca suffrutescens 1 } 400
Aplopappus gracilis Tr. }
Alternanthera repens 1 }
Tridens pulchella 1 }
Plantago ignota 33 }
Panicum hallii Tr. }
Fleshy fungi (puffballs) 2
---
Total 734
Fifteen species represented in addition to the fleshy fungi. No
perceptible grass growth from the summer rains here, therefore dependent
on a wide variety of scattering plants.
DEN 6.
October 17, 1918. Mixed type, partially denuded, no growth from summer
rains. Sandy soil.
Species stored. Grams.
Bouteloua rothrockii (crowns) (heads 1 to 2 per cent) 1,435
Bouteloua rothrockii (heads and crowns, about 50 per cent of each) 325
Bouteloua rothrockii (with small percentage of crowns) 315
Boerhaavia wrightii (with a few grass crowns) 150
Prosopis velutina 90
Solanum elaeagnifolium (3 fruits) Tr.
-----
Total 2,315
Four species represented. Count of 100 grams of stored _Bouteloua_
crowns gives 1,700, or 17 crowns per gram. At this rate there were at
least 27,000 crowns stored in this burrow. If a density of 250 plants to
the square yard be assumed (a high estimate) these crowns represent the
total _B. rothrockii_ on 104 square yards of range surface. Further
examination of the vicinity of this den showed that the surrounding area
was not completely cleared, but was devoid of _B. rothrockii_, while
still having _B. eriopoda_ with crowns undisturbed.
DEN 11.
April 9, 1919. In partially denuded land where good spring growth of
_Eschscholtzia_ was in bloom at time of excavation. Stomach of
_spectabilis_ killed in this burrow contained a mass of fresh but
finely comminuted green material, probably poppy leaves, strongly
colored with yellow from blossoms. No summer growth here in 1918.
Species stored. Grams.
Bouteloua rothrockii (crowns) (miscellaneous chaff, etc.) 107
Eschscholtzia mexicana (buds and flowers) }
Anisolotus trispermus (leaves and pods) }
Gaertneria tenuifolia (leaves) } 10
Lupinus sparsiflorus (flowers) }
Solanum elaeagnifolium (2 fruits) Tr.
---
Total 117
Six species represented, some only by leaves or flowers and not by
seeds. _Such storage is never in large quantity._ The fresh storage
material was weighed after becoming air dry. This illustrates a late
spring condition, storage running low.
DEN 14.
August 8, 1919. Excellent summer growth all over range. This burrow in
mixed growth, grasses and weeds.
Species stored. Grams.
Miscellaneous portions of _green plants_ of mixed species,
_no seeds_ 5
Representing minimum for any one of the 22 burrows studied. Active
storage does not begin until September.
DEN 16.
October 17, 1919. In good grass, but mound overrun by a large
_Apodanthera_ vine.
Species stored. Per cent. Grams.
Aristida divaricata 90 to 95 }
Chamaecrista leptadenia 10 to 5 } 58
Bouteloua rothrockii Tr. }
Prosopis velutina 200
Apodanthera undulata 55
---
Total 313
Five species represented. Two species, _Apodanthera_, and _Chamaecrista
leptadenia_, new to storage records. Several whole fruits of
_Apodanthera_, about 2 inches in diameter, stored in addition to seeds
alone; seeds of this form not previously noted in burrows, but very
abundant in this one, indicating importance of the factor of
accessibility in storage.
DEN 19.
October 31, November 1, 1919. In good grass. Entire burrow system mapped
(Fig. 2, p. 29).
Species stored. Per cent. Grams.
Aristida spp. (probably mostly _divaricata_) 98 }
Eriogonum sp Tr. }
Bouteloua rothrockii 1 } 1,813
Bouteloua aristidoides 1 }
Panicum sp Tr. }
Prosopis velutina 1,213
-----
Total 3,026
Five species represented, in addition to those of _Aristida_. Largest
storage of _Prosopis_ found. Mound was near a good-sized mesquite tree.
No storage in subsidiary burrows.
DEN 21.
January 31, 1920. Male trapped here night of January 29, and suckling
female trapped at same place and same opening of mound, night of January
30. Burrow excavated to secure young, which were found in special nest
chamber.
Species stored. Grams.
Aristida spp. (intimate mixture of undetermined species) 1,115
Eschscholtzia mexicana (from spring of 1919) 48
Opuntia (prickly pear, seeds only, no fruits) 10
-----
Total 1,173
Three species represented. Prickly pear hitherto found as fruits only.
DEN 22.
January 1, 1921. Rather good grass growth here in summer of 1920. Burrow
typical, sandy soil. Two skulls of former residents unearthed.
Species stored. Grams.
Aplopappus gracilis (some B. rothrockii) 1,030
Astragalus nuttallianus 630
Bouteloua rothrockii (some A. gracilis) 530
Sida diffusa 30
Solanum elaeagnifolium (282 fruits) 53
Loeflingia pusilla Tr.
Bouteloua aristidoides Tr.
Plantago ignota Tr.
Lupinus sparsiflorus Tr.
Old storage (mostly Bouteloua aristidoides with traces of B.
rothrockii and Aristida divaricata) 60
-----
Total 2,333
Eleven species represented. First instance of quantity storage of
_Aplopappus gracilis_. First occurrence of _Loeflingia pusilla_ and
_Astragalus nuttallianus_.
DEN 24.
November 8, 1921. On mesa northeast of Albuquerque, N. Mex., near base
of Sandia Mountains. Fair grass growth here during preceding summer.
Species stored. Grams.
Sporobolus cryptandrus strictus 5,455
Salsola pestifer 295
-----
Total 5,750
Two species represented. The heads of _Sporobolus cryptandrus strictus_
are retained to a great extent within the leaf sheaths. This
necessitates the cutting of the stems into suitable lengths for
carrying, and the stored material appears to be merely cut sections of
the stems. Close examination, however, discloses the heads within, and
shows that as in other instances seed storage is the end sought. These
pieces are packed beautifully parallel like so many matches, and vary
from a minimum length of 20 to a maximum of 37 millimeters, averaging
about 30. Count of 2 grams of the above _Sporobolus_ material shows that
there are 125 separate cut sections per gram, or a total of
approximately 680,000 pieces in this one lot of storage, indicating a
remarkable activity on the part of the individual rat (Pl. VIII, Fig.
1).
[Illustration: PLATE VII. FIG. 1.--DEN EXCAVATED ON RANGE RESERVE.
Vertical section through Den No. 1, of Table 1 (p. 20), showing the
complex system of burrows, some of them plugged with closely packed
storage (outlined in white), the depth of the den, and the widened
chambers centrally located.]
[Illustration: PLATE VII. FIG. 2.--CONTENT OF EXCAVATED DEN.
Storage content of Den No. 1 (Fig. 1, above), showing the separate
species of plants listed in Table 1. The rod is 1 meter long. The large
pile on the left is composed of seed-laden heads of crowfoot grama
(_Bouteloua rothrockii_), the large pile on the right consists of heads
of six-weeks grama (_Bouteloua aristidoides_), the pile of heads in the
center is desert plantain (_Plantago ignota_), and the smallest heap is
composed of buckwheat-bush seeds (_Eriogonum polycladon_).]
The number of lots of storage (24) studied in detail, extending as it
does over a period of three years with seasons of varying growth
conditions, is not sufficient to permit the construction of a curve
showing increase and decrease in quantity of stored material with
growing seasons and intervals between; but the results indicate a very
decided increase during the autumn storing season, and continuing large
well into the winter, since some outside material can still be obtained
until midwinter. From about February to April a decrease may be noted,
followed, if the spring growth of annuals be good, by a slight increase;
and we can very nearly predict the general character of the increases
and decreases by the precipitation and consequent growth conditions.
TABLE 2.--_Quantity of storage per den correlated with time of year and
growth conditions of preceding season (chiefly from United States Range
Reserve near the Santa Rita Mountains, Ariz.)._
------------+-----------+-----------+-----------
| | |
Den No. | Date. | Quantity. | Preceding
| | | season.
| | |
------------+-----------+-----------+-----------
| 1918. | _Grams._ |
| | |
1 | Feb. 7 | 4,127 | Good.
2 | Mar. 9 | 3,615 | Do.
3 | July 25 | 401 | Poor.
4 | Sept. 20 | 734 | Do.
5 | Sept. 21 | 2,520 | Do.
6 | Oct. 17 | 2,315 | Do.
7 | Dec. 20 | 1,247 | Do.
| | |
| 1919. | |
| | |
8 | Feb. 7 | 1,600 | Do.
9 | Mar. 13 | 370 | Do.
10 | Apr. 7 | 180 | Do.[3]
11 | Apr. 9 | 117 | Good.[3]
12 | May 7 | 298 | Do.[3]
13 | May 11 | 1,590 | Do.
14 | Aug. 8 | 5 | Good.
15 | Sept. 4 | 151 | Do.
16 | Oct. 17 | 313 | Do.
17 | Oct. 18 | 583 | Do.
18 | Oct. 25 | 3,410 | Do.
19 | Nov. 1 | 3,026 | Do.
20 | Dec. 13 | 2,816 | Do.
| | |
| 1920. | |
| | |
21 | Jan. 31 | 1,173 | Do.
| | |
| 1921. | |
| | |
22 | Jan. 1 | 2,333 | Fair.
23[4] | Nov. 7 | 1,685 | Good.
24[4] | Nov. 8 | 5,750 | Do.
| | |
------------+-----------+-----------+-----------
[Footnote 3: Changing from poor summer season of 1918 to excellent
spring growth of 1919.]
[Footnote 4: From near the Sandia Mountains, N. Mex.; others from
United States Range Reserve, near the Santa Rita Mountains, Ariz.]
In presenting Table 2, showing quantity of storage per burrow correlated
with the time of year and the character of the preceding growing season,
the fact may be emphasized that the growing seasons in southern Arizona
are two in number--early spring and midsummer. The spring season is the
less important, the plants consisting chiefly of a variety of small
annuals, while the important range grasses make their chief growth and
head out almost exclusively in the July-August rainy season. It may be
noted also that the actual increases in storage appear somewhat after
the growth period proper, since storing does not get well under way
until the seed crop is mature. The banner-tailed kangaroo rat shows a
marked adaptability to different foods available in the neighborhood of
its burrows. It must, perforce, adapt itself and its storage program to
the food that it can get, and this varies enormously with the climatic
conditions of successive seasons. The large numbers present in suitable
localities clearly indicate that the animal is successful in meeting the
changing and sometimes extremely adverse conditions of its environment.
[Illustration: PLATE VIII. FIG. 1.--CONTENT OF DEN EXCAVATED IN NEW MEXICO.
Storage content of Den No. 24, of Table 1, from Sandia Mountains, N.
Mex. This is the largest lot of storage taken in the course of the
investigations. The larger pile consists wholly of a valuable grass,
_Sporobolus cryptandrus strictus_: the smaller of Russian thistle
(_Salsola pestifer._)]
[Illustration: PLATE VIII. FIG. 2.--GROWTH FOLLOWING ELIMINATION OF
KANGAROO RATS.
The same mound as shown in Plate III, Figure 1, after three years of
protection, the rodents having been killed out. Nearly as good grass
recovery following poisoning operations occurred in the single excellent
season of 1921.]
At times, more especially in the seasons of active growth, some of the
green and succulent portions of plants are eaten. This was very
noticeable in the spring of 1919, when a most luxuriant growth of
Mexican poppy (_Eschscholtzia mexicana_) occurred. Stomachs at this time
were filled with the yellow and green mixture undoubtedly produced by
the grinding up of the buds and flowers of this plant. Small caches of
about a tablespoonful of these buds were also found in the burrows at
this time. Occasionally in spring one may find a few green leaves of
various plants, _Gaertneria_ very commonly, tucked away in small pockets
along the underground tunnels, indicating that such materials are used
to some extent. As has been shown in detail, however (Table 1), the
chief storage, and undoubtedly the chief food, consists of air-dry
seeds.
The character of the storage, the absence of rain for months at a time
in some years, and the consequent failure of green succulents show that
without doubt _spectabilis_ possesses remarkable power, as to its water
requirements, of existing largely if not wholly upon the water derived
from air-dry starchy foods, i.e., metabolic water serves it in lieu of
drink (Nelson, 1918, 400), this being formed in considerable quantities
by oxidation of carbohydrates and fats (Babcock, 1912, 159, 170). During
the long dry periods characteristic of southern Arizona, no evidence
that the animal seeks a supply of succulent food, as cactus, is found;
and if it may go for two, three, or six months without water or
succulent food, it is reasonable to suppose that it may do so
indefinitely. In the laboratory _spectabilis_ ordinarily does not drink,
but rather shows a dislike for getting its nose wet. During the periods
of drought the attacks upon the cactuses by other rodents of the same
region, as _Lepus_, _Sylvilagus_, _Neotoma_, and _Ammospermophilus_,
become increasingly evident. The list of plant species thus far found
represented in the storage materials of _spectabilis_ on the Range
Reserve is shown in Table 3.
TABLE 3.--_List of all plant species found in 22 dens of _Dipodomys
spectabilis_ on the United States Range Reserve, near the Santa Rita
Mountains, Ariz., with approximate total weights._
GRASSES.
Grams.
Aristida bromoides (six-weeks needlegrass) 536
Aristida divaricata (Humboldt needlegrass) 9,412
Aristida scabra (rough needlegrass) 344
Bouteloua aristidoides (six-weeks grama) 3,093
Bouteloua radicosa (grama) 1,269
Bouteloua eriopoda (black grama) Tr.
Bouteloua rothrockii (seeds, 8,495; crowns, 3,517 grams)
(crowfoot grama) 12,012
Festuca octoflora (fescue grass) 70
Panicum arizonicum (Arizona panic-grass) 11
Panicum hallii (Hall panic-grass) Tr.
Pappaphorum wrightii Tr.
Tridens pulchella Tr.
Valota saccharata Tr.
OTHER PLANTS.
Alternanthera repens Tr.
Anisolotus trispermus (bird's-foot trefoil) 186
Aplopappus gracilis 1,030
Apodanthera undulata (melon loco) 55
Astragalus nuttallianus (milk vetch) 630
Ayenia microphylla Tr.
Boerhaavia wrightii 885
Chamaecrista leptadenia (partridge pea) 5
Echinocactus wislizeni (visnaga) 5
Eriogonum polycladon 35
Eschscholtzia mexicana (Mexican poppy) 250
Gaertneria tenuifolia (franseria) Tr.
Collomia gracilis (false gilia) Tr.
Heterotheca subaxillaris Tr.
Kallstroemia laetevirens Tr.
Lupinus sparsiflorus (lupine) Tr.
Martynia altheaefolia (small devil's-horns) 12
Mollugo verticillata (carpetweed) 324
Oenothera primiverus (evening primrose) 15
Opuntia discata (prickly pear) 15
Loeflingia pusilla Tr.
Lepidium lasiocarpum (peppergrass) Tr.
Plantago ignota (plantain) 818
Polygala puberula (milkwort) Tr.
Portulaca suffrutescens (purslane) Tr.
Prosopis velutina (mesquite) 1,570
Sida diffusa (spreading sida) 30
Solanum elaeagnifolium (742 fruits) (trompillo, prickly solanum) 156
Puffballs and fleshy fungi (undetermined) 12
Total species, exclusive of fungi, 41.
It will be seen from Table 3 that while a large number of species of
plants are represented in the totals from so many dens, a majority of
them are of very minor importance, and that the seeds of grasses are the
principal storage and probably therefore the principal food material.
Six of the most important species of grasses (disregarding species
furnishing less than 5 grams) comprise 85.6 per cent of the total weight
of storage from 22 dens. Crowfoot grama (_Bouteloua rothrockii_) stands
first in quantity in the total, forming 39.4 per cent of all stored
material, 46 per cent of the six important grasses, and 45 per cent of
all grasses. The largest amount of storage of any one species of grass
in any one den on the Range Reserve also is of this species, 2,205
grams[5] (Table 1, den 1, p. 20, and Pl. VII, Fig. 2). This is exceeded
by a dropseed grass, _Sporobolus cryptandrus strictus_, which amounted
to 5,455 grams in a lot from Albuquerque, N. Mex. (Table 1, den 24, and
Pl. VIII, Fig. 1).
[Footnote 5: This amount of dry grama grass seed (heads) amounts to
approximately a bushel.]
Of the species other than grasses found stored in these dens, mesquite
beans (_Prosopis velutina_) are most important both by weight and number
of dens containing them. The total for the 22 Range Reserve dens is
1,570 grams, or 35.9 per cent of the seeds other than grasses, but only
5.1 per cent of the total storage. In bulk mesquite beans do not loom up
large, as they are probably the heaviest material stored. Sections of
pods which must have been dragged into the burrows are found, some of
them certainly being much too long for carriage in the pouches. The
species of plant other than grass found in the largest quantity in any
one den, however, was _Aplopappus gracilis_, not recorded in quantity
from any den until the excavation of the twenty-second, and then found
in a very large bulk of soft, fluffy material, with most of the seeds
separated from the heads, and weighing 1,030 grams (Table 1, den 22).
Any of the food materials above listed are likely to be found in the
cheek pouches, while in addition such extraneous matter as stones and
feces have also been found. All species of plants stored are accessible
in the immediate vicinity of the mound, and when any particular plant is
found seeding in abundance in the vicinity of the den it is likely to be
represented in the storage. Usually the animals can be readily trapped
with almost any kind of grain bait, as oats, rolled oats, rolled barley,
and wheat; and nut meats also are attractive, though we have no record
of the storing of any true nut in the dens, such not being available in
the range of the animal on the Range Reserve.
The following plants not represented in the list stored by the kangaroo
rat on the Range Reserve have been found in the cheek pouches or mounds
of _spectabilis_ in other localities:
_Amaranthus palmeri_, _Sesuvium portulacastrum_, and _Atriplex
wrightii_ (alluvial soil of Santa Cruz Valley, Continental, Ariz.,
Bailey).
Cut leaves and stems of a small sagebrush (Franklin Mountains,
Tex., Gaut).
_Gutierrezia_ heads (San Juan Valley, N. Mex., Birdseye).
_Verbesina enceliodes_, _Portulaca oleracea_, _Bouteloua gracilis_,
and _Munroa squarrosa_ (Rio Alamosa, N. Mex., Goldman).
Tops of buds of _Artemisia filifolia_ (Mesa Jumanes, N. Mex.,
Gaut).
Tumbleweed (_Amaranthus graecizans_), Russian thistle (_Salsola
pestifer_), _Munroa squarrosa_, and _Sporobolus cryptandrus
strictus_ (Sandia Mountains, Albuquerque, N. Mex., Vorhies).
BURROW SYSTEMS, OR DENS.
The burrow system, or den, in which _spectabilis_ stores its caches of
food materials, has its nest, and remains throughout the hours of
daylight is a complicated labyrinth of tunnels. Ejection of refuse and
soil from this retreat builds up the mound frequently referred to. These
mounds are, as Bailey says, characteristic of the species, and are as
unmistakable as muskrat houses or beaver dams, and as carefully planned
and built for as definite a purpose--home and shelter. They are,
furthermore, the most notable of all kangaroo rat dwelling places
(Nelson, 1918, 400). They range in height from 6 inches to approximately
4 feet and from 5 to 15 feet in diameter.
The mound is built up not only through the cleaning out of chaff and
other food refuse, but through extension and modification of the
tunnels; old tunnels, entrances, and caches of musty food material are
from time to time closed up and others excavated, repair and rebuilding
being especially necessary after the collapse of portions of the den as
a result of heavy rains or trampling by cattle. Ejected material is most
commonly simply thrown out fan-wise from the openings, without much
apparent effort to add to the height of the mound.
There are usually from 6 to 12 entrance holes in each mound opening into
the subterranean burrow system, each hole from 4 to 5-1/2 inches in
diameter. These holes are nearly all situated a little above the surface
of the surrounding soil, and as Price has suggested (in Allen, 1895,
213), this is doubtless a wise provision against flooding, as torrential
rains sometimes occur in the kangaroo rat country.
Both Bailey and Nelson state that as a rule several of the holes are
closed with sand or miscellaneous earth and old storage material during
the daytime, but our observations on the Range Reserve are that such
closing is only occasional. Many occupied dens have not a single
opening closed. Further, night observations disclose that the inhabitant
of the mound will appear from some one of the two or three most-used
openings when night falls, and not necessarily from one which has been
closed by day. Recently an opening closed one day was observed in use
during the night, but was left open all the next day.
In attempting to determine whether there exist similarities of plan or
system in the dens, it was considered advisable to map them with some
degree of accuracy. This we were enabled to do by laying off a square
about a given mound, 2-1/2 or 3 meters each way, and subdividing it into
a series of small squares of half a meter on each side by drawing
cross-lines on the surface of the ground over the top of the mound. One
person then did the digging and exploring of the tunnels, as to
direction and depth, while the other noted the results on coordinate
paper (Figs. 2 and 3); the proper excavation and mapping of one of these
workings occupied from four to eight hours for the two.
[Illustration: FIG. 2.--Diagram of a typical den of _Dipodomys
spectabilis spectabilis_. Double shading indicates where one portion of
tunnel lies above another and solid black a three-story arrangement;
_A_, _B_, _C_, etc., active openings to surface; figures without arrows,
depths in centimeters to tunnel roofs; figures with arrows, tunnel
widths in centimeters; _N._ nest chamber; _S_, storage; _OS_, old
storage; _Y_, probably an old nest chamber; _Z_, old, unused, or
partially plugged openings.]
While there is greater complexity in the larger, and probably older,
mounds than in the smaller, all are extremely complicated and can only
be described as labyrinthine in character. The tunnels wind about
through the mound, rising and falling in vertical depth,
intercommunicating frequently, but with occasional cul-de-sacs, and in
places expanding into chambers, of which there may be three or four
large ones. The stored materials are found in some, but not necessarily
all, of these chambers, and may also occupy considerable lengths of
ordinary tunnel, especially when the quantity present is large. Small
evaginations of the tunnels frequently contain lesser caches, and it is
in such pockets that bits of fresh material are placed during a growing
season, or that grain supplied the previous night for bait is usually
found.
The main masses of storage are most often found centrally located at
depths of from 15 to 57 centimeters, although at times one may find a
cache near the periphery of the system and as near the surface as 2 or 3
centimeters. In the latter case the materials are subject to wetting
from rains, and consequent spoilage.
The major portion of the whole tunnel system is within about 50
centimeters of the surface of the mound, but usually some one branch
tunnel goes to somewhat greater depth, and this is likely to be the one
containing the nest; this is also likely to extend toward or beyond the
periphery of the main system, and always ends blindly. Such a one, from
which two young were taken on January 31, 1920, was at a depth of about
65 centimeters, and about 1-1/2 meters beyond the periphery of the mound
itself.
The individual tunnels average about 8 centimeters in height, and about
11 centimeters in width, though the variation, especially in width, is
considerable. The expansions mentioned as being the chief places of
storage are from 15 to 25 centimeters in diameter, and may or may not
involve a considerable increase in height. They are frequently located
at junction points of two or more branches of the tunnel system.
The nest cavity is a chamber of approximately spherical shape and from
17 to 23 centimeters in diameter. Chambers of this character were
observed and noted as "old storage" in a number of cases. They were
sometimes cut off from the rest of the habitation, and at first were
supposed to contain abandoned musty storage. As experience in excavating
and interpreting results has been gained we have concluded that these
chambers in fact represent abandoned nests.
[Illustration: FIG. 3.--Diagram of the system of surface runways and
subsidiary dens of _Dipodomys spectabilis spectabilis_. The underground
tunnels of the main den were too complicated to illustrate on this
scale, being very similar to those of Figure 2. The underground tunnels
of the subsidiaries are shown in solid black. Some runways fade out in
the grass in a manner that can not be indicated in a line drawing.]
Bailey gives the dimensions of nest chambers observed in New Mexico as
about 6 by 8 inches to 8 by 10 inches. The nest is composed of finer,
softer, and more chaffy material than the regular storage. The chaff
refuse from the food probably contributes largely to it, though some
leaves of grasses not stored for food may also be found, and a nest,
especially the one in use, may be distinguished, if excavating is
carefully done, by the distinct cavity about the size of a fist in its
interior (Pl. IX, Fig. 1). One may sometimes find this cavity distinctly
warm from the recent presence of the inhabitant.
The walls or partitions between the chambers and tunnels are in places
surprisingly thin, and it is no wonder that one is almost certain to
break through in stepping on a mound, since the whole is a honeycomblike
structure of from two to four stories in vertical plan, as shown by the
transect of a mound in Plate VII, Figure 1. As Bailey writes, these
partition walls are a mixture of earth and old food and nest material
discarded years ago, resembling the adobe walls of the Mexican houses
built of chopped earth and straw. This is the result of the continual
ejection of refuse and earth as before mentioned, combined with the
caving action of rains and disturbances from larger animals.
Apparently there are no special pockets for deposit of feces in
_Dipodomys_ burrows; such matter may be found throughout the den, and is
more or less mixed with the food refuse which carpets practically the
entire tunnel system. The nest and food stores are, however, clean and
neat, the droppings being dry and, though present on the floor of a
storage chamber, not actually mingled with the food. Evidently the
animal does not clean up the floor litter before storing food material.
The entire system for any one den seems to consist not only of the
burrows within the mound itself, as described, but of those small
outlying ones which we have referred to as subsidiary burrows. These are
two to four in number, and are connected with the main mound by the
runways already mentioned. They often seem to be way stations on the
runways connecting main mounds, and there is seldom any mound of earth
whatever in connection with them. One entire den system, the home mound
and three subsidiaries, was mapped after being excavated (Fig. 3), all
having been carefully gassed with carbon bisulphide. The subsidiaries
were simple and contained no storage. Two of them were shallow, while in
the third a depth of 48 centimeters was reached. They appear to be
merely places of refuge, though the well-worn trails connecting them
with the main mound indicate regular use. These runways are conspicuous
on the Range Reserve, and are apparently characteristic of mounds
throughout the range of the animal. Dwellers in different mounds must
have rather extensive social contacts, notwithstanding the enmity of
individuals toward each other in captivity. The main mound, in this
instance very complicated, was in one place three stories high, and we
have found as many as four utilized stories; but as a rule there are two
or three only.
Since collapses are rather frequent during rainy seasons, aside from the
trampling previously referred to, the kangaroo rats, where abundant,
as on the Range Reserve, may well be a factor in increasing soil
porosity and fertility; for in the course of time they probably have
succeeded in plowing and cultivating the whole surface layer of the
soil. They may thus be a factor in ecologic succession, tending to
improve the character of the soil and adapt it to another stage.
Doubtless their own workings afford the only shelter the animals know.
In the course of our digging in one mound, the occupant, an adult male,
did not forsake the den until the excavation was three-fourths
completed; and even then it did not leave by a burrow leading away from
our operations, but came toward us, escaped the active efforts of four
individuals bent on its capture, and ran speedily along a used runway
toward another burrow several meters distant. A sack had been stuffed in
the mouth of this, however, and, baffled, the rat then returned to the
original burrow and was captured. Observations on other rats thus driven
from the home mound indicate that they are very familiar with the
runways of the vicinity of the mound and the various subsidiary burrows,
and it is a question whether they need to see clearly to follow these
runs. Apparently they never attempt to escape by forsaking their
well-traveled runways. Tests of the maze-running ability of these
animals by animal-behavior experts would be of extraordinary interest,
in view of the character of the homes which they always inhabit and the
network of runs on the outside.
[Illustration: PLATE IX. FIG. 1.--KANGAROO RAT NEST AND YOUNG.
Nest and the two young, the ordinary number in the litter, of _Dipodomys
s. spectabilis_, taken from den on January 31, 1920.]
[Illustration: PLATE IX. FIG. 2.--YOUNG OF THE KANGAROO RAT.
The same young as shown in Figure 1, above. They were probably about two
weeks old, the pelage being short but with the white markings of the
adult; the tails are relatively short and with scarcely any hair.]
COMMENSALS AND ENEMIES.
COMMENSALS.
It is doubtful whether any animals live in a truly commensal
relationship with _spectabilis_, but of not unfriendly associates there
are a great number. It is the experience of Bailey, corroborated by
observations of Vorhies on living animals, that these kangaroo rats are
active in defending their caches of food, and will even fight
individuals of the same species savagely and to the death. One moonlight
night a strange individual was liberated on a mound. It deliberately
entered one of the openings, but after about two minutes' time made an
exceedingly rapid exit, running rapidly out of sight as if pursued,
though the owner of the home did not appear outside of the burrow. There
can be little doubt that the stranger was precipitately ejected by the
owner. We suspect, though this is a point difficult to prove
satisfactorily, that _merriami_ does not always store food supplies for
itself, but visits the burrows of _spectabilis_ regularly to pilfer the
seed stored therein. The observed facts thus far recorded which suggest
this are that in no _merriami_ burrow examined has a store of food been
found, and also that in trapping for _spectabilis_ on its own
characteristic mounds one catches a large percentage of _merriami_.
On two separate occasions Vorhies has observed the smaller species
running over the mounds of the larger, actually carrying away the grain
which had been placed to entice the larger when it might appear. (In
these cases the larger species did not put in an appearance until near
morning.) Furthermore, the dens of _merriami_ are often connected by
distinct runways with those of _spectabilis_, indicating much traveling
or visiting. That this is probably not friendly visiting is suggested by
the certainty with which an individual of the larger species will strike
and kill one of the smaller when they are placed together in the same
inclosure. The word "thief" expresses this suspected relationship better
than would the term "parasite."
It is not to be expected that such obvious shelter retreats as the
mounds of _spectabilis_ should fail to attract the attention of other
animals. We have found a small gecko (_Coleonyx variegatus_), scorpions
of two or three undetermined species, and certain insects (of the Order
Orthoptera) to be very common inhabitants of the dens. With the
exception of the parasitic insects the most common are wingless
locustids (_Ceuthophilus_ spp.) and the peculiar wingless females of a
species of cockroach (_Arenivaga erratica_). These two are seldom absent
when a burrow is excavated, the female cockroaches being abundant,
although the winged males have never been taken in the burrows.
Cary's observations at Monahans, Tex., and those of others at numerous
localities, combined with our own, show that at various times the dens
furnish protection and shelter for various species of cottontail rabbits
(_Sylvilagus_), ground squirrels (_Citellus_ and _Ammospermophilus_),
wood rats (_Neotoma_), grasshopper mice (_Onychomys_), rattlesnakes
(_Crotalus_), and most of the common lizards. Of these the ground
squirrels _Citellus tereticaudus_ and _Ammospermophilus harrisii_ are
most often noted on the Range Reserve using the dens as a retreat, the
_Ammospermophilus_ seldom being observed to enter any other kind of
burrow. It should be added that the total observations include dens
which have been deserted by their rightful owners.
NATURAL CHECKS.
The enemies of the kangaroo rat are not determined in detail, or as to
relative importance, but the badger (_Taxidea taxus berlandieri_) and
the kit fox, or swift (_Vulpes macrotis neomexicana_), may well be
foremost. Dens which have been deeply excavated by badgers are
frequently seen, and sometimes two or three badger tunnels penetrate one
burrow system. Dens thus despoiled are probably soon reoccupied even if
the original owner is captured, and in the course of a few months the
reworking of the abode obliterates the signs of destruction.
Droppings of the kit fox show an abundance of bones of small mammals of
kangaroo rat size, among them those of _spectabilis_.
Bobcats (_Lynx baileyi_) and coyotes (_Canis mearnsi_) probably are a
prejudicial factor. Skunks may sometimes be able to surprise the
kangaroo rats, but probably not often. The western horned owl (_Bubo
virginianus pallescens_), the barn owl (_Tyto alba pratincola_), and
perhaps others may well be among the most feared enemies, but no special
investigation of owl pellets on the reserve has been possible. In 592
barn-owl pellets from California were found remains of 230 kangaroo
rats, only one other rodent being represented by a larger number
(McAtee, 1921, 258).
Much more information on enemies is needed. The relatively low rate of
reproduction (see p. 18) indicates comparative freedom from inimical
factors.
PARASITES.
_Dipodomys s. spectabilis_ is regularly infested with a species of flea,
_Ctenophthalmus_ sp. Seldom or never is a specimen taken in reasonably
fresh condition without some of these parasites present on its body,
though of course they desert the body of the host after it becomes cold,
and hence dead specimens left too long may be free from them. The den
conditions are ideal for the breeding of this parasite, because of the
great quantities of fine, dusty, organic refuse littering the tunnels
and furnishing food and refuge for the larvæ. As demonstrated to us by
F. C. Bishopp, of the Bureau of Entomology, a handful of this refuse
taken from the floor of a burrow within arm's length of the entrance is
almost certain to contain these larvæ.
Less regularly present, perhaps because of its different life history,
is a small tick, _Trombicula_ sp. At times this parasite is very common,
being present on nearly every individual rat, and at other times
specimens are difficult to find; it appears to be more commonly present
in summer and fall than at other seasons, and is found attached chiefly
to the ears.
No internal parasites have been detected. The nocturnal and fossorial
habits of the animal seem to give complete protection against a form of
parasite which is very common among some other rodents of the Range
Reserve, notably _Lepus_ and _Sylvilagus_. Nearly all rabbits are
infested with "warbles," the larvæ of a species of bot-fly, _Cuterebra_
(family Oestridae). Other small mammals also are occasionally
parasitized by the _Cuterebra_, but in the handling and examination of
perhaps 200 or more individuals of _spectabilis_ and _merriami_, we have
yet to find a single case of infestation by an oestrid fly.
ABUNDANCE.
One's first impression of a well-occupied _spectabilis_ area is that a
large family must inhabit each den, but, as previously mentioned, we
have gradually been compelled to shift from this conception to the idea
of but a single animal to a mound, except when the young are present.
Therefore a census of the adult kangaroo rat population can readily be
made, simply by counting the mounds. Such a census affords at least a
conservative estimate of the number of adult individuals occupying a
given area.
The first estimates of abundance on the Range Reserve were from actual
counts of dens on areas measured off for experimental fencing, and gave
the figure of about two mounds to the acre. From time to time rough
estimates were made on different portions of the pastures, and these
checked well with the above. Later still, a careful count showed 300
mounds on approximately 160 acres (see p. 8), or 1.87 mounds per acre.
Nine areas of 2 acres each, representing different environmental
conditions, were later selected in different portions of the Range
Reserve, and the dens accurately counted. The number of dens per 2 acres
varied from none to a maximum infestation of 12, neither extreme
occurring over large areas. The total number of dens was found to be 43
on the 18 acres, or an average of 2.38 dens per acre.
From all these estimates it may fairly be concluded that two mounds, or
two animals, per acre is a conservative estimate for the infestation of
the entire Range Reserve, with the possible exception of small areas at
its upper edges, where the altitude limit of _spectabilis_ is passed. It
is, however, impossible to estimate the area of the State infested with
kangaroo rats, for some large stretches of fine grassland show no
kangaroo rats whatever, while others have more than are present on the
reserve; and we have no estimates of the extent of either type.
ECONOMIC CONSIDERATIONS.
In May, 1894, Fisher found a ranchman at Willcox, Ariz., who complained
more bitterly of the depredations of _spectabilis_ than of those of any
other mammal.
On the United States Range Reserve the food material appropriated by the
kangaroo rat during good years is inappreciable. There is such an excess
of forage grass produced that all the rodents together make very little
difference. But with the periodic recurrence of lean years, when drought
conditions are such that little or no grass grows, the effects of rodent
damage not only become apparent, but may be a critical factor
determining whether a given number of domestic animals can be grazed on
the area (Pl. VIII, Fig. 2).
With two kangaroo rats to the acre (1,280 per square mile), there would
be 64,000 animals on the 50 square miles of the Range Reserve. If each
rat stores 4 pounds of grass seeds and crowns and other edible forage
during the season (and in severe seasons we find that more crowns are
stored than under ordinary conditions), a total of 256,000 pounds, or
128 tons, of edible forage are rendered unavailable to stock. In dry
years it is probable that this amount of forage would be of critical
importance. Allowing 50 pounds of food a day for each steer, the forage
destroyed would be sufficient to provide for the needs of one steer for
5,120 days, or for the needs of 14 steers for one year. On a stock ranch
the size of the Range Reserve this might mean the difference between
success and failure.
It seems not unlikely, therefore, that during seasons of drought the
banner-tailed kangaroo rat, where it is abundant on the grazing ranges
of the Southwest, may be a factor of critical importance in relation to
forage production and carrying capacity. It must be remembered,
moreover, that the stored material consists largely of seeds, so that
this loss is of greater importance than would be the case were it
ordinary forage. Some of the range grasses of this region found in
greatest quantity in the stored material depend in large part, under
certain conditions, upon seed reproduction. Rehabilitation of a depleted
range after severe drought and consequent close grazing and trampling is
retarded by the heavy toll of seed taken by the kangaroo rats.
CONTROL.
Kangaroo rats may be easily eradicated by the use of the poisoned grain
used for prairie-dog control by the Biological Survey and the University
of Arizona Extension Service. This can be obtained by application to the
State representative of the Biological Survey or to the local county
agricultural agent, or may be mixed as follows:
_Formula for poisoned bait._--Dissolve 1 ounce of strychnine
sulphate in 1-1/2 pints of boiling water. Add 1 heaping
tablespoonful of gloss starch, previously mixed with a little cold
water, and boil until a clear paste is formed. Add 1 ounce of
baking soda and stir to a creamy mass. Add 1/2 ounce of glycerine
and 1/4 pint of corn sirup and stir thoroughly. Pour over 16 quarts
of rolled barley and mix well until every grain is evenly coated.
Allow to dry before using.
In bushel quantities use as above directed, 2 ounces of strychnine,
2 ounces of soda, 1 ounce of glycerin, 1-1/4 ounces of starch,
1-1/2 quarts of boiling water, and 5/8 pint of corn sirup.
Scatter poison, when the natural food of the kangaroo rat is
scarce, on clean hard places near the holes, 1 quart to 50 holes.
If powdered strychnine alkaloid is used, prepare the hot starch
paste first. Then sift strychnine and baking soda, previously
thoroughly mixed, into the hot starch paste and stir to a creamy
mass. Proceed as in the above directions with sirup, glycerin,
etc.
Use this poison within five days after mixing or retain in
air-tight containers.
_Caution._--All poison containers and all utensils used in the
preparation of poison should be kept _plainly labeled_ and _out of
reach of children_, irresponsible persons, and live stock.
A spoonful of the poisoned grain scattered about the used entrances
of a mound is sufficient, and prebaiting is not necessary, as with
prairie dogs.
A word of caution should perhaps be offered in connection with control
measures. As man has come to occupy a greater portion of the earth's
surface, and as he has become more and more the master of his
environment, he has inevitably disturbed the relationships of the birds
and mammals about him, has upset the balance of nature. If he kills the
carnivorous species because of their depredations on game and live stock
he must be prepared to cope with the increased hordes of rodents which
feed on vegetation and on which the carnivorous animals act as a check.
If he destroys the rodents, he may remove the checks on certain noxious
plants or insects. One control measure often necessitates the adoption
of another.
This is not to argue against control measures, for if our harmful
species were not controlled, agriculture in many sections would be
impossible. Control measures, however, should be scientifically founded
and applied. The indiscriminate slaughter of supposedly harmful species
of birds and mammals in the guise of benefiting agriculture may do far
more harm than good. Many of the species which do some harm do far more
good. The exact status of each suspected species should be carefully
determined through an adequate scientific investigation. If the species
is condemned, sound control measures should be thoroughly applied.
In grazing districts or in areas devoted to intensive agriculture the
death sentence should probably be passed on the banner-tailed kangaroo
rat. It should be recalled, however, that this is the largest and one of
the handsomest of all its family, and that it is one of the most
characteristic and interesting of all the desert fauna; where extensive
grazing or agricultural operations are not undertaken, therefore, we
feel that the kangaroo rat should be let alone, unless its presence
threatens infestation of valuable agricultural or grazing lands.
SUMMARY.
(1) Kangaroo rats may be separated with ease from all other mammals; the
long tail and short and weak fore feet separate them from the pocket
gophers; the white hip-stripe distinguishes them from the pocket mice.
The decidedly larger size and the white-tipped tail separate _Dipodomys
spectabilis spectabilis_ and _D. deserti_ from _D._ _merriami_ and _D.
ordii_. The darker color and vividly contrasted black-and-white tail of
_spectabilis_ distinguish it from _deserti_.
(2) _Dipodomys s. spectabilis_ occurs in the open arid country of
portions of the Lower and Upper Sonoran Zones of Arizona, New Mexico,
Texas, Sonora, and Chihuahua. It lives in harder soil than does
_deserti_, and builds more conspicuous mounds.
(3) There is no evidence of intergradation or hybridization between
_spectabilis_ and _deserti_.
(4) _Dipodomys s. spectabilis_ is nocturnal; it is gentle, and does not
offer to bite when taken in the hand; is silent for the most part;
active; somewhat sociable with its fellows, but fights in defense of its
food stores; progresses chiefly by leaping; signals by a drumming or
tapping on the ground with its hind feet.
(5) The breeding season of _spectabilis_ begins in January and continues
into August. Whether more than one litter is raised in a single season
is unknown. The number of young in each litter varies from 1 to 3,
averaging 2.
(6) _Dipodomys s. spectabilis_ does not hibernate, but provides food
stores, mostly seeds, for use during seasons when food would be
otherwise unavailable. Storage in each den varies in quantity from 5
grams (about 1/6 ounce) to 5,750 grams (12.67 pounds). Materials stored
include several important forage plants; for example, various species of
_Bouteloua_ and _Aristida_, with _B. rothrockii_ (crowfoot grama) the
most important. Accessibility and abundance of different plants have
much to do with the kinds of storage found.
(7) The dens of _spectabilis_ are the most notable of all kangaroo rat
dwelling places. They range from 6 inches to 4 feet in vertical height,
and from 5 to 15 feet in diameter. Here the kangaroo rat has its home,
shelter, and food-storage chambers. Within the den is found a tortuous
network of burrows, with many storage and some nest chambers, the whole
arranged so as to be two to four stories high.
(8) _Dipodomys s. spectabilis_ is not of great economic significance,
except locally, in ordinary seasons. During periods of extreme drought
it may be of critical importance on grazing areas from the standpoint of
the carrying capacity of the range.
(9) Kangaroo rats are easy to poison by following the same formula as
that used by the Biological Survey for destroying prairie dogs.
(10) In many places unsuited to extensive grazing or agriculture
_spectabilis_ does no appreciable damage. It is one of the most
interesting of all the rodents peculiar to our Southwestern deserts, and
should not be molested except where it is destructive.
BIBLIOGRAPHY.
ALLEN, J. A.
1895. On a collection of mammals from Arizona and Mexico, made by Mr.
W. W. Price, with field notes by the collector. Bull. Amer. Mus. Nat.
Hist., vol. 7, art. 6, pp. 193-258. 17 figs. in text.
BABCOCK, S. M.
1912. Metabolic water: Its production and rôle in vital phenomena.
Research Bull. No. 22, Univ. Wisconsin Agr. Exp. Station, pp. 159 and
170, March.
BAILEY, V.
1905. Biological survey of Texas. North Amer. Fauna No. 25, Biol. Surv.,
U. S. Dept. Agr., pp. 222, 16 pls., 24 figs. in text.
CLEMENTS, F. E.
1905. Research methods in ecology. Lincoln, Univ. Pub. Co., pp. xvii,
334, 85 figs. in text.
GRIFFITHS, D.
1910. A protected stock range in Arizona. Bull. No. 177, Bur. Plant
Ind., U. S. Dept. Agr., pp. 28, 6 pls., 1 fig. in text.
GRINNELL, JOSEPH.
1921. Revised list of the species in the genus _Dipodomys_. Journal of
Mammalogy, vol. 2, No. 2, pp. 94-97, May 2.
MCATEE, W. L.
1921. Farm help from the birds. In Yearbook of the U. S. Dept. Agr. for
1920, pp. 253-270; unnumbered figs. in text.
MERRIAM, C. H.
1890. Description of three new kangaroo rats, with remarks on the
identity of _Dipodomys ordii_ of Woodhouse. In North Amer. Fauna No. 4,
Div. Ornith. and Mamm. (Biol. Surv.), U. S. Dept. Agr., 41-49.
NELSON, E. W.
1918. Smaller mammals of North America. Nat. Geog. Mag., vol. 33, No. 5,
pp. 371-493; numerous unnumbered figs. and colored pls. in text.
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