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.]