y 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