_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.)._