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_.]