Life History of the Kangaroo Rat

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).

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. 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.

a¹. Size much larger (hind foot and greatest length of skull more than 42 millimeters); tail tipped with white.

b¹. 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.

b². 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.

a². Size much smaller (hind foot and greatest length of skull less than 42 millimeters); tail not tipped with white.

b¹. Hind foot with four toes
Dipodomys merriami.

b². 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).

Plate II. Fig. 1.—Winter View of Area Inhabited by Kangaroo Rats. 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).

Plate II. Fig. 2.—Kangaroo Rat Country Following Summer Drought. 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.

Plate III. Fig. 1.—Kangaroo Rat Mound (Dipodomys s. spectabilis). 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.)

Plate III. Fig. 2.—Kangaroo Rat Mound (Dipodomys deserti). 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.

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