Part 2

For some years an intensive study of the forage and other vegetative conditions of this area has been made, the permanent vegetation quadrat, as proposed by Dr. F. E. Clements (1905, 161-175), being largely utilized. During the autumn of 1917 representatives of the Carnegie Institution and the Arizona Agricultural Experiment Station visited the Reserve and were impressed with the evidence of rodent damage to the grass cover. The most conspicuous appearance of damage was noted about the habitations of the banner-tailed kangaroo rat (Dipodomys spectabilis spectabilis Merriam), although it was observed also that jack rabbits of two species (Lepus californicus eremicus Allen and L. alleni alleni Mearns), which were very abundant in some portions of the reserve, were apparently affecting adversely the forage conditions in particular localities. Accordingly, the Biological Survey, the Agricultural Experiment Station of the University of Arizona, the Carnegie Institution of Washington, and the U. S. Forest Service have undertaken a study of the relation of the more important rodents to the forage crop of the Range Reserve in Arizona.

The present paper is a first step in this larger investigation. In this work the authors have made no attempt to deal with the taxonomic side of the kangaroo rat problem. It is not unlikely that intensive studies will show that the form now known as Dipodomys spectabilis spectabilis is made up of a number of local variants, some of them perhaps worthy of recognition as additional subspecies. But it is felt that the conclusions here reached will be little, if at all, affected by such developments.

Color descriptions are based on Ridgway's Color Standards and Color Nomenclature published in 1912.

IDENTIFICATION.

There are only three groups of mammals in the Southwest having external cheek pouches. These are (a) the pocket gophers (Geomyidae), 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).

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).

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).]

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.]

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.)]

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.]

DESCRIPTION.

GENERAL CHARACTERS.

Size large; ears moderate, ear from crown (taken in dry skin) 9 or 10 millimeters; eyes prominent; whiskers long and sensitive; fore feet short and weak; hind feet long and powerful, provided with four well-developed toes; tail very long, usually 30 to 40 per cent longer than the body. Cranium triangular, the occiput forming the base and the point of the nose the apex of the triangle, much flattened, auditory and particularly mastoid bullae conspicuously inflated.

COLOR.

General color above, brownish buffy, varying in some specimens to lighter buffy tints, grizzled with black; oblique hip stripes white; tail with dark-brown or blackish stripes above and below, running into blackish about halfway between base and tip, and with two lateral side stripes of white to a point about halfway back; tail tipped with pure white for about 40 millimeters (Pl. I). Underparts white, hairs white to bases, with some plumbeous and buffy hairs about base of tail; fore legs and fore feet white all around; hind legs like back, brown above, hairs with gray bases, becoming blackish (fuscous-black or chaetura-black) about ankles, hairs on under side white to bases; hind feet white above, dark-brown or blackish (near fuscous) below.

Color variations in a series of 12 specimens from the type locality and points widely scattered through the range of spectabilis consist in minor modifications of the degree of coloration, length of white tip of tail, and length of white lateral tail stripes. In general the color pattern and characters are remarkably uniform. Young specimens, while exhibiting the color pattern and general color of adults, are conspicuously less brown, and more grayish.

There appears to be little variation in color with season. In the series at hand, most specimens taken during the fall, winter, and spring are very slightly browner than those of summer, suggesting that the fresh pelage following the fall molt is a little brighter than is the pelage after being worn all winter and into the following summer. But at most the difference is slight.

OIL GLAND.