Life History of the Kangaroo Rat Part 1

Life History of the Kangaroo Rat.

by Charles T. Vorhies and Walter P. Taylor.

IMPORTANCE OF RODENT GROUPS.

As the serious character of the depredations by harmful rodents is recognized, State, Federal, and private expenditures for their control increase year by year. These depredations include not only the attacks by introduced rats and mice on food materials stored in granaries, warehouses, commercial establishments, docks, and private houses, but also, particularly in the Western States, the ravages of several groups of native ground squirrels and other noxious rodents in grain and certain other field crops. Nor is this all, for it has been found that such rodents as prairie dogs, pocket gophers, marmots, ground squirrels, and rabbits take appreciable and serious toll of the forage on the open grazing range; in fact, that they reduce the carrying capacity of the range to such an extent that expenditures for control measures are amply justified. Current estimates place the loss of goods due to rats and mice in warehouses and stores throughout the United States at no less than $200,000,000 annually, and damage to the carrying capacity of the open range and to cultivated crops generally by native rodents in the Western States at $300,000,000 additional; added together, we have an impressive total from depredations of rodents.

The distribution and life habits of rodents and the general consideration of their relation to agriculture, forestry, and grazing, with special reference to the carrying capacity of stock ranges, is a subject that has received attention for many years from the Biological Survey of the United States Department of Agriculture. As a result of the investigations conducted much has been learned concerning the economic status of most of the more important groups, and the knowledge already gained forms the basis of the extensive rodent-control work already in progress, and in which many States are cooperating with the bureau. If the work is to be prosecuted intelligently and the fullest measure of success achieved, it is essential that the consideration largely of groups as a whole be supplemented by more exhaustive treatment of the life histories of individual species and of their place in the biological complex. The present report is based upon investigations, chiefly in Arizona, of the life history, habits, and economic status of the banner-tailed kangaroo rat, _Dipodomys spectabilis spectabilis_ Merriam (Pl. I).

INVESTIGATIONAL METHODS.

Some 18 years ago (in 1903) a tract of land 49.2 square miles in area on the Coronado National Forest near the Santa Rita Mountains, Pima County, southern Arizona, was closed to grazing by arrangement between the Forest Service and the Agricultural Experiment Station of the University of Arizona. Since that time another small tract of nearly a section has been inclosed (Griffiths, 1910, 7[1]). This total area of approximately 50 square miles is known as the United States Range Reserve, and is being devoted to a study of grazing conditions in this section and to working out the best methods of administering the range (Pl. II, Fig.

1).

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 Inst.i.tution and the Arizona Agricultural Experiment Station visited the Reserve and were impressed with the evidence of rodent damage to the gra.s.s 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 Inst.i.tution 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.[2] 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.

[Footnote 1: References in parentheses are to the Bibliography, p.

40 (the last figure being to the page of the publication).

References to authorities where no citation of literature is appended relate for the most part to ma.n.u.script notes in the files of the Biological Survey or the University of Arizona Agricultural Experiment Station.]

[Footnote 2: In addition to a.s.sistance rendered by officials of the Biological Survey and the University of Arizona, which is hereby acknowledged, the authors are indebted to the following persons for helpful suggestions and a.s.sistance: G. S. Miller and J. W. Gidley, of the U. S. National Museum; Dr. Frederic E. Clements and Gorm Loftfield, of the Carnegie Inst.i.tution; Morgan Hebard, of the Academy of Natural Sciences of Philadelphia; James T. Jardine and R. L. Hensel, both formerly connected with the U. S. Forest Service; and R. R. Hill, of the Forest Service. They are also indebted to William Nicholson, of Continental, Ariz., for many courtesies extended in connection with work on the Reserve.]

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_).

[Ill.u.s.tration: 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 princ.i.p.ally 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).

[Ill.u.s.tration: 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_).]

[Ill.u.s.tration: 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 compet.i.tion between them as to which shall benefit by what the range has to offer.]

[Ill.u.s.tration: 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.)]

[Ill.u.s.tration: 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_.]

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.