Natural History of the Ground Squirrels of California/Yuma Round-tailed Ground Squirrel
Other names.—Round-tailed Spermophile; Yuma Ground Squirrel; Mohave Desert Ground Squirrel, part; Spermophilus tereticaudus; Xerospermophilus tereticaudus; Citellus tereticaudus mohavensis, part.
Field characters.—A small ground squirrel, of slender build, and of pale brown color (no stripes or other markings); tail long and slender, not broadly haired; ears very small, mere rims; length of body without tail about 6 inches, with tail about 3¾ inches more.
Description.—Adult in winter pelage: Whole upper surface from nose to and including tail, light pinkish cinnamon in general tone; individual hairs lead-colored at extreme bases, then dull white, then pinkish cinnamon, and with tipping of white. Eyelids and whole lower surface to root of tail, white; side of head and neck including ear dull white; whiskers black; hairs of belly lead-colored at extreme bases. Upper surfaces of feet dull buffy white; claws dark brown basally, becoming horn-color at tips; soles of feet haired save for under sides of toes. Tail cylindrical in shape, a little more heavily haired toward end than at base; under side dull buff, with black mottlings in fine pattern toward end; above like back on basal half, becoming black and buff mottled toward end; hairs at tip of tail brown at their bases, then buff, then broadly black, and with white ends.
Adult in summer pelage: Coat short and harsh as compared with winter coat; color above brighter pinkish cinnamon. Otherwise as in winter, but the tail, which apparently does not molt, pale brown and still slenderer, due to the fading out of the dark colors and to the wear and consequent shortening of the hairs.
Color variations.—Young partly grown are colored like summer adults, but the pelage is not quite so harsh and on the under surface is so sparse as to allow the bare skin to show through in places. There are two molts in the adults each year and two distinct pelages separated by these. The spring molt occurs during April (March 28 to May 1 according to specimens at hand), and the fall molt probably during October though there are no specimens available to show its extent. The remnants of the winter pelage during the spring molt become faded in some specimens to a dull yellowish tone. This molt advances in a general way from the front backward, but specimens often show a patchy or mixed coat on the back and rump.
Measurements.—Average and extreme measurements, in millimeters, of seventeen adult specimens from the Colorado and Imperial valleys are as follows: seven males: total length, 247 (225–261); tail vertebræ, 95 (85–107); hind foot, 36 (34–37); greatest length of skull, 37.1 (35.2–38.2); zygomatic breadth, 23.5 (22.5–24.4); interorbital width, 8.9 (8.3–9.5). Ten females: total length, 241 (216–258); tail vertebræ, 91 (75–102); hind foot, 36 (33–38); greatest length of skull, 36.0 (34.3–38.2); zygomatic breadth, 22.2 (21.3–23.4); interorbital width, 8.6 (8.0–9.5).
Males are seen to be slightly larger than females. The ears in this species are small, the rims rising not more than 3 millimeters (? inch) above their inner base. In but few specimens did the collector attempt to secure the measurement of the ear.
Weights.—Stephens (MS) found two females and a male to weigh together 12 ounces, an average of 4 ounces each. "All were thin."
Type locality.—Fort Yuma [Imperial County], California (Baird, 1857, pp. 315–316).
Distribution area.—Low-lying sandy areas on the Colorado and Mohave deserts. Life-zone, Lower Sonoran (see fig. 23). More specifically: the Imperial Valley west as far as La Puerta (Mus. Vert. Zool.) in extreme eastern San Diego County, north to the southern end of Salton Sea, and east to old Fort Yuma; thence north along the Colorado River nearly or quite to the Nevada line; and from the vicinity of Needles and Blythe, in the Colorado Valley, northwestward across the central part of the Mohave Desert to at least as far as Kramer (Grinnell, MS), in west-central San Bernardino County. Altitudes of occurrence, from 200 feet below sea-level to 2,300 feet above. The range of this species is not continuous over the area just indicated (see fig. 18), but consists of many colonies more or less distantly isolated from one another.
Specimens examined.—A total of 28 from the following localities in California: San Bernardino County, one-half mile north of Barstow, 1; Daggett, 1; Blythe Junction, 4; Needles, 3. Imperial County: south end of Salton Lake, 6; six miles south of Holtville, 2; Coyote Well, 4; Pilot Knob, 4; Colorado River opposite Cibola, 2. San Diego County: La Puerta, 1.
The Yuma Round-tailed Ground Squirrel was first made known to science in 1857 from specimens taken by an army officer stationed at old Fort Yuma, which was situated on the California side of the Colorado River opposite the present town of Yuma. It inhabits the hottest of our southeastern desert valleys. Its metropolis lies in the Imperial Valley and thence north along the valley of the Colorado; a few colonies occur also on suitable parts of the Mohave Desert. Over this general region the species is by no means continuously distributed. It seems to be very particular in its requirements, only level ground of a sandy nature being as a rule inhabited at all. Places are preferred where wind-drifted sand has been accumulated into mounds about the bases of mesquite, creasote bushes, or salt bushes. Here the burrows are to be seen opening up among the stems of the partly buried shrubs; and the animals, if not actually seen themselves, are shown by their tracks in the sand surface to be in the habit of foraging out across the bare intervals for the seeds which are to be found sifted among the sand particles.
The squirrels themselves are usually shy and by reason of their obscure coloration and especially the shimmering glare on the desert surface are not readily observed unless particularly sought for. Neither are their total numbers very great even where conditions are fairly favorable. While our observations show them to be strictly diurnal in habits, we have noted an apparent aversion to direct sunshine. Perhaps this is because the sunshine on the desert is in summer so intense as to be quickly fatal to any small animal exposed to it for long. We know for a fact of squirrels caught in traps by one foot or merely a toe, on open ground, which have quickly succumbed–"sun-cooked" is the
Fig. 23. Diagram showing the ranges of the ground squirrels of California according to life-zones. (For life-zone map of California, see Grinnell, 1913, pl. 15.)
term we use for such victims. At any rate, the squirrels are seen crossing open spaces but momentarily, and thenceforth they remain in the shade of bushes until they take final alarm and descend into their burrows.
The long, slender, ratlike tail is exclusively characteristic of this species of ground squirrel. The body, too, is rather slender, though after a full meal of green stuff individuals have been seen which showed a rather pot-bellied outline. The mere rims of ears give the animal a round-headed look. The movements are rather more agile than in most of its relatives. It not infrequently climbs up into bushes to a height of four or five feet, but here it becomes clumsy.
The voice of the Round-tailed Ground Squirrel is unmistakable when once learned. As far as known to us, but one kind of note is uttered, a single high-pitched squeak or shrill whistle, seep, uttered only at rather long intervals, never in a series as with some others of the ground squirrels. The quality of this call is such that the direction from which it emanates is difficult to fix; also the distance is hard to determine. It seems to be given as a warning by an individual, either located within the mouth of its burrow (Stephens, 1906, p. 70) or when standing motionless under a bush. C. L. Camp (MS) records that he has seen an individual, when its curiosity was aroused, stand high up on its hind legs and utter its "sharp squeak" with the mouth wide open, at the same time "giving the thorax a violent contraction."
Facts in regard to the breeding of this squirrel are shown in the following data. On March 15 (1914) near Barstow a male Round-tailed Ground Squirrel was seen abroad which proved astonishingly indifferent. "It came up to where we were digging out a kangaroo rat colony, smelling into various burrows, evidently intently hunting for a female. The testes of this animal were enormous, dragging on the ground behind it as it waddled along" (Grinnell, MS). Two females captured in the valley of the Colorado River opposite Cibola, April 3 and 4 (1910), were found to contain six and four embryos, respectively; young about half grown were taken at Needles July 15 and 19 (1909) (Grinnell, 1914, p. 224). Stephens (1906, p. 70) says that the breeding season falls in March and April and that the number of young in a litter is four to seven. In spite of the long hot period each year in the habitat of this species, there is no evidence to show that more than one litter is reared annually.
There is a period of inactivity during midwinter, when these animals are not seen abroad. Whether or not there is regular hibernation, as with the species of colder regions, we do not definitely know; but this, seems to be the case.
"The food is seeds the greater part of the year; these are stored to some extent. In the spring, during the few weeks when green vegetation is obtainable, leaves and buds are eaten voraciously" (Stephens, 1906, p. 70). In our experience, stems of the squaw-tea (Ephedra) and leaves of the mesquite form an important element of the diet wherever and whenever obtainable.
In June, 1918, W. C. Jacobsen (MS) found several colonies of Yuma Round-tailed Ground Squirrels in the Imperial Valley within five or six miles south of Holtville. The interesting thing was that here the animals were invading the cultivated fields and were finding alfalfa suited to their tastes. Individuals were seen to eat the leaves of the alfalfa with avidity, but left the stems uneaten. Many dry stems were found lying about near the mouths of their burrows. At another point, near Bond Corners, oat hulls were seen around burrows.
There is a possibility, therefore, that this strictly desert rodent might come to have an economic bearing on the reclaimed sections of the desert. Whether or not it will become a serious pest remains to be seen. One note of interest in this connection is furnished by Stephens (MS), who says that at Silsbee, while occasionally getting into the fields, the Round-tails were easily drowned out. As far as known to us, this species never drinks water even when within reach of it. In most parts of its range and at most seasons of the year water is secured only through chemical elaboration from its dry or nearly dry food materials. It is currently reported by the farmers in Imperial Valley that the native desert animal life quickly disappears when the land is brought under cultivation and especially irrigation.