Ecology of the California Ground Squirrel on Grazing Lands

Abstract

[longdash]A population of the ground squirrel (Citellus beecheyi) was studied in 1938, 1939, 1940, 1941, 1942, and 1946 at the San Joaquin Exptl. Range, California, by live-trapping and marking and by direct observation. A total of 1552 squirrels were trapped and examined 15,936 times. 63 kinds of plants were recorded as food, but filaree (Erodium) and acorn (Quercus) made up the bulk of the diet. Quantity of food recorded in several individuals for 143 squirrel-days of feeding in confinement, and 6 in the field under natural conditions, varied widely. 22.5 g. of dry wheat was the avg. requirement in summer, and 115 g. (nearly 80% moisture) was the largest meal taken. In a squirrel enclosure stocked at the rate of 12 per acre, the yield of range vegetation was decreased on the avg. by more than 1,000 lb. per acre annually. On a selected area the population density of squirrels at the annual low point in late winter averaged 2.3 per acre for 5 yrs. Breeding occurs in late winter. Young averaging 7 per litter mature by the following yr., and make up nearly half the breeding population. Individual squirrels generally stay in home ranges 100 to 150 yards in diam., but may move and settle in new areas. Males move about more than females, and young wander more than adults. Territorial-ity is not developed, but fighting is frequent, especially in the breeding season. From June until the latter half of Jan., aestivation and hibernation occur in part of the squirrel population. At this locality some individuals do not become dormant; others, especially adult females, apparently are dormant continuously for 7 months or more. Extent of dormancy varies greatly from yr. to yr., depending on weather and food supply. Principal natural enemies of ground squirrels on the area are the rattlesnake, red-tailed hawk, coyote, badger, and gray fox; 1971 records of their predation on squirrels were obtained. Crudely measured predation factors seem to account for about half the annual increase of the squirrels.