Social Behavior and Social Organization in an Unconfined Population of Uinta Ground Squirrels

Social interactions and relationships in an unconfined population of Uinta ground squirrels were studied for two years following a 60% reduction in population size. Frequencies and patterns of interactions were evaluated in the context of the breeding cycle and compared to similar data collected before the population reduction. Males defended territories in early spring, within which they courted females. They were highly aggressive toward other males. After the first month of activity, male aggressiveness declined and they occupied individual home ranges for the remainder of the year. Females were tolerant of other squirrels until early in pregnancy, when they became intolerant and established territories of their own. All other squirrels were excluded. Yearling females emerged later than adults, were bred later, and became territorial later. Females maintained territories through gestation and lactation, until the young appeared above ground. Juveniles interacted chiefly with siblings for the first three weeks after emergence. Nuzzles and play bouts were the primary forms of interaction observed. As older juveniles gradually expanded their home ranges, they began to encounter other squirrels. Most such interactions were aggressive. Some juveniles disappeared, apparently due to dispersal. Males were more likely to disperse than females. Frequency of aggressive encounters in the population was affected more by the time of season than by population density per se. Still, there was generally a lower frequency of aggressive encounters under low density conditions. Not all age-sex groups were affected uniformly. Adult and yearling males were involved in encounters less frequently at low density. Late in the season, females of all ages and juvenile males were involved in encounters more frequently despite lower density. Frequency of sexual encounters per initiating male was higher at low density for the last two months of the year, due primarily to unusual yearling activity. At low density there was a fundamental change in the social role of yearling males in the population. Where at high density they were non-breeders and entirely subordinate, yearlings now competed for and courted females, and defended territories against other males. Late in the 1970 season, the exceptionally vigorous yearling male cohort harassed juveniles aggressively and sexually, causing increased dispersal of juvenile males and retarded sexual development of males returning in 1971 as yearlings. Activity of a single cohort at low density thus caused social stresses previously noted only at high density