Population Composition of Richardson's Ground Squirrels
(also known as gophers)
The typical social structure of a population of Richardson's ground squirrels consists of mother-offspring kin clusters with unrelated immigrant males scattered throughout the area. This social structure arises because most males either die or disperse before they reach adulthood, whereas females that survive to adulthood usually reside near their natal site throughout life. Composition of the above-ground population varies with time of the year, according to which animals are in hibernation and which are active. See Annual Activity Cycle.
Richardson's ground squirrels emerge from hibernation early in spring, with males emerging about 2 weeks before females. At emergence, yearling ground squirrels of both sexes are sexually mature and classified as adults. Yearlings outnumber older adult squirrels, with 80-90% of adult males and 60-70 % of adult females in the population being yearling adults. See Survival & Longevity.
A major characteristic of populations of Richardson's ground squirrels is the seasonal change in sex ratio of the population. In spring, the above-ground population initially consists only of adult males. Once females begin to emerge, they quickly outnumber males by more than 3:1. Throughout the mating season, mortality of males greatly exceeds that of females due to fierce intra-sexual competition between males. At the end of the mating season, females outnumber males by 10:1 or more.
Despite the abundance of adult females in the population, males compete vigorously and aggressively for mates. Even though the population sex ratio is female biased, the operational sex ratio is male biased. The operational sex ratio is the ratio of estrous females per breeding male. Because only females in estrus are available for mating, and each female is in estrus for only 1-3 hours on a single afternoon of the year, estrous females are a commodity in short supply. When viewed in this manner, males outnumber estrous females on almost all days of the mating season.
After the emergence of litters in May, the population includes all age and sex classes of squirrels for a short time. Then, adult male Richardson's ground squirrels enter hibernation in early June, leaving only adult females and juveniles in the active above-ground population. Adult females enter hibernation about 2 weeks after males, usually in late June and early July, and normally all adult Richardson's ground squirrels are in hibernation by late July. This now leaves juveniles as the only active squirrels in the population. Juvenile females enter hibernation in August, but their brothers stay above ground until October.
Sources: PDFs of many of these articles can be downloaded from the Michener Publications page
- Michener, G. R. 1998. Sexual differences in reproductive effort of Richardson's ground squirrels. Journal of Mammalogy, 79: 1-19.
- Michener, G. R. and I. G. McLean. 1996. Reproductive behaviour and operational sex ratio in Richardson's ground squirrels. Animal Behaviour, 52: 743-758.
- Michener, G. R. 1990. Use of body mass and sex ratio to interpret the behavioral ecology of Richardson's ground squirrels. pp. 304-338 in Interpretation and explanation in the study of behavior. (eds. M. Bekoff and D. Jamieson). Westview Press, Boulder.
- Michener, G. R. 1983. Spring emergence schedules and vernal behavior of Richardson's ground squirrels: why do males emerge from hibernation before females? Behavioral Ecology and Sociobiology, 14: 29-38.
- Michener, D. R. and G. R. Michener. 1971. Sex ratio and interyear residence in a population of Spermophilus richardsonii. Journal of Mammalogy, 52: 835.