Although the sex ratio of male and female Richardson’s ground squirrels is 1:1 at birth and at weaning, males experience higher mortality than females thereafter such that the sex ratio among adults is strongly biased toward females, often with 3 to 5 times as many females as males. Most young males do not survive to adulthood, and those that do have less than a 50% chance of surviving another year. Females have higher survival rates than males throughout life so they have greater longevity, some living 3 or 4 years and a few living for 5 years. (see Survival and Longevity) This sexual difference in life spans can be used as an indicator for the different life strategies of the two sexes, with females being conservative and remaining near their natal territory whereas males pursue the high risk strategy of dispersing (see Dispersal) and searching for mates.
Because most sons either die or disperse before reaching adult age whereas many daughters live near their natal site throughout life, the social structure of a population consists of mother-offspring kin clusters with unrelated immigrant males scattered throughout the area. (see Kinship and Social Organization)
Ground squirrels emerge from hibernation in the spring, with males emerging before females (see Hibernation). Because they are reproductively mature on emergence from their first hibernation yearling ground squirrels are classified as adults. Yearlings outnumber older adult squirrels, with 80-90% of adult males and 60-70% of adult females in the population being of yearling age. data
In addition to the bias towards the yearling age class, another major trend characteristic of Richardson's ground squirrels is the seasonal change in sex ratio of the population. When males emerge from hibernation in spring all females are still underground in hibernation, so the above-ground population initially consists only of adult males. (see Hibernation) Once females begin to emerge they quickly outnumber males by more than 3:1. As the mating season progresses the mortality of males greatly exceeds that of females, so females outnumber males at the end of the mating season by 10:1 or even more. Adult male Richardson’s ground squirrels enter hibernation before adult females, so the above-ground population of adults eventually includes only females by July. Then the adult females enter hibernation, so the only animals active above ground by August are the juveniles. data
Despite the abundance of adult females in the population, adult males compete vigorously and aggressively for access to mates. This intense male-male competition for mating opportunities can be interpreted in the context of the operational sex ratio. The operational sex ratio is the ratio of estrous females per breeding male, as it is only the estrous females that are available as potential mates. Thus, it is a measure of the number of receptive females available per male. Because a female Richardson’s ground squirrel is in estrus for only 2-3 hours on just one afternoon a year, estrous females are actually a commodity in short supply. When viewed in this manner, males outnumber estrous females on almost all days of the mating season. data In other words, the population sex ratio is female biased but the operational sex ratio is male biased.
These trends in population sex ratio
and operational sex ratio are not reflected in the litters, which have
an average 1:1 ratio of males to females. (see Reproductive
Physiology) However, because juvenile females hibernate before juvenile
males, by September the above-ground population consists only of juvenile
males.