Sexual Differences in Reproductive Effort of Richardson's Ground Squirrels

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Abstract
Male and female Richardson's ground squirrels exhibit differences in a constellation of behavioral, ecological, and physiological characteristics related to a fundamental sexual difference in reproductive effort, viz. males primarily expend energy on mate acquisition whereas females primarily expend energy on parenting. Male Richardson's ground squirrels emerge from hibernation earlier than females, at a heavier body mass, and with larger fat reserves. Once females emerge, males reduce time foraging and they compete aggressively and intensively for access to estrous females with resultant physical wounding, loss of body mass, depletion of fat reserves, and high mortality. Females retain post-hibernation fat reserves during gestation and even increase in personal body mass. Although mass of the litter exceeds the mother's own mass by late lactation, mothers are largely able to meet the demands of milk production through foraging, and their body mass remains fairly constant through lactation. Adult males immerge into hibernation earlier than adult females, and are heavier and fatter on immergence. In their first active season, juvenile males complete growth to adult size, whereas juvenile females attain only 80% of adult size; females then complete growth during pregnancy the following spring. Juvenile males attain adult size by delaying entry into hibernation until 7–9 weeks later than juvenile females. During hibernation, both juvenile and adult male Richardson's ground squirrels spend substantially less time than females in the physiological state of torpor. In particular, males terminate torpor >1 week before emergence, during which time they subsist on seeds cached in the hibernaculum, replenish fat, and initiate spermatogenesis in preparation for the forthcoming mating season. In contrast, female Richardson's ground squirrels do not store food in the hibernaculum, they emerge from hibernation <2 days after termination of torpor, and they are impregnated 2–4 days after emergence from hibernation when at their lowest annual body mass. On an annual basis, adult females have a higher survival rate than adult males, largely because females are more likely to survive through the reproductive period.

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