SEXUAL DIFFERENCES IN REPRODUCTIVE EFFORT OF RICHARDSONS GROUND-SQUIRRELS

Male and female Richardson's ground squirrels exhibit differences in a constellation of behavioral, ecological, and physiological characteri stics related to a fundamental sexual difference in reproductive effor t, viz. males primarily expend energy on mate acquisition whereas fema les primarily expend energy on parenting. Male Richardson's ground squ irrels 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 acces s to estrous females with resultant physical wounding, loss of body ma ss, depletion of fat reserves, and high mortality. Females retain post -hibernation fat reserves during gestation and even increase in person al body mass. Although mass of the litter exceeds the mother's own mas s by late lactation, mothers are largely able to meet the demands of m ilk production through foraging, and their body mass remains fairly co nstant through lactation. Adult males immerge into hibernation earlier than adult females, and are heavier and fatter on immergence. In thei r first active season, juvenile males complete growth to adult size, w hereas juvenile females attain only 80% of adult size; females then co mplete growth during pregnancy the following spring. Juvenile males at tain adult size by delaying entry into hibernation until 7-9 weeks lat er 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 ter minate torpor >1 week before emergence, during which time they subsist on seeds cached in the hibernaculum, replenish fat, and initiate sper matogenesis in preparation for the forthcoming mating season. In contr ast, female Richardson's ground squirrels do not store food in the hib ernaculum, they emerge from hibernation <2 days after termination of t orpor, and they are impregnated 2-4 days after emergence from hibernat ion when at their lowest annual body mass. On an annual basis, adult f emales have a higher survival rate than adult males, largely because f emales are more likely to survive through the reproductive period.