Differential Costs of Reproductive Effort for Male and Female Richardson's Ground Squirrels

Abstract

The costs of reproductive effort for adult male and female Richardson's ground squirrels were compared to determine whether these costs differed in timing or magnitude in a manner related to sexual differences in mating and parental effort. Reproductive effort was assessed from 1982 to 1986 for a population of Richardson's ground squirrels in southern Alberta, Canada, by monitoring seasonal mortality schedules, fecundity, and changes in body mass and fat content. Although the adult sex ratio was female—biased, the operational sex ratio was male—biased. Males lost mass and sustained injuries as a result of male—male conflict during the mating season. From 50 to 79% of males disappeared annually during reproduction (between spring emergence and the end of the mating season), whereas <20% of females disappeared during reproduction (between spring emergence and litter emergence). Overwinter mortality rates were similar for adult males and females. Active seasons of adult males and females were of similar durations, but because reproductive effort finished earlier for males, they had a longer period in which to prepare for hibernation, and they entered hibernation (immerged) with larger fat stores. Juvenile males had a longer active season and attained a greater proportion of adult structural size than juvenile females (90 vs. 75%). Although males emerged from hibernation with more residual fat than females, fat stores were rapidly depleted during mating 2—4 wk postemergence, whereas females did not deplete residual fat until late lactation 7—8 wk postemergence. Survival rates of females that did and did not wean litters did not differ significantly, and survival rates of mothers were not significantly or negatively correlated with litter size or litter mass. Although litter size tended to regress toward the mean in consecutive years of reproduction, neither size nor mass of litter weaned negatively influenced litter size or mass in the subsequent year. Depletion of larger stored fat reserves and lower survival rate during reproduction indicate that, from both energetic and survival perspectives, mating effort is more costly for male Richardson's ground squirrels than parental effort is for females.