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.

Mortality in relation to season, age, sex, and reproduction in Columbian ground squirrels (<i>Spermophilus columbianus</i>)

“…This result is consistent with that in Richardson's ground squirrels, for which mortality during hibernation was low and similar in both sexes (Michener and Locklear 1990). We did not, however, find increased mortality in females during lactation compared with mating and gestation as we predicted.…”

“…The much higher male disappearance between emergence from their first and second hibernation is likely due to the male-biased dispersal found in Columbian ground squirrels (Boag and Murie 1981;Festa-Bianchet and King 1984;Wiggett and Boag 1993) rather than to mortality per se. A tendency for males to die younger than females has been shown in Richardson's and Belding's ground squirrels and black-tailed prairie dogs (Cynomys ludovicianus) (Sherman and Morton 1984;Michener and Locklear 1990;Hoogland 1995). In our study, however, male Columbian ground squirrels after the age of dispersal do not have higher mortality than females up to the age of 4 (Table 1).…”

“…These differences are not only due to the male-biased dispersal (Boag and Murie 1981;Festa-Bianchet and King 1984) but might also reflect high reproductive costs during the mating season (Neuhaus et al 1999). In Richardson's ground squirrels (Spermophilus richardsonii), adult mortality was higher in males than in females during the mating season, active season, and the whole year (Michener and Locklear 1990;Michener 1998). Trade-offs between predation risk and foraging have been shown to exist in Belding's ground squirrels (Spermophilus beldingi), where well-fed animals showed more vigilance than deprived individuals (Bachman 1993).…”

Reproduction at All Costs: The Adaptive Stress Response of Male Arctic Ground Squirrels

“…In many ground squirrel species, the breeding period may last no more than 2-3 weeks; during this period, males roam widely, sustain more injuries, eat less, and lose more mass than females. This intense mating period is followed by higher mortality rates immediately after breeding in males than in females (Morton and Sherman 1978, McLean and Towns 1981, Michener and Locklear 1990, Michener and McLean 1996. Our objective was to examine postmating survival in male arctic ground squirrels (Spermophilus parryii plesius) and to test the hypothesis that breeding males during the mating period exhibit a stress response that may compromise STRESS IN MALE ARCTIC GROUND SQUIRRELS their survival, whereas males at other times (nonreproductive adult males and juvenile males in late summer) have a normal stress response.…”

High Spring Mortality of Adult Richardson’s Ground Squirrels, <em>Urocitellus richardsonii</em>, Associated with a Severe Rainstorm in Southwestern Saskatchewan

“…It is unlikely that the disappearance rate observed in spring 2010 was due to dispersal because, at this time of year, Richardson's Ground Squirrels do not disperse (Michener and Locklear 1990) and the fields surrounding the study plots were also flooded. It is also unlikely that predators killed many Richardson's Ground Squirrels during the rainstorm.…”