Meeting Abstract
38.3 Wednesday, Jan. 5 Struggling mothers, Strong Sons: Optimization and Sex Allocation in Richardson’s Ground Squirrels (Spermophilus richardsonii) RYAN, C.P.*; ANDERSON, W.G.; HARE, J.F.; Simon Fraser University, Canada; University of Manitoba; University of Manitoba calenryan@hotmail.com
Among vertebrates, skewed sex ratios at birth and weaning are generally attributed to sex-differential costs for successfully rearing offspring and to condition-dependent differences in the relative reproductive value of each sex. Theory predicts that systematic deviations in sex ratio may be adaptive in the face of prevailing environmental or physiological cues experienced by the mother, yet scant research has examined this directly in free-living mammals. This study examined adaptive sex allocation in a free-living polygynandrous sciurid, Richardson’s ground squirrel (Spermophilus richardsonii), by examining the relationship between offspring sex ratio and indicators of maternal condition and investment. These included litter size and juvenile mass at emergence, as well as maternal age, changes in maternal body mass, and maternal fecal glucocorticoid metabolite levels during gestation and rearing. Males born of small litters were heavier at emergence than males from larger litters (P < 0.001), which were significantly female-biased (P < 0.050), whereas females showed no significant change in mass with litter size. Mothers with higher fecal cortisol levels during the gestation period (but not during the rearing period) were more likely to produce male biased litters (P < 0.05), whereas females producing larger litters showed significantly higher cortisol levels during rearing (but not gestation) than mothers producing smaller litters (P < 0.01). These results support state dependent optimization and an adapted Trivers Willard hypothesis, whereby litter size and sex ratio are simultaneously optimized in response to maternal condition, and are supported by emerging mechanistic theories for ASA in mammals.
