Stress-induced sex ratios in ground squirrels: support for a mechanistic hypothesis

Systematic deviations in sex ratio may be adaptive in the face of the prevailing environmental or physiological cues experienced by the mother; yet some theoretical and mechanistic hypotheses remain at odds and are rarely examined noninvasively under natural conditions. Conventional interpretations of the Trivers and Willard (Trivers RL, Willard DE. 1973. Natural selection of parental ability to vary the sex ratio of offspring. Science 179:90-92) hypothesis (TWH) predict that higher stress should be associated with female-biased litters; yet, several mechanistic hypotheses predict the opposite. We tested the predictions of the TWH and 2 stress-related mechanistic hypotheses in a free-living polygynandrous sciurid, Richardson's ground squirrel, Urocitellus richardsonii. We examined the relationship between litter sex ratio and indicators of maternal condition and investment. These included litter size, juvenile mass at emergence, as well as maternal age, changes in maternal body mass, and maternal fecal glucocorticoid (cortisol and corticosterone) levels during gestation, lactation, and post-weaning. Males born of small litters were significantly heavier at emergence than males from larger litters, which were female biased, whereas females showed no significant change in mass with litter size. Mothers with higher fecal cortisol levels during the gestation period (but not during lactation or post-weaning) were more likely to produce male-biased litters, whereas females producing larger litters showed significantly higher cortisol levels during lactation (but not gestation or post-weaning) than mothers producing smaller litters. Our results provide support for both the proximate glucose-mediated mechanistic model of Cameron (Cameron EZ. 2004. Facultative adjustment of mammalian sex ratios in support of the Trivers-Willard hypothesis: evidence for a mechanism. Proc R Soc Ser B Biol Sci. 271:1723-1728) and the broad-sense TWH, where social and life-history characteristics play an integral role.