Do Female Yellow-Bellied Marmots Adjust the Sex Ratios of Their Offspring?

The overall sex ratio of weaned yellow-bellied marmots does not differ significantly from one. Litter size has no effect on the sex ratio of young. Stress, measured by eosinophil concentration and mirror-image stimulation, is not associated with biased sex ratios. Three-year-old females and females living in matrilines of two produce more daughters than expected. Females whose age is below the mean produce female-biased litters. Young females that are members of matrilineal social groups produce significantly more daughters than sons. By contrast, the lifetime sex ratios of young produced by subordinate females is malebiased. These results are contrary to those predicted from the model of Trivers and Willard and the model of local resource competition. The patterns of sex-ratio variation in marmots, primates, red deer, Antechinus, and white-tailed deer suggest that females produce the sex that has a higher probability of future reproductive success, regardless of the costs of producing individuals of that sex.