Male-biased sex ratio in litters of Alpine marmots supports the helper repayment hypothesis

In a French population of Alpine marmots (Marmota marmota), the sex ratio at weaning was biased in favor of males. This bias also seemed to exist at birth. Under Fisher's equal allocation principle, this means that daughters should be more costly to produce than sons. Because the Alpine marmot can be considered a cooperative breeding species, we investigated whether the differential cost between sons and daughters may be explained by the helper repayment hypothesis. The Alpine marmot uses social thermoregulation during hibernation, allowing juveniles to better survive over winter. In the study population, juvenile survival during winter increased with group size. More precisely, juvenile survival during winter increased with the number and with the proportion of subordinate males in the hibernating group, but juvenile survival did not depend on the number of subordinate females. As our results did nut support alternative hypotheses to explain the observed bias in sex ratio among offspring at emergence, we conclude that the helper repayment hypothesis is the best candidate to explain the observed offspring sex ratio bias in Alpine marmots. By participating in social thermoregulation, subordinate males may repay part of the investment they received from their parents and thus become less costly to produce. We suggest that only subordinate males helped because they may gain direct fitness benefits, whereas subordinate females may only expect indirect fitness benefits from helping. Finally, the offspring sex ratio per individual parent was male biased, but mothers adjusted the size and the sex composition of their litters according to their phenotypic condition as expected from the Trivers-Willard hypothesis.