Proximate and ultimate explanations of mammalian sex allocation in a marsupial model

Offspring sex ratios in mammals vary in potentially adaptive yet unpredictable ways. An integrative approach that simultaneously examines proximate and ultimate explanations of mammalian sex ratios would greatly advance the field. We examined the importance of maternal glucose and stress hormones for offspring sex (male or female) as mechanisms associated with the Trivers–Willard and the local resource competition hypotheses of sex allocation. We tested this framework in a marsupial mammal, the tammar wallaby (Macropus eugenii). Mothers that were better able to maintain body condition over the driest part of the year, a presumptive proxy for local resource availability, were more likely to produce daughters (the philopatric sex), consistent with local resource competition. Maternal glucose was correlated with offspring sex, but in the opposite direction than we predicted—higher maternal glucose was associated with female pouch young. These patterns, however, were not consistent across the 2 years of our study. Maternal stress hormone metabolites measured from fecal samples did not predict glucose or offspring sex. A causative glucose mechanism may underlie an adaptive strategy for mothers with high local resources (high glucose) to produce philopatric daughters that will benefit from inheriting resource access. Examining species-specific relationships between glucose and offspring sex across mammals could provide crucial insight into the disparate ecological and selective pressures faced by mammals with respect to offspring sex ratio.