Sex allocation in a colobine monkey

In polygynous, sexual dimorphic species with higher variance in male reproductive success compared with females, females are expected to invest more heavily in sons than daughters within the constraints imposed by their physical condition (Science 1973; 179:90). Mothers in good condition, usually those of high rank, should produce more sons than females in poor condition or of low rank. We investigated sex allocation and sex-biased maternal investment in a population of wild Hanuman langurs using rank and group size as approximations of female physical condition. Our results show that reproductive costs of sons were higher with both significantly longer interbirth intervals following male births and longer lactational periods for sons. Not in all groups did analyses of rank-dependent sex allocation reveal the expected pattern of high-ranking mothers producing more sons. However, sex ratio was significantly influenced by group size, with females from larger groups, i.e., in worse physical condition, producing a daughter-biased sex ratio. In fact, only females of population-wide superior physical condition can be expected to produce sons, because in Hanuman langurs males disperse and compete population-wide. Thus, our results support the Trivers-Willard model and may explain the mixed evidence accruing from studies of single groups. We present a graphical model of how group size and dominance-related differences in energy gain may influence sex allocation under different competitive regimes relative to overall resource availability. Tests of adaptive sex allocation models should consider whether reproductive competition of the preferred sex takes place primarily within a group or within the population.