The partitioning of reproduction in a group, or reproductive skew, is affected by multiple ecological, genetic, and population parameters. The ways in which these factors interact to determine reproductive skew have been formally modeled, but few attempts have been made to assess the utility of skew theory in advancing our understanding of primate social systems and observed levels of skew. In the study presented here, I demonstrate how reproductive skew among male Hanuman langurs (Semnopithecus entellus) can be calculated using paternity data. In contrast to expectations, skew was found to increase with the number of males in a group. In a further analysis, I estimated skew for 21 populations of Asian langurs and investigated the ecological and population factors influencing variation in this parameter. The results lend support to model predictions that future mating prospects, as measured in chances of individual survival and number of potential mates, can be a significant predictor of skew. Furthermore, increased population density positively affected skew, which may reflect ecological constraints on dispersal for subordinates. These results suggest that skew theory may offer a useful framework for understanding the effects of multiple factors determining variation in reproductive skew.
One of the key aims of studies in evolutionary biology, behavioral ecology, and primatology is to analyze the factors influencing variation in individual fitness and associated behavioral traits (e.g. Clutton-Brock & Harvey 1979, Krebs & Davies 1993). The interactions between key genetic and ecological parameters and features of animal societies were formally modeled in the late 1970s in the pioneering work of Sandra Vehrencamp, who developed a model of the degree to which fitness can be biased among group members (Vehrencamp 1979, 1983a, 1983b).