Males matter: Increased home range size is associated with the number of resident males after controlling for ecological factors in wild Assamese macaques

Objectives: There is increasing evidence of male resource defense during intergroup encounters in non-human primates. Only few studies showed a reproductive benefit of having more males in a group, and evidence only comes from territorial species, or from species with relatively small male group sizes where males are less prone to suffer from collective action problems. We investigated the effect of male group size on home range size and female reproductive success in a non-territorial species with male dispersal and large male group sizes. Methods: We studied one wild group of Assamese macaques (Macaca assamensis) by following them almost daily (June 2006–September 2012) and collected spatial, behavioral, climate and spatiotemporal data on food plants. Results: Among ecological factors, decreasing rainfall and a statistical interaction between food abundance and distribution were positively related to home range size. After controlling for ecological predictors, we found that male group size but not overall group size had a significant positive effect on full and core home range size. A simple correlation analysis suggests that such an increase in home range area, presumably increasing access to food resources, can be associated with increased female fecundity measured as the proportion or the number of females conceiving in a given year. Discussion: Within the limitations of this study, the results suggest that male resource defense could be a strategy benefitting both sexes if male reproductive skew was low and many males benefited from increased female fertility. Am J Phys Anthropol 159:52–62, 2016.