More sociable female eastern grey kangaroos are less likely to wean young (#384)
Several studies of sociable mammals in the wild have demonstrated that social integration and strong bonds with others can be beneficial for females’ reproductive success. However, strong social ties can also reduce reproductive success in certain contexts, such as in high-density environments. The consequences of individuals’ patterns of sociability in gregarious species that form preferential associations in the absence of cooperative behaviours, as is the case for female eastern grey kangaroos (Macropus giganteus), are less well known than for species with cooperative behaviours. We have analysed nearly five years of reproductive and social data from over 120 individually-recognized wild eastern grey kangaroos to test whether measures of individual females’ sociability were related to their reproductive success. We used generalised linear mixed models to explore how sociability indices and lagged rainfall (a proxy for grass abundance) influenced the reproductive outputs and offspring survival of females. Females with larger social networks and those who formed preferential associations with a higher number of other individuals had fewer young that survived to weaning than did less sociable females. Offspring survival to weaning was also higher for females who maintained larger distances to their nearest neighbour during foraging. In contrast to these results, there were no effects of mothers’ sociability on the survival of their young to the large pouch-young stage. Thus female kangaroos’ sociability affected the survival of their young only once the young had begun leaving the pouch, suggesting that this negative effect may have resulted from an increased risk to the young of losing their mothers in higher density situations. Our findings thus suggest that the benefits of sociability in mammals may be affected by density-dependent processes. The maintenance of individual differences in sociability may involve further life-history trade-offs or variation in environmental conditions.