Higher sociability leads to lower reproductive success in female kangaroos.

C S Menz,R G Dwyer,E C Best,A J Carter,A W Goldizen,S P Blomberg,N J Freeman

doi:10.1098/rsos.200950

Abstract

In social mammals, social integration is generally assumed to improve females' reproductive success. Most species demonstrating this relationship exhibit complex forms of social bonds and interactions. However, female eastern grey kangaroos (Macropus giganteus) exhibit differentiated social relationships, yet do not appear to cooperate directly. It is unclear what the fitness consequences of such sociability could be in species that do not exhibit obvious forms of cooperation. Using 4 years of life history, spatial and social data from a wild population of approximately 200 individually recognizable female eastern grey kangaroos, we tested whether higher levels of sociability are associated with greater reproductive success. Contrary to expectations, we found that the size of a female's social network, her numbers of preferential associations with other females and her group sizes all negatively influenced her reproductive success. These factors influenced the survival of dependent young that had left the pouch rather than those that were still in the pouch. We also show that primiparous females (first-time breeders) were less likely to have surviving young. Our findings suggest that social bonds are not always beneficial for reproductive success in group-living species, and that female kangaroos may experience trade-offs between successfully rearing young and maintaining affiliative relationships.

Highlights

Group living, or sociality, is thought to evolve when the net benefits of associating with conspecifics outweigh the costs [1]
By contrast to most studies on other mammals, we found robust negative relationships between multiple dimensions of females’ sociability and their reproductive success, while controlling for other factors including a female’s parity
By using a combination of social network analysis (SNA) techniques and analyses of social preferences, we were able to extend this earlier finding to show that female kangaroos with (i) larger social networks, (ii) greater numbers of preferred associates and (iii) larger median group sizes produced fewer young that survived to weaning

Summary

Introduction

Sociality, is thought to evolve when the net benefits of associating with conspecifics outweigh the costs [1]. Group living, the fitness costs and benefits of differentiated social relationships—where individuals associate 2 with differing degrees with different conspecifics—have only recently received attention [3]. We use the term ‘sociability’ as an overarching term describing individual animals’ social patterns, including two quite different components. The second, ‘differentiated social relationships’ describes an individual’s direct and indirect relationships with particular conspecifics and can be quantified using a range of social network metrics and measures of relationship preferences. Differentiated social relationships can arise from individuals’ preferences for or avoidances of particular other conspecifics, but may reflect pairs of individuals spending more or less time together because of patterns of home range overlap. Conspecifics vary consistently in their patterns of both gregariousness and differentiated social relationships [4,5] and this variation is known to be heritable in other species [6,7], suggesting that these traits can be a target for selection

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