Higher sociability leads to lower reproductive success in female kangaroos

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.

Keywords: animal personality; fitness; parity; reproductive success; social bonds; social networks.

Figure 1.

The effects of female sociability on the probability of weaning a young for Approach 1. Predicted probabilities (solid lines) and 95% confidence intervals (shading) of a multiparous female kangaroo successfully weaning a young in each of the 4 years from 2010 to 2013 against standardized strength scores. Frequencies of observed weaning outcomes (0 = failure, 1 = success) for individual females are shown in the histograms.

Figure 2.

The effects of female sociability on the probability of weaning a young for Approach 2. Predicted probabilities (solid lines) and 95% confidence intervals (shading) of a multiparous female weaning a young in the second year of a 2-year period (period A = 2010–2011, period B = 2011–2012, period C = 2012–2013) against (a) standardized strength scores, (b) number of preferred associates, (c) standardized group size residuals. Frequencies of observed weaning outcomes (0 = failure, 1 = success) for individual females are shown in the histograms.