Higher sociability leads to lower reproductive success in female kangaroos

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 social
preferences, yet do not appear to cooperate directly. It is unclear what
the fitness consequences of sociability could be in species that do not
exhibit obvious forms of cooperation. Using four 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 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.