Social effects on adult survival in a wild cooperative bird

Abstract

Explaining the evolution of cooperation is a key challenge in evolutionary biology. It is widely recognised that cooperation often yields indirect benefits to helpers through their effects on offspring survival, however less well known are the effects of helping on breeder survival, partly as studies often struggle to disentangle helping behaviour from associated effects of group size. It is also possible that cooperation could yield direct benefits to helpers. One potentially widespread mechanism by which this could happen is ‘group augmentation’: where cooperation increases group size (by increasing the recruitment or survival of group members), co-operators will benefit directly from their actions if living in a larger group enhances their survival or future reproduction. Using long-term field data on white-browed sparrow-weavers, I investigated these two mechanisms by which helping behaviour could yield fitness benefits for helpers. Specifically, does helping yield (i) indirect fitness benefits by improving the survival of related breeders and (ii) direct fitness benefits via group augmentation. I present evidence that helping behaviour (not group size) can improve dominant female survival, most likely as a result of load-lightening, and that this should yield indirect fitness benefits to helpers. I then present evidence that living in a larger group actually has negative effects on helper survival. The results suggest that this may be a consequence of foraging competition, as group size also negatively predicted territory size per capita and helper body condition. Thus, group augmentation mechanisms could lead to selection against cooperation, rather than for it. Unlike the results of many studies on these topics, my findings cannot be readily attributed to differences among territories, as they hold when we examine the effects of within-territory variation in group size and/or helper number over time. Together, my results are particularly interesting as they show opposing effects of helping on the indirect and direct fitness of helpers.