Cooperation in animal societies: integrating ecology, hormones and early development

Why do some individuals contribute much more to cooperative activities than others? Answering this intriguing question is one of the key challenges facing evolutionary ecologists and economists who seek to understand the cooperative behaviour of humans and animals. Like humans, members of animal societies show enormous variation in the amount that they invest in cooperative activities, such as helping to rear each other's young. It is clear that some of this variation can be attributed to characteristics of the individual (e.g. their age and sex) and their environment (e.g. group size or the availability of food), but a considerable amount of variation remains entirely unexplained. If individual cooperative tendencies are determined in part by their underlying hormone levels (as many other behaviours are), much of this unexplained variation could be due to hormonal differences between individuals, arising from processes such as formative experiences during their early lives. However, the roles that hormones and early development play in shaping cooperative tendencies are currently poorly understood. I propose a programme of work that will advance our understanding of vertebrate cooperation substantially by addressing this critical shortfall in our knowledge. I will study cooperation in the white-browed sparrow weaver, a social bird. In sparrow weaver groups, the dominant pair breed and subordinate group members help to feed their offspring. First, I will measure the helpers' contributions to nestling-feeding and relate these to variation in their prolactin, testosterone and corticosterone levels (hormones that are thought to regulate parental care and so are good candidates for the regulation of cooperative care). Investigating the roles of multiple hormones simultaneously is important as variation in cooperation due to one hormone may otherwise obscure the effects of another. This work will establish which hormones are associated with higher or lower helper contributions. To check that these relationships reflect effects of hormones on cooperation (rather than the other way around) I will experimentally alter the hormone levels of helpers and verify that their cooperative behaviour changes as expected. Second, I will establish whether the cooperative tendencies of helpers (and their 'helper hormone' levels) are determined by aspects of their early life (e.g. their mother's number of helpers at laying, or the number of helpers rearing them as a nestling). As the number of helpers at laying and rearing are usually very similar, their roles can be very difficult to tease apart. I will resolve this problem by swapping clutches of eggs between groups with different numbers of helpers (after they have been laid and before they are reared) and testing whether the cooperative tendencies of the chicks are determined by aspects of their actual mother and her group (at laying) or their foster mother and group (at rearing). This will allow me to investigate the intriguing possibility that vertebrate mothers might produce more-cooperative offspring when they have fewer helpers to assist them (from which they would stand to benefit). Sparrow weavers are an ideal species for this study, as their nests are easy to find and the birds can be easily trapped from their roost chambers for hormone sampling. In addition, examining the effects of their early life on the cooperative tendencies is straightforward as youngsters typically stay on to help their parents and feed nestlings from six months of age. As the key aims of the work will use much of the same hormonal and behavioural data, they can be readily conducted alongside each other. Finally, as I have extensive experience with behavioural and hormonal fieldwork, have specialist collaborators in place to conduct the laboratory analyses and have demonstrated the feasibility of all necessary protocols at a densely-populated study site, the project has the best possible chance of success.