Reproductive conflict and the costs of social status in cooperatively breeding vertebrates

Conflict over reproduction is an inherent part of group living. In many social vertebrates, conflict may be reflected as allostatic load, or the costs of social status and dominance rank, which may be quantified by measuring glucocorticoid stress hormones. Here, we develop the first quantitative model of allostatic load based on the tug-of-war model of reproductive skew to generate insights into the mechanisms underlying reproductive conflict in cooperative breeders and to determine whether glucocorticoids can be used to assess conflict levels in group-living vertebrates. It predicts that subordinates have higher allostatic loads than dominants under most conditions, but when body condition is lower in dominants than in subordinates, dominants experience higher allostatic load. Group structure is also important, as dominants generally have higher allostatic loads than subordinates when there is a large number of subordinates in the group, but this cost can be reduced by increasing the number of dominants, as in plural breeding societies. Using glucocorticoid data from cooperatively breeding superb starlings Lamprotornis superbus, we found empirical support for both predictions. Our model is useful for understanding how the costs of social status influence reproductive sharing, and it suggests that glucocorticoids can be used to examine reproductive conflict and cooperation in social species.