The influence of stress hormones and aggression on cooperative behaviour in subordinate meerkats

In cooperative breeders, aggression from dominant breeders directed at subordinates may raise subordinate stress hormone (glucocorticoid) concentrations. This may benefit dominants by suppressing subordinate reproduction but it is uncertain whether aggression from dominants can elevate subordinate cooperative behaviour, or how resulting changes in subordinate glucocorticoid concentrations affect their cooperative behaviour. We show here that the effects of manipulating glucocorticoid concentrations in wild meerkats (Suricata suricatta) varied between cooperative activities as well as between the sexes. Subordinates of both sexes that were treated with a glucocorticoid receptor antagonist (mifepristone) exhibited significantly more pup protection behaviour (‘babysitting’) compared to those treated with glucocorticoids (cortisol) or controls. Females treated with mifepristone had a higher probability of exhibiting pup food provisioning (‘pupfeeding’) compared to those treated with cortisol. In males, there were no treatment effects on the probability of pupfeeding, but those treated with cortisol fed a higher proportion of the food they found to pups than those treated with mifepristone. We also used 19 years of behavioural data to show that dominant females did not increase the frequency with which they directed aggression at subordinates at times when their need for assistance was highest. Our results suggest that it is unlikely that dominant females manipulate the cooperative behavior of subordinates through the effects of aggression on their glucocorticoid levels and that the function of aggression directed at subordinates is probably to reduce the probability they will breed.