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

Abstract

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) on cooperative behaviour varied between cooperative activities as well as between the sexes. Subordinates of both sexes 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 (pup-feeding) compared to those treated with cortisol. In males, there were no treatment effects on the probability of pup-feeding, but those treated with cortisol gave a higher proportion of the food they found to pups than those treated with mifepristone. Using 19 years of behavioural data, we also show that dominant females did not increase the frequency with which they directed aggression at subordinates at times when the need for assistance was highest. Our results suggest that it is unlikely that dominant females manipulate the cooperative behaviour 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.

Keywords: aggression; behavioural plasticity; cooperation; glucocorticoids; sociality; stress.

Figure 1.

(a) Subordinate female and male meerkats fed mifepristone (n = 9) exhibited significantly more babysitting (proportion of total babysitting provided by subordinate) than did those fed cortisol (n = 9) or the controls (n = 8). (b,c) Unlike babysitting, the effects of the treatments on pup-feeding were sex-specific (electronic supplementary material, tables S2–S3). Subordinate females treated with mifespristone (n = 8) were significantly more likely to feed pups compared to those treated with cortisol (n = 10) but not the controls (n = 7). However, the amount of prey biomass found by subordinate females and fed to pups did not differ among the treatment groups. The probability of pup-feeding by subordinate males was not affected by the treatments but males treated with cortisol (n = 4) fed significantly more of the prey biomass they found to pups compared to those fed mifepristone (n = 7) but not controls (n = 7). (Online version in colour.)

Figure 2.

Behavioural data collected over 19 years (greater than 110 483 h) indicated that the amount of aggression subordinate female meerkats (n = 713) received from the dominant female (n = 98 females from 40 groups) varied according to whether the dominant female was pregnant and whether there were offspring in the group that subordinates were taking care of (babysitting and pup-feeding, electronic supplementary material, table S7). Subordinate females received the highest levels of aggression from the dominant female when she was pregnant and there were no pups in the group. The amount of aggression directed at subordinate males (n = 807) was not significantly influenced by the pregnancy status of the dominant female or whether there were offspring in the group being babysat or fed. (Online version in colour.)