Submissive behaviour is mediated by sex, social status, and relative body size in a cooperatively breeding fish.

Acting submissively may inhibit aggression and facilitate the termination of contests without further escalation. The need to minimize conflict is vital in highly social species where within-group interactions are frequent, and aggression can dampen group productivity. Within social groups, individual group members may modulate their use of submissive signals depending on their phenotype, the value of the contested resource, their relationship to the receiver of the signal and the characteristics of the local environment. We predicted that submissive behaviour would be more common when signallers had limited ability to flee from conflict, when signallers were of a low rank within the group, when signallers and receivers differed substantially in body size (and thus in fighting ability), and when signallers and receivers were of opposite sex and therefore not directly in competition over reproductive opportunities. We tested these predictions using social network analyses on detailed behavioural observations from 27 social groups of the cooperatively breeding cichlid fish Neolamprologus pulcher. Congruent with our prediction, submissive behaviour was more common when there were fewer shelters available, sug- gesting that constraints on fleeing behaviour may increase the use of submission. Also fitting with predictions, submissive behaviour was more common with increasing body size asymmetry between the competitors, among lower ranked fish and in interactions between opposite-sex dyads, which supports the idea that signalling submission is adaptive in contests over low-value resources. Our findings suggest that subordinate N. pulcher are primarily concerned with being tolerated within the social group and may use submissive behaviour to avoid escalated conflict. Our findings offer a window into the factors that influence signals of submission in a highly social vertebrate.