Punishment from dominant breeders increases helping effort of subordinates in a cooperatively breeding cichlid

Prosocial punishment is a process of enforcement that promotes a partner's cooperation in a sustained relationship and is an important mechanism in the evolution of cooperative behaviour. The ‘pay-to-stay’ hypothesis predicts that in cooperatively breeding animals, dominant breeders punish idle subordinates (helpers) that assist breeders in offspring care to promote their cooperation. However, limited evidence demonstrates that subordinates increase helping effort in response to breeder aggression; therefore, whether breeder aggression functions as punishment remains unclear. In the laboratory, we observed idle helpers of the cooperatively breeding cichlid fish Neolamprologus savoryi and whether dominant breeders punished them. Breeders increased their aggression towards subordinates that were prevented from helping by 2h removal from the tank, and attacked helpers subsequently increased territorial defence depending on breeder aggression level, suggesting that breeder aggression functions as punishment. No breeder aggression was observed when helpers performed pre-emptive helping behaviour prior to dominant breeder attacks. Notably, dominant breeders punished even their offspring when idle, suggesting that the helping behaviours of relatives may be facilitated by parental punishment. Our study shows that dominant group members can use punishment to elicit help from subordinates, even in nonhuman animals.