Social rank and species-specific differences influence cooperation and conflict in anemonefish societies

Many animal groups consist of dominance hierarchies. Theoretical models suggest that higher-ranked individuals should increase aggression but should decrease helping unless the cost of helping decreases with higher rank. Most empirical tests focus on systems with kin selection, whereas tests for groups in which individuals are unrelated are rare. Here we used two anemonefish species to test hypotheses of variation in helping and aggression with respect to social rank. We assessed behavioural frequencies for each rank in Amphiprion percula and A. perideraion groups and performed a removal experiment to determine whether behaviours were rank- or size-specific. Overall, we found A. perideraion performed more aggression and less helping than A. percula, possibly due to a difference in ecological constraints. In both species, we found consistent differences in behaviour between ranks, with higher ranks performing both aggression and helping more often. Despite low relatedness within groups, we found that lower-ranked individuals helped in both species. When we experimentally promoted individuals and controlled for group size, their behaviour became indistinguishable from that of individuals established in the higher rank. Thus, for the first time, we quantified helping for nonbreeding individuals in a marine fish, showed that behavioural patterns are rank-specific, and confirmed that individuals adjust both aggression and helping to their likelihood of inheriting a breeding position.