Animal social groups can be organized in hierarchies where individual status determines privileges within the group, and stability is maintained through conflict (aggression–submission) and cooperation. Aggression, submission and cooperation are not homogeneous among group members and are influenced by social context and associated trade-offs. However, studies of rank-specific behaviours are rare which limits our understanding of these patterns. Here, we performed rank ascension experiments using 15 groups of Amphiprion clarkii, a relatively mobile anemonefish, to assess rank-specific behaviour related to social context. We showed that promoted ranks increased cooperation rates compared to nonpromoted ranks to fulfil the tasks associated with their new status within the group. Group size had no effect on cooperation rates, although it did influence rates of submission and neutral interactions, and subordinates did not increase cooperation after group size reduction. Thus, subordinate
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,