Effect of social context on behaviour in anemonefish hierarchies

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 non-promoted ranks to fulfil the tasks associated with their new status within the group; group size had no effect on group cooperation rates and subordinates did not increase cooperation after group size reduction; and frequency of one-on-one agonistic encounters within the group was asymmetric and higher for lower ranks. Thus, subordinates modulate rates of cooperative behaviour according to their status, irrespective of their body size or group size, and experience more conflict than dominants as they attempt to maintain their position in the hierarchy and avoid eviction. We provide evidence that social context in the form of rank has an important effect on individual behaviour and appears to be the key driver of cooperation and within-group conflict.