Cooperative and aggressive behaviours vary between ranks in anemonefish social hierarchies

Many animal groups consist of individuals organised in dominance hierarchies, based on age, size or fighting ability. Lower ranked individuals often do not reproduce themselves but perform cooperative behaviours to help the reproductive output of dominant individuals or the group as a whole. Theoretical models suggest that individuals of higher rank should show increased amounts of aggressive behaviours, such as aggressions towards other group members, but should decrease the amount of cooperative behaviours, such as brood care or territory maintenance. Most empirical tests of these models focus on insect or mammalian systems where kin selection plays a large role, rather than animals that live in groups of unrelated individuals. Here we use two anemonefish species to test hypotheses of variation in cooperation and aggression with respect to social rank and species, for social systems where group members are unrelated. We assessed the behaviours of each rank in 20 groups of Amphiprion percula and 12 groups of A. perideraion in Kimbe Bay, Papua New Guinea. We also performed a removal experiment to test if cooperative and aggressive behaviours are likely adaptive, i.e., if they change as an individual ascends in rank. Our results show differences between the two species, with A. percula showing more cooperative behaviours and A. perideraion showing more aggressive behaviours, despite them being closely related and sharing a very similar ecology. With respect to both cooperation and aggression we found consistent differences between ranks in both species, with higher ranks performing more aggressive as well as more cooperative behaviours. When we experimentally provided lower ranked individuals (rank 4) an opportunity to ascend in the hierarchy, they showed more aggression and more cooperation in line with our observations for rank 3 individuals. Thus, we show that rank specific behavioural patterns are likely adaptive in anemonefishes and that some model predictions do not hold in systems where kin selection benefits are absent. Rather, future fitness benefits through territory inheritance and group augmentation likely motivate cooperative and aggressive behaviours by subordinates in groups of unrelated vertebrates.