Dominance hierarchies and social structure in captive groups of the Mozambique tilapia<i>Oreochromis mossambicus</i>(Teleostei Cichlidae)

Abstract

Groups of O. mossambicus were formed 10 days after the end of mouthbrooding and were observed at the onset of sexual maturity by focal sampling. The agonistic interactions were recorded and used to construct sociometric matrices. The dominant hierarchies were found to be linear (h = 0.94 ± 0.06) and semi-despotic (alpha individuals participated in more than half of the group interactions). Size and sex were important factors in determining the outcome of agonistic interactions. The alpha individuals were males and the largest members of their groups. Despite the fish being raised together for a long period, agonistic interactions involving high intensity aggression were common which indicates that at this stage frequent assessment of competitive ability is performed by the group members. Agonistic interactions are especially common among males and less frequent than expected among females and between males and females. Omega individuals participated in fewer agonistic encounters than expected. According to rank distance, hierarchical neighbours were involved in less interactions than expected except for symmetrical interactions that made up a low proportion of the total number of interactions. This finding is contrary to expectations, but maybe explained by the fact that aggression is initiated by dominants presumably to maintain their status against individuals that are clearly subordinates. It was found that a single dominance index (victories/victories + defeats) was a very good predictor (rs = 0.963, n = 45) of the rank order positions of the individuals within their groups. It is argued that in species where agonistic interactions are not controlled by processes involving complex cognitive operations this index may be a biologically realistic indicator of aggressive motivation.