DOMINANCE HIERARCHIES AND SOCIAL-STRUCTURE IN CAPTIVE GROUPS OF THE MOZAMBIQUE TILAPIA OREOCHROMIS-MOSSAMBICUS (TELEOSTEI-CICHLIDAE)

Groups of O. mossambicus were formed 10 days after the end of mouth-br ooding and were observed at the onset of sexual maturity by focal samp ling. The agonistic interactions were recorded and used to construct s ociometric 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 import ant 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 in teractions involving high intensity aggression were common which indic ates 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 b etween males and females. Omega individuals participated in fewer agon istic encounters than expected. According to rank distance, hierarchic al neighbours were involved in less interactions than expected except for symmetrical interactions that made up a low proportion of the tota l number of interactions. This finding is contrary to expectations, bu t maybe explained by the fact that aggression is initiated by dominant s presumably to maintain their status against individuals that are cle arly subordinates. It was found that a single dominance index (victori es/victories + defeats) was a very good predictor (r(s) = 0.963, n = 4 5) of the rank order positions of the individuals within their groups. It is argued that in species where agonistic interactions are not con trolled by processes involving complex cognitive operations this index may be a biologically realistic indicator of aggressive motivation.