Abstract

Previous studies indicate that dominant fish grow faster than subordinate fish when fed equal rations. It is unclear, however, whether this growth differential is caused by intrinsic differences related to their propensity to become dominant, or by the extrinsic effect of the social stress experienced by subordinates. We first tested whether dominant convict cichlids (Amatitlania nigrofasciata) grew faster than subordinates when fed an equal amount of food. Second, we tested whether the growth advantage of dominants occurred when only visual interactions were allowed between pairs of fish. Third, we randomly assigned social status to the fish to rule out the possibility that intrinsic differences between fish were responsible for both the establishment of dominance and the growth differences. In three separate experiments, dominant fish grew faster than size-matched subordinate convict cichlids, but the growth advantage of dominants was higher when there were direct interactions between fish compared to only visual interactions. Our results provide strong support for the hypothesis that the slower growth rate of subordinate fish was due to the physiological costs of stress.