A behavioral logic underlying aggression in an African cichlid fish

Social rank in a hierarchy determines which individuals have access to important resources such as food, shelter, and mates. In the African cichlid fish Astatotilapia burtoni, rank is under social control, such that larger males are more likely than smaller males to be dominant in rank. Although it is well known that the relative size of A. burtoni males is critical in controlling social rank, the specific behaviors underlying responses to males of different sizes are not well understood. In many other fish species, when size asymmetries between competing males are small, physical aggression is likely to be used to settle territorial disputes. Our goal was to characterize these responses in male A. burtoni by performing resident-intruder assays, in which aggressive behaviors were measured in territorial males in response to the introduction of unfamiliar males that differed in relative standard length (SL). We found that the relative SL of intruders played an important role in determining behavioral performance. Resident males that were smaller than the intruder (resident >5% smaller in SL) or matched (resident between 0 and 5% smaller or larger in SL) in size to the intruder performed more lateral displays, a type of non-physical aggression, than resident males that were larger than the intruder (resident >5% larger in SL). However, physical aggression, such as chases and bites, did not differ as a function of relative SL. Thus, contrary to what was expected based on work in other species, our results demonstrate that dominant resident A. burtoni increase non-physical aggression to settle territorial disputes when non-dominant intruders are closer in size. This behavioral strategy may aid in readily settling conflicts when size asymmetries are small.