Males are quicker to adjust aggression towards heterospecific intruders in a cichlid fish

To manage the costs of aggression, territory holders confronted by intruders commonly adjust their aggression according to the perceived level of threat. Yet, we currently know surprisingly little about heterospecific interactions or sex differences with regard to adjustment of aggression, particularly in the context of the 'dear enemy' phenomenon, in which familiar individuals are treated less aggressively than unfamiliar ones. To address these knowledge gaps, we experimentally manipulated territorial intrusions in a biparental cichlid fish, the moga, Hypsophrys nicaraguensis, in their natural habitat. We found that aggression by both females and males decreased quicker when the focal fish was sequentially presented with the same heterospecific intruder stimulus than when it was presented on each round with a different stimulus. We also found a significant sex difference: the decrease in aggression over subsequent encounters was quicker in males. Such patterns of adjustment in aggression can have important ecological implications by affecting the territory-holding success of the interacting individuals, and, in the case of heterospecific interactions, patterns of species coexistence at the community level.