Team colours matter when playing away from home: aggression biases in geographically isolated Mbuna cichlid populations

The rocky shore habitats of the African Great Lakes support high densities of cichlid fishes, including many closely-related/ecologically similar species. Aggressive behaviours between conspecifics, and perhaps heterospecifics, influences this unusually high level of species coexistence. In dichotomous choice aggression trials, male Maylandia thapsinogen were presented simultaneously with two heterospecific intruders (Maylandia emmiltos and Maylandia zebra). M. thapsinogen were significantly more aggressive towards intruders from an allopatric species (similar orange dorsal fin colour - M. emmiltos), than towards a different allopatric species (blue dorsal fin - M. zebra). Aggression biases disappeared when colour differences were masked using monochromatic lighting. A second experiment compared female aggression biases between M. emmiltos with M. thapsinogen, species similarly coloured to one another, the former possessing a yellow, as opposed to a black throat as the latter does. M. thapsinogen preferentially attacked females of their own species in full but not monochromatic light, while female M. emmiltos showed no significant bias in aggression under any lighting. Responses were not affected by olfactory cues provided by the stimulus fish. These results indicate that divergence in colour might facilitate species co-existence in some cases, but not all, which could be important should populations re-join through lake level drops.