Divergent Evolution of Male Aggressive Behaviour: Another Reproductive Isolation Barrier in Extremophile Poeciliid Fishes?

Reproductive isolation among locally adapted populations may arise when immigrants from foreign habitats are selected against via natural or (inter-) sexual selection (female mate choice). We asked whether also intrasexual selection through male-male competition could promote reproductive isolation among populations of poeciliid fishes that are locally adapted to extreme environmental conditions [i.e., darkness in caves and/or toxic hydrogen sulphide (H₂S)]. We found strongly reduced aggressiveness in extremophile Poecilia mexicana, and darkness was the best predictor for the evolutionary reduction of aggressiveness, especially when combined with presence of H₂S. We demonstrate that reduced aggression directly translates into migrant males being inferior when paired with males from nonsulphidic surface habitats. By contrast, the phylogenetically old sulphur-endemic P. sulphuraria from another sulphide spring area showed no overall reduced aggressiveness, possibly indicating evolved mechanisms to better cope with H₂S.