Size-dependent male mating behaviour in the Cave molly, Poecilia mexicana (Poeciliidae, Teleostei)

We investigated male mating behaviour in a population of Poecilia mexicana from a Mexican limestone cave. A previous study has shown that unlike surface-dwelling P. mexicana, Cave molly males show surprisingly low frequencies of sexual behaviour when together with a female. In surface-living mollies, small males are typically more sexually active than large ones. By contrast, the frequency of sexual behaviour was found to be correlated positively with male body size in the Cave molly. We asked whether the presence of a larger male competitor would influence the mating tactic of small males. In full contact experiments with one large male, one small male and one female interacting, we found: (1) small males did not switch to the pattern of sexual behaviour known from surface-living mollies. (2) In light, the frequency of sexual behaviour was found to be significantly correlated positively with male body size. In darkness, sexual behaviour tended to increase with increasing male body size but no significant correlation was found. Apparently, small Cave molly males do not use “alternative mating behaviour” even in the presence of a larger competitor. The size-dependent pattern of mating behaviour seems paradoxical because small males appear not to compensate behaviourally for the mating disadvantage they have due to female choice.