Sexual selection and predator response in a male-polymorphic livebearing fish

Sexual polymorphism describes discrete variation among individuals of one sex, often involving conspicuously colored, displaying male morphs and inconspicuous, sneaking male morphs. Sexual polymorphism may be maintained over evolutionary time if the displaying morph is favored by sexual selection and the sneaking morph experiences reduced predation. We tested these ideas using 3D printed models and live males in the sexually polymorphic poeciliid fish, Girardinus metallicus. Females did not prefer the displaying black morph; however, black morphs exhibited more male-male aggression, and dominant black morph males achieved higher mating success than all plain morph males, suggesting a sexual selection advantage. Predatory blue tilapia (Oreochromis aureus) did not show a preference for either morph, suggesting no plain morph advantage in this regard that would maintain the polymorphism. It is possible that the polymorphism is instead maintained because as black morphs become common, aggressive interference among them causes their fitness to decline, thereby keeping black morphs rare relative to plain morphs, but not eliminating them entirely. Our results underscore the need to further explore the function of the black morph display, as it appears not to be sexually selected via female choice, in contrast to mating displays in similar species.