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.