Spawning coloration, female choice and sperm competition in the redside dace, Clinostomus elongatus

Abstract

Understanding pre- and postcopulatory mechanisms of sexual selection (male–male competition and female choice) can provide insight into the evolution of male ornamentation. In this study, we use an integrative approach to study sexual selection by incorporating measures of both mechanisms using the redside dace, Clinostomus elongatus, a freshwater, externally fertilizing fish found in streams of eastern North America. We measured the success of individual male redside dace in both pre- and postspawning sexual selection and related the success of males to a range of traits, including spectral properties and the size of their red spawning coloration, body size metrics and sperm quality. We found significant sexual dimorphism in the relative amount and spectral properties (saturation and brightness) of the red spawning coloration, with males being more ornamented than females. Using dichotomous mate choice trials, we found that females did not favour males with relatively larger red spawning coloration area and more saturated or brighter red spawning coloration. Males with relatively more red spawning area would be favoured in male–male postspawning sperm competition because they possess sperm with higher velocity, and sperm velocity was determined to be the key predictor of sperm competition success as assessed by paternity analyses related to in vitro sperm competition trials. These results suggest that prespawning selection via female choice is not driving the evolution of red spawning coloration and that postspawning selection via male–male competition is primarily determined by sperm velocity, which was correlated with the relative amount of red spawning coloration.