Relationships between Pre- and Postcopulatory Sexually Selected Traits in Green Frogs (Lithobates clamitans)

Sexual selection is thought to be a primary driver of trait evolution. The expression of traits that improve mate acquisition or fertilization success may be linked in organisms that experience intense sexual selection as they must invest sufficient energy into many different sexual traits in order to reproduce. Two prevailing models-the phenotype-linked fertility hypothesis and the trade-off hypothesis-describe potential positive or negative correlations, respectively, between pre- and postcopulatory traits. We examined the interaction between precopulatory traits (belly coloration and forearm size) and postcopulatory traits (sperm morphology, sperm velocity, and sperm count) in male Green Frogs (Lithobates clamitans). We found multiple positive and negative correlations between pre- and postcopulatory sexual traits. Interestingly, belly saturation and hue, which are precopulatory traits, were negatively correlated with sperm count and sperm length, suggesting that the expression of these traits is constrained by energetic resources, fitting the trade-off hypothesis. However, male forearm size was positively correlated with both sperm count and sperm velocity but negatively correlated with sperm length. These results suggest that males must make complex investments in suites of sexual traits to maximize fitness in the face of energetic trade-offs.