The phenotype-linked fertility hypothesis predicts that male sexual traits indicate fertility, and that the coevolution of male ornaments and female preference for such traits is driven by female pursuit of fertility benefits. Females may minimize the number of energetically costly matings in which they engage by selecting males with more viable sperm or numbers of sperm sufficient to fertilize all their eggs. Female gray tree frogs strongly prefer males that produce long duration advertisement calls. Past studies tested a good genes hypothesis using gray tree frogs and found that in certain environments, tadpoles were more fit if sired by males with longer call durations than males with shorter call durations. Although this evidence supports the good genes hypothesis, results varied considerably across years and environments, and it is unclear whether variation in male call duration can also be explained by the phenotype-linked fertility hypothesis. I therefore quantified sperm number, testes mass and sperm viability in males and tested the prediction that call duration was an indicator of any of these three measures.
Results/Conclusions
Sperm number increased with testes mass and decreased with date. Males may decrease their investment in sperm production as the breeding season progresses and fewer accommodating females are present. Call duration was best explained by a model that included collection site and the interaction between date and male body mass. The interaction suggests that in contrast to medium and large males, smaller males decrease call duration as the season progresses. Gray tree frog calls are intense and energetically expensive, and smaller males may become energetically drained before larger males. Smaller males may also be more likely to bet-hedge in favor of future breeding seasons, resulting in a decrease in call duration. My study found no support for the phenotype-linked fertility hypothesis, as neither sperm number nor viability was significantly related to call duration. Fertilizing efficiency may not be a limiting factor for females, and gray tree frog call duration may only signal the male’s good genes.
