Sexual size dimorphism in the Tyrrhenian tree frog: a life-history perspective

Sexual size dimorphism (SSD) is often explained as the differential equilibrium between stabilizing survival selection and directional sexual/fecundity selection on the body size of males and females. Provided that survival selection is similar in both sexes, female-biased SSD is thought to occur when fecundity selection on female body size is stronger than sexual selection on male body size. However, in animals with indeterminate growth, body size depends on several life-history traits, thus, to understand why SSD has evolved, one should understand how it arises. We investigate SSD in the Tyrrhenian tree frog, Hyla sarda, by describing sexual dimorphism in age and growth and by assessing how body size affects their reproductive success. Females are 16% larger than males because they mature 1 year later, live 1 year longer and reach a larger asymptotic body size. Furthermore, body size correlates positively with female fecundity, but not with male mating success. These results suggest that SSD arises from differential optimal trade-offs between the expected number of reproductive episodes (which decreases with prolonging growth) and the expected success in each reproductive episode (which increases with prolonging growth).