Secondary Sexual Traits and Individual Phenotype in Male Green Frogs (Rana clamitans)

Secondary sexual characteristics are often under intense sexual selection as ornaments for attracting mates or as traits associated with male-male competition such as combat. Secondary sexual traits under sexual selection are predicted to be related to male phenotype, because only high-quality males should be capable of developing these costly and exaggerated traits. Therefore, high-quality males should have larger or more elaborate secondary sexual traits than poor-quality males. We used the Green Frog (Rana clamitans) to test this hypothesis using three sexually dimorphic traits (the enlarged forelimbs, enlarged tympanum, and yellow throat of the male Green Frog) and two measures of male phenotype (body condition and size). Forelimb size was condition-dependent—males with larger forelimbs were in better condition than males with small forelimbs. Males with an intermediate shade of yellow on their throat tended to be in better condition than males with dark and light yellow throats, but the degree of yellow was related to body size—males with a dark yellow throat were larger than males with a light yellow throat. There was no evidence of condition-dependence of tympanum diameter; although after correcting for body size, males with a large tympanum had large forelimbs suggesting that tympanum size reflected some other, unmeasured, indicator of male quality. These results are consistent with the hypothesis that males produce multiple signals and cues that reflect different aspects of male quality. Females can maximize the benefits of mate choice by using multiple traits to assess male quality.