Camouflage, conspicuousness, and inducible color change in a polymorphic, sexually dichromatic frog

Sexual dichromatism is prevalent throughout the animal tree of life and can play an important role in visual signaling and mate choice in many species. Some instances of sexual dichromatism, however, result from a combination of mechanisms including sexual niche partitioning and intrasexual signaling to identify competitors. Sexual dichromatism is relatively rare in anuran amphibians (frogs and toads) but is striking and prevalent in the African reed frogs (Hyperoliidae). In sexually dichromatic hyperoliids, males and females exhibit shared coloration post-metamorphosis, but at the onset of maturity, females undergo a change in color and/or color pattern whereas males typically retain the juvenile coloration. Hypothesized functions of dichromatism in reed frogs include sexual niche partitioning such that males and females use different habitats and their different colorations provide more effective camouflage in their respective habitats or alternatively, that color patterns play a role in sex and/or mate recognition in dense breeding choruses. To test these hypotheses, we characterized several aspects of natural history, ecology, and physiology in a population of the sexually dichromatic forest reed frog (Hyperolius tuberculatus) on Bioko Island, Equatorial Guinea. We found that frogs were predominantly observed on green foliage regardless of coloration or sex, providing no support for sex and/or morph differences in habitat use. In addition, our visual modeling analyses demonstrated that both color morphs are likely detectable for typical vertebrate predators in bright light regardless of background foliage coloration. Analyses using a custom reed frog dual-rod photoreceptor visual model do not support the hypothesis that H. tuberculatus perceive chromatic (hue) differences between conspecific color morphs in dim light conditions, but instead suggest that both color morphs may be more conspicuous to conspecifics than to potential predators in dim light conditions. Finally, we documented multiple instances of mature males exhibiting female coloration and our preliminary steroid hormone exposure experiments indicate that exposure to estradiol induces a color change in adult male H. tuberculatus, as demonstrated in other members of the Hyperolius viridiflavus species complex. Collectively, our results demonstrate that the population of H. tuberculatus on Bioko Island is an excellent system for future studies investigating the behavioral, physiological, and molecular mechanisms underlying sexual dichromatism in reed frogs. Furthermore, the foundational data we present set the stage to characterize species discrimination and mate choice among sympatric and closely related species of reed frogs to investigate the roles of intra- or inter-specific female signaling in the evolution of dichromatism.