Sex-specific ventral dichromatism and melanization in harlequin toads (Atelopus): a common but overlooked character of unknown function

Hidden colors are a widespread phenomenon in the animal kingdom, particularly in anurans. In some cases, hidden colors are suddenly exposed during defensive displays to startle predators, others seemingly remain hidden—particularly from researchers. Amazonian species of Neotropical harlequin toads (genus Atelopus) show striking and consistent ventral sexual dichromatism where females show and males generally lack melanization. Inspired by these observations we undertook a deeper inquiry across this species-rich genus. We collected data on ventral sexual dichromatism in Atelopus species and scored expression of sex-specific ventral melanization (i.e. black, brown and/or grey coloration). Ventral sexual dichromatism was present throughout the entire range of the genus and in almost all phylogenetic groups. However, there was a clear geographic signal with this trait being most common and widespread in Amazonian Atelopus species. Ventral melanization was correlated with temperature and elevation. Focusing on the Amazonian species, we present hypotheses on potential functions of sexually dimorphic ventral patterns and sex-specific ventral melanization as a baseline to further investigate the dynamics of sexual and natural selection as potential drivers of these traits. Selective pressures on less exposed body parts, such as ventral sides, likely differ considerably from those on dorsal appearance. Given the amount of research on amphibian coloration, it is remarkable how little we know about the evolution, function and underlying mechanisms of ventral appearance. We hope our work will spark more interest in the flip side of amphibians, thereby broadening our understanding of animal coloration.