Sexual dichromatism in agile frogs: sex-reversed males closely resemble typical male coloration

Edina Nemesházi , Veronika Bókony, Ossi Nokelainen, Zsanett Mikó, Nikolett Ujhegyi, Andrea Kásler, Bibiana Rojas

In species with sexual dichromatism, colouration can play an important role in intraspecific communication and affect breeding success. Communication by visual signals during the breeding season has been increasingly recognized to occur in anuran amphibians. However, most studies of sexual dichromatism have focused on consistent differences between males and females, and species with varying combinations of sexual genotypes and phenotypes have so far been overlooked. We studied dorsal and ventral colouration of the agile frog (Rana dalmatina), a species where genetically female (XX genotype) tadpoles experiencing warm environmental temperatures tend to undergo sex reversal. In accordance with this laboratory-based observation, phenotypic males with both XX and XY genotype occur in free-ranging populations, but ‘XX males’ have reduced breeding success despite being fertile. We compared different aspects of colouration between females, ‘XX males’ and ‘XY males’ to assess whether there is sexual dichromatism between the phenotypic sexes, and whether ‘XX males’ differ from ‘XY males’ in any of the measured parameters. We found that females differed from phenotypic males in both dorsal and ventral colouration. These differences were also supported by the frog vision model. Females showed a trend towards darker, more red-shifted dorsal and throat coloration, and featured substantially more ventral pigmentation near the rostral tip of the mouth relative to phenotypic males. Most females featured a red ventral pattern on the anterior part of the trunk that almost never occurred in males. Interestingly, ‘XX males’ showed a shift towards yellower average throat colouration relative to ‘XY males’; however, the difference was small and not detectable under the frog vision model. Our results suggest that agile frogs may rely on colouration for intraspecific communication during the breeding season, similarly to other species in this genus, but visual cues are unlikely to mediate the recognition of sex-reversed males.