Ontogeny of Sexual Dichromatism in the Explosively Breeding Wood Frog

—Some species exhibit sex-specific color differences (sexual dichromatism) that are considered important for reproduction. The developmental timing of sexual dichromatism is largely unknown, particularly in amphibians, and is important for informing our basic understanding of sex-specific divergence in the physiology, anatomy, and ecology of species more broadly. Using a controlled laboratory environment, we explored the onset of sexual dichromatism in Wood Frogs (Rana sylvatica), a species in which adult females typically appear redder than males. We photographed juvenile Wood Frogs approximately every two months for one year and quantified coloration (hue, saturation, brightness). Our results show that Wood Frog hue becomes sexually dichromatic before their first winter, with yellower hues in males and redder hues in females. Hue differences persist throughout the following spring and summer, with the exception of immediately after emergence from overwintering when both sexes are dramatically less bright. Male and female Wood Frogs typically do not join breeding aggregations until their first and second year, respectively; our results therefore indicate that sexual dichromatism is established soon after metamorphosis and substantially before first breeding attempts. We also noted seasonal fluctuations in hue and luminance, both of which increased during the first fall, decreased during winter, and increased again during the second spring and summer. These fluctuations suggest that color plays a role not only in sexual signaling but also in terrestrial camouflage. Future work should explore the ecological and evolutionary relevance of the early onset of sexual dichromatism and the function of seasonal variation in frog coloration.