For the last seven years, I’ve been catching, counting and measuring adult wood frogs in the Midwest. Even after handling thousands of these frogs, I still find the diversity of color among individual adults to be fascinating. The image above is a great illustration of this. It shows two female wood frogs, of approximately equal mass, captured on the same day from the same pond. One female is bright red, while the other is dark brown. This color polymorphism is widespread in the populations I study, and is most pronounced in the female frogs. However, the red color does not appear to be present in recently metamorphosed wood frogs. I have observed tens of thousands of metamorphosing wood frogs, and none have been red.
While the presence of red and brown color forms has been documented in other populations (e.g., King and King. 1991. Canadian Journal of Zoology. 69:1963-1968), I am not aware of any study investigating its cause. Is it genetically based, or environmentally-induced? Does diet affect the development of the red color? Does the color pattern of wood frogs affect the probability that predators will see them?