Color Variation in Mountain Yellow-legged Frogs

I've visited a lot of mountain yellow-legged frog populations this summer during my surveys conducted throughout the Sierra Nevada, and have been amazed (as always) at the variation in frog color patterns that I've seen. Sierra Nevada yellow-legged frogs (Rana sierrae) seem to have relatively invariant color patterns, with their characteristic dorsal dark blotches and smaller cream-colored spots on a dark brown background, yellow bellies, and dark speckling under the chin. In some populations the cream-colored spots are absent and the dark blotches are more obvious, but this variation is pretty subtle. In contrast, Southern mountain yellow-legged frogs (Rana muscosa) have wildly variable colorations. The most common dorsal color pattern is one with prominent dark blotches on a tan background, cream-colored bellies, and no spots under the chin. However, I just surveyed an R. muscosa population in which the frogs had an R. sierrae-like dorsal coloration and bellies that were markedly orange and semi-transparent (to the point where internal organs were distinctly visible through the skin!). At another site, the R. muscosa were highly variable within a population, some frogs having cream-colored or silverish vermiculations across their backs and others having the dorsal colorations more typical of R. sierrae.

What is the source of this variation? Does it have a genetic basis, and if so why the occasionally high levels of within-site variation? Given that the ranges of R. muscosa and R. sierrae abut each other along the Monarch Divide and Cirque Crest in the southern Sierra Nevada (R. sierrae north of the divide/crest, R. muscosa to the south), why the color variation despite virtually identical habitat conditions? Clearly these differences in coloration are driven not simply by natural selection for effective camouflage. Instead, I suspect that the color variation we see across the landscape is a consequence of the post-glacial frog colonization histories of these different parts of the Sierra Nevada.

If only the frogs could talk, they'd undoubtedly have quite a tale to tell....

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