Only one amphibian, the wood frog (Rana sylvtica) is found in on the refuge. They inhabit a variety of habitats including mixed forests, open meadows,wetlands and tundra, but are commonly found along shorelines. Wood frogs are highly terrestrial, and are only found in water during breeding and early development. Although adults can be as long as 3 inches (7.6 cm), they are frequently smaller. This smooth skinned frog may be brown, tan, grey, or green above, with a uniformly cream colored underside. It hibernates in shallow depressions under layers of dead vegetation. The wood frog is able to survive winter cold by increasing the glucose stored in the cell fluids; the glucose acts as an antifreeze and prevents skin cells from freezing. Recent studies of wood frogs in Tetlin and other National Wildlife Refuges in Alaska have found some of the highest rates of physical abnormalities (missing, shrunken, or misshaped limbs, or abnormal eyes) documented in the published literature. The cause for the high prevalence of abnormalities is unknown, but hypotheses include chemical contaminants, parasites, ultraviolet radiation, predators, extreme temperatures during development, or a combination of these factors.