The Alaskan wood frog survives the winter by freezing solid
During the bitter Alaskan winter, the wood frog survives by allowing up to sixty percent of its body water to freeze. Its heart stops beating, and its breathing ceases. To prevent its cells from bursting, the frog floods its tissues with glucose, which acts as a natural antifreeze, before thawing completely in the spring.
Life at the Northern Edge of the Amphibian World
Amphibians are ectothermic animals whose body temperatures fluctuate with their immediate surroundings. Because their thin, permeable skins offer little barrier against environmental extremes, most frog species are confined to temperate and tropical regions where sub-zero temperatures are rare or absent. The wood frog (*Lithobates sylvaticus*), however, defies this geographical constraint. Its range spans from the southern Appalachian Mountains in the United States across vast swathes of Canada and into the interior of Alaska, making it the only known amphibian species to live north of the Arctic Circle.
Unlike many northern animals that migrate south or excavate burrows deep below the subterranean frost line, the wood frog remains on the forest floor throughout the winter. It hibernates in the subnivean zone—the narrow space between the soil surface and the blanket of snow above—tucked beneath a shallow layer of fallen leaves and forest duff. In northern latitudes, temperatures in this shallow layer regularly plunge far below the freezing point of water. To survive months of severe cold without the ability to burrow beneath the permafrost, the wood frog relies on a radical physiological strategy: it allows its own body to freeze.
Recognizable by its tan, brown, or rust-colored body and a distinct dark marking across its eyes resembling a robber's mask, the wood frog is an unassuming creature during the warmer months. It spends the summer hunting small forest invertebrates like beetles, spiders, and caterpillars across the forest floor. Yet underneath this conventional exterior lies one of the most sophisticated natural freeze-tolerance systems ever documented in the animal kingdom.