Wood frogs can survive being frozen solid for months and thaw back to life in the spring.

The wood frog (Lithobates sylvaticus) is the only frog species found north of the Arctic Circle. To survive temperatures as low as -18°C (0°F), these amphibians have evolved a sophisticated cryoprotectant system. Researchers at the University of Alaska Fairbanks have documented that wood frogs can remain in a frozen state for up to seven months.When ice begins to form on the frog's skin, its liver undergoes a massive metabolic shift. It converts stored glycogen into glucose, flooding the bloodstream with sugar levels up to 100 times higher than normal. This extreme concentration of glucose, combined with accumulated urea, lowers the freezing point inside the cells and prevents them from dehydrating or shrinking.While the water between the cells turns into solid ice, the vital organs remain protected by this biological antifreeze. During this period, the frog is clinically dead with no detectable brain activity or heartbeat. As the ground warms in the spring, the frog thaws from the inside out, and its heart spontaneously restarts within a few hours.This remarkable adaptation allows wood frogs to inhabit a range spanning from Georgia to the Alaskan interior. Their ability to survive such extreme conditions is a primary subject of study for cryobiology and organ preservation research. Scientists hope that understanding this process could one day lead to breakthroughs in human organ transplant storage.