Some Frogs Can Freeze Solid and Survive

When winter's chill arrives, most cold-blooded creatures seek refuge by burrowing deep into the soil or mud. The North American wood frog, however, employs a far more radical survival strategy. It simply finds a shallow hiding spot under leaf litter and allows itself to freeze solid. During this time, its heart stops beating, its lungs cease to function, and it becomes completely unresponsive. To any observer, the frog would appear to be a tiny, frozen (Review) statue, clinically dead by most measures.

The secret to this incredible feat of cryopreservation lies in a biological antifreeze system. As ice crystals start forming on its skin, a signal triggers the frog's liver to rapidly flood its bloodstream with massive amounts of glucose. This sugar-saturated fluid circulates throughout the body, packing into every cell. While the water (Review) in the body cavity and between the cells turns to solid ice, the high glucose concentration inside the vital cells prevents them from freezing and being shredded by sharp ice crystals. This process effectively dehydrates the cells and puts the frog into a state of suspended animation.

When spring temperatures return, the "frogsicle" begins to thaw from the inside out (Review). Its heart is one of the first organs to reactivate, starting to beat slowly and pump blood through its veins, reviving the rest of its body. Within just a few hours of thawing completely, the wood frog is hopping around as if nothing happened, often becoming one of the first frogs to begin the spring breeding season.