Meet The Frog That Can Freeze In Winter And Defrost In Spring, No Problem

Walking in Interior Alaska in winter is an astonishing experience. The snowy landscape, the crisp air, the frozen frogs… Wait, what?

Yes, were you to feel about in the leaf litter during the region’s coldest months, you just might be lucky enough to find a frog frozen solid. Typically, you'd expect this to be sad news. A frog has just died, right? Its cells destroyed by the destructive process of ice formation, but you would be wrong. You see, you’ve just found a wood frog.

The wood frog, Lithobates sylvaticus, looks like any other frog in summer. A pleasant combination of greeny browns, but nothing you’d be writing home about. Come winter, however, they pull off something of a miracle: they freeze solid, heart stopping, before thawing out again come spring like it’s no big deal.

Freezing in this way would kill most other animals, not least because of the horrific damage that ice formation does to cells (which is why we can't survive freezing like human popsicles). The wood frog is different, however, as it's evolved a unique cascade of physiological processes that effectively enable it to produce its own antifreeze.

When they freeze, the water in their bodies starts to freeze too, pulling out fluid from their cells. However, at the same time, they produce lots of urine that stays in their blood rather than being excreted, and their livers release large amounts of glucose. When this curious combination of sugar and pee meet, it forms a kind of cryprotectant antifreeze that prevents cell shrinkage, preserving them until the frog warms up again. The protective strategy may be helped along by the fact that out in the wild, they experience multiple episodes of freezing and thawing as the weather fluctuates.

“We hypothesize that it is the pattern of freezing under natural conditions, which includes multiple freezing and thawing cycles, that causes the high concentrations of glucose that accumulate in tissues of Alaskan wood frogs, and that these high glucose concentrations contribute to the enhanced tolerance to cold that we have demonstrated,” wrote Don Larson and colleagues in their 2014 paper, which was the first to examine the ecological physiology, biochemistry, and behavior of freeze-tolerant wood frogs overwintering under natural conditions.

“Our results demonstrate that Alaskan wood frogs can survive being frozen for up to 7 months with minimum temperatures below -18°C [-0.4°F]. Only the Siberian salamanders Salamandrella schrenckii and S. keyserlingii, which endure 4–5 months frozen with survival of individuals to -35°C [-31°F], are comparable to the capabilities of North American wood frogs.”

As extremophiles go, a frogsicle is right up there.

What’s that? You’re hungry for more frog news. Oh, go on then: The Myth And Mystery Of The Living Frogs Entombed In Rocks.