The Animal That Can Survive Being Frozen Solid

For most animals, freezing solid is a death sentence, ice crystals rupture cells and destroy the delicate machinery inside them. The wood frog treats it as an annual routine instead, freezing every winter and thawing back to normal life every spring — a feat of extreme survival rivaled by few animals besides the tardigrade, which can endure the vacuum of space itself.

Turning Into a Frogsicle

The wood frog, Lithobates sylvaticus, is the only North American amphibian whose range extends north of the Arctic Circle, a feat made possible entirely by its ability to survive freezing, according to the National Wildlife Federation. As temperatures drop, the frog burrows into leaf litter and simply lets the cold take over, with up to 70% of the water in its body turning to ice, according to Discover Wildlife.

The freezing process itself is dramatic. Ice first forms on the frog’s skin, then works inward through its veins and arteries, eventually pushing blood toward the heart before that too freezes solid, according to National Geographic. The frog’s eyes glaze over and its brain freezes along with everything else. By any conventional measure, it looks dead.

The Antifreeze Trick

The reason the wood frog survives what would kill almost any other animal comes down to glucose. As freezing begins, the frog’s liver rapidly converts stored glycogen into massive amounts of glucose, flooding its bloodstream and the inside of its own cells, according to National Geographic. That sugar acts as a natural antifreeze, preventing the water inside cells from crystallizing and rupturing them, even as ice fills the spaces between cells, between organs, and beneath the skin.

Researchers have found that wood frogs actually cycle through repeated freeze-thaw episodes over the course of a winter, sometimes 10 to 15 times, before settling into their final deep freeze for the season, according to Audubon. Each cycle appears to build up glucose concentrations even further, potentially strengthening the frog’s tolerance as the season goes on.

Coming Back to Life

When spring arrives and temperatures rise, the frog thaws from the outside in. Its heart is among the first organs to restart, beginning sluggishly before returning to a normal rhythm within about 10 hours, according to Discover Wildlife. Within a day, the frog is back to hopping around and calling for mates, as if the past several months of being biologically frozen never happened.

Why Scientists Are Paying Attention

Beyond being a genuinely strange survival trick, wood frog biology has real medical relevance. Scientists are studying the freeze-thaw process in the hope of developing better methods for preserving human organs for transplant, since human tissue currently can’t survive being frozen the way a wood frog’s can, according to Discover Wildlife.

Frequently Asked Questions

Does the wood frog’s heart really stop completely? Yes. During deep freezing, the frog’s heart stops beating entirely and only resumes once the thawing process begins in spring.

Are wood frogs the only animals that can freeze and survive? They’re one of the most well-studied examples, but a small number of other cold-tolerant amphibians, including certain Siberian salamanders, have shown similar freeze tolerance.

The wood frog’s ability to treat freezing as a survivable pause rather than a fatal event remains one of the more remarkable adaptations in the animal kingdom, and one science hasn’t fully finished learning from.