At the start of winter, the wood frog of North America stops breathing, lets ice form in the spaces between its cells, and its heart stops entirely for up to seven months — then in spring it thaws, its heart restarts, and it hops away as if nothing happened

The wood frog of North America, Lithobates sylvaticus, spends part of every winter frozen. As the cold sets in, ice forms in the spaces between its cells, its breathing stops, and its heart stops. It stays that way, without a heartbeat and without a breath, through the coldest months. When the ground warms in spring, it thaws, the heart starts again, and the frog moves off to breed.

This is not a figure of speech. The animal is genuinely frozen, with a large share of its body water turned to ice, and it recovers.

The wood frog is the best-studied of a small number of vertebrates that survive whole-body freezing, and most of what is known about it comes from decades of work by a handful of laboratories, including that of Kenneth Storey at Carleton University in Canada and the team of Jon Costanzo and Richard Lee in the United States. What follows is drawn from their published work and from more recent fieldwork in Alaska.

What freezing actually does to the frog

The trick is not to avoid ice but to control where it forms. According to Storey’s reviews of the physiology, the wood frog endures the freezing of roughly 65 to 70 per cent of its total body water, and that ice is confined to the spaces between cells rather than inside them. Ice inside a cell tears it apart. Ice outside it, managed carefully, does not.

To keep the water inside its cells from freezing, the frog floods its tissues with sugar. Within minutes of ice beginning to form on its skin, the liver breaks down stored glycogen and releases glucose in quantities that would be dangerous in almost any other animal. Urea, which builds up in the blood as the frog stops excreting it in the weeks before winter, works alongside the glucose. Both lower the fraction of water that can freeze and help stabilise the cell structures left behind. As the process runs its course, circulation, breathing and heartbeat all come to a stop, and the frog passes the winter with no vital signs at all.

How long, and how cold, in the Alaskan case

The most detailed picture of this happening in the wild comes from a single 2014 study in the Journal of Experimental Biology, led by Don Larson and Brian Barnes at the University of Alaska Fairbanks. Earlier work had mostly relied on frogs cooled under controlled laboratory conditions. Larson and his colleagues instead tracked free-living animals into their natural winter shelters under the leaf litter and recorded what actually happened to them.

The frogs they followed stayed frozen for an average of 193 days, roughly six and a half months, through a winter in which their shelters averaged around minus 6 degrees Celsius and dropped as low as minus 18. The wild animals carried far more glucose than their laboratory counterparts, with concentrations roughly ten to thirteen times higher in heart and muscle tissue, and the authors suggested that the repeated freezing and thawing of a natural winter is what drives those levels up.

All eighteen of the tracked frogs survived.

This is one study, and its authors are careful about its scope. It measured a particular population in interior Alaska over a handful of winters, and it describes what those frogs did, not a fixed rule for the species everywhere.

Not every wood frog is this hardy

Freeze tolerance varies sharply across the animal’s range. Work by Costanzo and Lee on a northern Alaskan population, also in the Journal of Experimental Biology, found frogs surviving down to around minus 16 degrees, well below the limit their relatives in the milder south could tolerate. The Alaskan frogs stockpiled more glycogen and more urea, and appeared to hold a larger share of their body water in a bound state that does not freeze.

A later study of subarctic frogs, published in 2015 and available through the US National Library of Medicine, put a small group through several freeze and thaw cycles before cooling them to minus 8 degrees. They recovered normal behaviour and stayed healthy through seven weeks of observation afterwards.

The chemistry is not universal even among freeze-tolerant frogs. Cope’s gray tree frog, Hyla chrysoscelis, manages the same feat using glycerol rather than glucose as its main cryoprotectant. Different lineages have reached the same outcome by different biochemical routes.

Why the frog interests cryobiologists, and what it does not promise

The wood frog draws attention from researchers working on organ preservation, where the central problem is keeping tissue viable through cooling and rewarming without the ice damage that ordinarily destroys it. A vertebrate that shuts down its heart and circulation for months and then restarts them shows that recovery from such a state is at least possible.

It is not a template for freezing people. The frog does not simply endure the cold. It prepares for it across an entire body at once, coordinating where ice forms, how much sugar reaches each organ, how far each tissue dehydrates, and how deeply its metabolism drops. A human organ placed in the same conditions has none of that integrated machinery. What the frog demonstrates is narrower: in an animal built for it, a stopped heart and stalled circulation need not be the end.

Several questions about the mechanism remain open, including exactly how the frog senses the first ice and triggers the surge of glucose so quickly, and why natural freeze-thaw cycling produces higher glucose levels than a single laboratory freeze. Those are the details the Alaskan fieldwork was designed to chase, and the ones most likely to occupy the next round of study.