How wood frogs survive freezing for most of the winter

How wood frogs survive freezing becomes a remarkable question in Interior Alaska. In natural overwintering sites, the frogs studied by researchers remained frozen for an average of 193 consecutive days. All 18 animals that were followed through the winter survived.

Some experienced minimum temperatures close to −18°C. The longest estimated period below their freezing point was 218 days — more than seven months.

These were not frogs sitting in carefully controlled laboratory freezers. They spent winter in shallow depressions in the forest floor near breeding ponds, covered by leaf litter and eventually insulated by snow.

Researchers from the University of Alaska Fairbanks located the animals in autumn, tracked some of them with radio transmitters, placed temperature sensors at their hibernation sites and checked whether the frogs emerged alive in spring.

What they found pushed the known limits of wood-frog freeze tolerance well beyond what many laboratory experiments had suggested.

Freezing without letting ice destroy the cells

A wood frog does allow ice to form in its body. The crucial issue is where that ice forms.

Much of the freezing occurs outside cells. As extracellular water turns to ice, water is drawn out of the cells, producing severe dehydration and osmotic stress. Ice growing inside cells, by contrast, can be lethal.

One major defence is glucose.

When freezing begins, wood frogs mobilize glycogen stores, particularly from the liver, and distribute large amounts of glucose through their tissues. The sugar acts as a cryoprotectant, helping cells tolerate water loss and stabilizing membranes and other cellular structures.

The Alaskan frogs accumulated strikingly high amounts.

Compared with frogs frozen under standard laboratory conditions, naturally freezing animals had about 13 times as much glucose in muscle, roughly 10 times as much in the heart and more than three times as much in the liver.

The laboratory freezer, in other words, was not recreating everything that happened during a real Alaskan autumn.

When one freeze may prepare the frog for the next

Before temperatures stayed below freezing for the winter, the frogs repeatedly froze at night and thawed again during warmer periods. Researchers recorded an average of about 12 freeze–thaw episodes, with some animals experiencing as many as 17.

That pattern suggested a possible explanation for the extraordinary glucose levels.

The authors proposed that each freezing event could trigger another release of glucose. During the relatively brief thaw, the sugar may not be fully removed or converted back into storage form before the next freeze arrives. Repeated cycles could therefore allow cryoprotectant levels to build progressively.

The study did not directly prove that this sequence causes the frogs' extreme winter survival. It showed that frogs freezing naturally accumulated much more glucose than animals frozen under laboratory protocols, while the freeze–thaw mechanism remained a hypothesis.

The researchers also detected an antifreeze glycolipid in muscle and internal-organ extracts. Related molecules can influence ice growth in other freeze-tolerant organisms, although this study did not establish exactly what the glycolipid does in wood frogs.

There is another part of the story: the frog does not experience the full severity of the Alaskan air temperature.

During the winters studied, the air dropped to roughly −37°C and −41°C. Leaf litter and snow created a much more buffered microclimate around the animals. The average minimum temperature at their overwintering sites was −14.6°C.

Survival therefore depends on more than one trick. The frog chooses a protected site, snow provides insulation, and its physiology helps its cells tolerate extracellular ice and severe water loss.

Then spring asks the system to work in reverse.

After months below their freezing point, the 18 frogs followed in natural hibernacula thawed, survived and began moving again.