Biological Miracle

Boreal forest
Wood frog's winter habitat: Boreal Forest

NPS Photo

Freezing Solid

The most remarkable thing about wood frogs happens not in the summer when they transform from tadpoles to frogs, but in the winter when they hibernate.

The northern forests of Alaska and Canada have some of the most dramatic temperature ranges in the world. Summer days stretch to 24 hours and temperatures can climb into the 90’s. Winter brings some of the earth’s coldest temperatures: it is not unusual to have temperatures of minus 50 degrees Fahrenheit. Prospect Creek, just south of the Brooks Range, had the coldest temperature ever recorded in Alaska—minus 80 degrees Fahrenheit. Almost certainly, there were hibernating wood frogs near Prospect Creek when that record was set.

This is the winter world in which the wood frog must survive. Remember, frogs are cold blooded, so their body temperature is about the same as the surrounding air. How do these delicate little creatures endure the intense, protracted, iron-cold subarctic winter?

This mystery has intrigued scientists for a long time. Biologists have studied wood frog hibernation and what they’ve learned is truly astonishing.

Most frogs survive northern winters by hibernating deep under water, in ponds, lakes and streams—they are cold and dormant but their body temperature never falls below freezing.

Wood frogs have a different strategy. They hibernate by nestling down into the leafy litter on the forest floor. The leaves, duff and overlying snow give some insulation from extreme cold, but the frogs are not protected from subfreezing temperatures as they would be if they chose the underwater strategy.


Researchers have found that wood frogs spend the winter frozen! This amazing strategy allows wood frogs to become active very early in spring, because the land thaws and warms more quickly than the ice-covered lakes The newly active frogs can mate and lay eggs in small ponds and even in melt water pools that dry up by midsummer. By contrast, frogs that hibernate underwater take longer to become active, so they must breed later. These frogs need permanent water that won’t dry out.

For most other animals, survival depends on protecting themselves from any condition that could freeze their flesh. Why is freezing so dangerous? Several things can happen: If ice crystals form inside an animal they can puncture blood vessels. When blood freezes, there is no mechanism to deliver oxygen and nutrients to organs, so extreme metabolic damage occurs. And ice severely injures cells by drawing out water and causing dehydration, scrambling the interior structure of cells and fracturing the cell walls. The result is pervasive and deadly internal damage.


Yet wood frogs have evolved ways to freeze solid for up to eight months each year. They’ve accomplished what would seem to be a biological miracle. How do they pull this off?

At the beginning of winter, ice quickly fills the wood frog’s abdominal cavity and encases the internal organs. Ice crystals form between layers of skin and muscle. The eyes turn white because the lens freezes.

At the same time, the wood frog’s liver produces large amounts of glucose that flushes into every cell in its body. This syrupy sugar solution prevents the cells from freezing and binds the water molecules inside the cells to prevent dehydration.

So on the one hand, the wood frog’s body allows ice to form around the outsides of cells and organs; and on the other hand, it prevents ice from forming inside the cells--thus avoiding the lethal damage suffered by most animals when they freeze.

What does a hibernating wood frog look like? There is no muscle movement. No heartbeat. No breathing. For the entire winter, the wood frog is like a lump of hard, frigid, icy stone carved in the shape of a frog. But it’s alive, in a state of suspended animation.

In spring, the wood frog thaws from the inside outward. First the heart starts beating. Then the brain activates. Finally, the legs move.

Nobody yet understands what starts the wood frog’s heart after being frozen and inert for the entire northern winter. Once the frog is fully thawed, it heads off through the woods to find a breeding pond or other suitable water.

The wood frog is completely undamaged by conditions that would be fatal to nearly all other animals.


Wood Frogs and Humans

Glucose in the wood frog's blood keeps it from freezing during the extreme arctic winter temperatures. This is the same as the blood sugar in all vertebrate animals, including humans.

Hibernating wood frogs can tolerate blood sugar levels 100 times higher than normal without the damage suffered by human diabetics when their blood sugar is only 2 to 10 times above normal. Understanding how frogs can do this might provide valuable knowledge to help in the management of high blood sugar in people with diabetes.

Also, the wood frog's ability to withstand freezing may help researchers discover how human organs used for transplants could be frozen and thawed without damage. This would increase the allowable time between removing an organ from a donor and implanting it within the recipient, which could make many more transplants possible.

Researchers are also interested in how the wood frog's body can stop blood circulation and start it again many months later without blood clots or other injuries. Understanding the mechanism which allows this could be valuable for treating people after their blood flow is temporarily halted by heart attack or stroke.


Last updated: October 6, 2022

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