How Do Wood Frogs Survive Freezing? Frozen Frogs Explained

What happens when a wood frog freezes?

The wood frog spends the winter on land, tucked just under the leaf litter of the forest floor. Unlike toads, it does not burrow below the frost line, so when temperatures fall well below 0 °C (32 °F), it freezes along with the leaves around it.

Freezing usually begins when ice crystals in the damp litter touch the frog’s skin. From there, ice spreads through the body over a period of hours. It forms under the skin, in the body cavity, around the organs and in the blood, but not inside the cells. As freezing goes on, the heart stops beating and the blood stops flowing.

A completely frozen wood frog feels hard and looks dead. It has no heartbeat, no breathing, no blood circulation, no muscle movement and no detectable brain activity, and it can stay like this for weeks or months.

Sugar and urea: a natural antifreeze

The key to survival is keeping ice out of the cells, because ice crystals inside a cell tear it apart. Wood frogs do this with cryoprotectants, substances that protect living tissue from freezing damage.

The most important is glucose. Before winter, the frog stores large amounts of glycogen, a starch-like fuel, in its liver. As soon as ice starts to form, the liver rapidly breaks this glycogen down into glucose and releases it into the blood, which carries it to the organs while the heart is still beating. Glucose levels soar to many times their normal value.

The second cryoprotectant is urea, a waste product the frog builds up in its tissues during autumn. Together, glucose and urea lower the freezing point of the fluid inside the cells, limit how much ice can form, and stop the cells from shrinking too much. As ice forms outside the cells, it draws water out of them: in frozen frogs, skeletal muscle can lose up to about half of its water and the liver about two-thirds. The cells survive this partial drying because the sugar and urea keep them stable.

There is a limit. A wood frog survives as long as no more than about 65–70% of its body water turns to ice. Its cells must also cope for months without oxygen, and researchers have found other helpers, including enzymes that keep working at low temperatures and an antifreeze compound in some tissues.

Thawing out in spring

When temperatures rise above freezing, the frog thaws. Its heart starts beating again, breathing resumes, and over the following hours it regains its reflexes and the ability to move. In regions where winter temperatures go up and down, wood frogs can freeze and thaw several times in one season without harm.

Soon after the final thaw, wood frogs head for the nearest temporary pool to breed. They are among the first frogs to call in spring, sometimes while ice still covers part of the water.

How cold can a wood frog survive?

It depends on where the frog comes from. Wood frogs from southern Canada and the US Midwest survive freezing down to about −3 to −6 °C (27–21 °F). Wood frogs from interior Alaska are far tougher: in laboratory tests they survived −16 °C (3 °F), a temperature 10–13 °C colder than their southern relatives can bear.

In a study published in 2014, scientists in interior Alaska tracked 18 wood frogs to their natural winter hiding places. The frogs stayed frozen for about 193 days in a row, with average October–May temperatures of −6.3 °C (21 °F) and lowest temperatures between about −9 and −18 °C (16 and 0 °F). Every frog survived. Alaskan frogs store more glycogen and build up more urea than southern ones, and repeated freezing and thawing in autumn seems to raise their glucose levels further.

Where wood frogs spend the winter

In autumn, adult wood frogs leave their summer haunts and settle in the leaf litter, often on slightly higher ground within about 65 m (210 ft) of a breeding pool, so they have only a short trip in spring. A blanket of snow on top helps insulate them from the coldest air.

Other frogs solve winter differently. Many pond frogs rest underwater at the bottom of ponds, and toads dig down below the frost line. These strategies are compared in Where do frogs go in winter? The wood frog’s freeze tolerance is what allows it to live so far north; see Where do wood frogs live?

Other animals that survive freezing

Only a few vertebrates can survive being partly frozen. In North America they include the spring peeper, gray treefrogs and chorus frogs, as well as the hatchlings of some turtles, such as the painted turtle. The Siberian salamander of northern Asia is another famous freeze-tolerant amphibian.

Many other animals take the opposite approach and avoid freezing altogether. Some polar fish make antifreeze proteins that stop ice crystals from growing in their blood, and birds keep their feet just above freezing with a clever blood-flow system, explained in Why don’t ducks’ feet freeze?

Why scientists study frozen frogs

Wood frogs have been studied for decades, notably by the Canadian biologist Kenneth Storey and his colleagues. Scientists want to understand how cells survive ice, drying out and months without oxygen, and then recover. Some researchers hope these lessons could one day help improve the cold storage of human organs for transplants, although that goal is still far off.