When the temperature outside drops, we head inside to our warm, cozy houses or bundle up in layers of clothing to go outside. What do animals do? Some migrate to warmer climes, some hibernate in caves, some burrow deep down into the earth, and some rest at the bottom of frozen lakes and rivers. And in the case of some frogs, they spend the winter frozen. 

Like something out of a science fiction story, Wood Frogs (Lithobates sylvaticus) freeze during the winter and survive into the following spring. How is this possible?

Wood Frogs have a unique adaptation that allows them to survive freezing temperatures. As temperatures begin to cool in the fall, the Wood Frog hunkers down into leaf litter. While this provides some insulation from the cold, it isn’t enough for it to survive the coldest days of winter. Wood Frogs have a type of special protein, called a nucleating protein, that causes the water in their blood to freeze first and ice to grow in between cells where it isn’t a danger to the frog, rather than inside the cells where the damage would be lethal. Because the freezing process begins at the earliest sign of wintry weather, the rate of ice formation is slowed, giving the frog more time to adjust to freezing temperatures. This also gives the amphibian the ability to survive the many freeze-thaw cycles of winter.

As the process begins, the frog’s brain sends a message to its liver telling it to convert stored glycogen into glucose, a type of sugar. The glucose begins to circulate through the frog’s bloodstream and into cells, protecting the cells from damage and minimizing the effects of dehydration. Ice forms all around the outsides of the internal organs, sucking water out of them, but leaving behind a thick, syrupy solution inside cells. The frog also begins to conserve its urine, which raises levels of plasma urea. The plasma urea acts as a strong cryoprotectant (a substance that protects cells or tissues from damage when frozen). Next, the frog’s heartbeat slows and eventually stops. Its breathing stops and there is no activity in its brain. The frog is “frozen” and appears to be dead.

In the spring, the frog begins the thawing process. Water flows back into cells, the heart restarts, blood flows again, the lungs take in air, and the brain becomes active. The complete thawing process takes a few days to complete. Once defrosted, the frog is off to eat, mate, and start the process over again the next winter.

Wood Frogs occur in the Ozark Highlands of Arkansas in the northwest and north-central part of the state. They are a tracked species, considered vulnerable in Arkansas.

Watch a video of wood frogs freezing and thawing here.

Sources

Photos:
Top — A Wood Frog (Lithobates sylvaticus). Photo by Kory Roberts, used with permission under a CC BY-NC license.

Bottom — A Wood Frog (Lithobates sylvaticus) sitting among leaves. Photo by Todd Fitzgerald, used with permission under a CC BY-NC license.

Main — A Wood Frog (Lithobates sylvaticus). Photo by Kory Roberts, used with permission under a CC BY-NC license.