Winter is Coming: How Do Frogs Avoid Freezing?

Yesterday while shopping at the mall, I noticed that some stores had already begun putting up holiday decorations. The racks were filled with sweaters and down coats. We humans (at least in the northeast U.S.) are preparing for winter. But what about our amphibian friends? How do they prepare for winter? After all, frogs would seem vulnerable to extreme cold with their thin skins and their need to constantly stay moist.

Actually, we don’t need to worry about the frogs. They are well-equipped to deal with the cold weather, even with Arctic temperatures.

Frogs are ectothermic, which means that they rely on their environment to regulate their body temperature. Birds and mammals, including humans, are endotherms. We generate heat chemically and internally by breaking down food. The bodies of ectotherms reflect the air, ground, and water temperatures around them. One advantage that ectotherms have over mammals is that they can survive for long periods without eating.

In the fall, frogs first need to find a place to make their winter home, a living space called a hibernaculum that will protect them from weather extremes and from predators. The frog then “sleeps” away the winter by slowing down its metabolism. When spring arrives, it wakes up and leaves the hibernaculum, immediately ready for mating and eating.

Aquatic frogs and toads such as the leopard frog and American bullfrog usually hibernate underwater in streambeds or on pond bottoms. Because aquatic frogs need oxygen, they lie just above the mud, or only partially buried in the mud, so they are near the oxygen-rich water. They may even occasionally slowly swim around.

Terrestrial frogs and toads typically hibernate on land. Those frogs and toads that are good diggers like the American toads burrow deep into the soil, safely below the frost line. Other frogs, such as the wood frog and the spring peeper, aren’t good diggers and so must scout out their winter homes in deep cracks and crevices in logs or rocks, or they might dig down into the leaf litter.

These frozen peepers and wood frogs might look dead; their hearts have actually stopped beating. But the partially frozen frogs aren’t dead. Instead, they have a kind of natural anti-freeze in their bodies. Ice crystals form in their organs and body cavity, but a high concentration of glucose in the frogs’ vital organs prevents freezing. When spring approaches and its hibernaculum warms up above freezing, a frog’s frozen body will thaw, and it will come back to life.

As you go about preparing for winter, think of the frogs with their amazing adaptations for survival, safe in their winter homes, waiting for spring.

Here’s a video from YouTube about the hibernation of a wood frog. It’s pretty amazing—take a look!

This is a partial repost of an earlier post from December 2010. Most of the information from the post came from an article in Scientific American, How Do Frogs Survive the Winter? by Rick Emmer.