Dear Landon,

When the cold weather comes, I bundle up in a sweater and explore the snow.

But my frog friends are never around then. In fact, I’ve never seen a frog in a sweater.

I asked my friend Erica Crespi why that is. She’s a biologist at Washington State University. She studies frogs and other amphibians.

She told me frogs are different from you and me. We’re warm-blooded. Our bodies use energy to make heat. When it’s cold outside, it’s still warm inside our bodies. We just put on warm clothes to keep our heat from escaping.

But frogs are cold-blooded. They don’t spend a lot of energy making heat. So, when it’s cold outside, it’s cold inside a frog’s body, too. That could be bad news during a winter freeze.

Crespi told me that’s why most frogs living in places with cold winters find a slightly warmer place to hunker down.

It’s warm inside the Earth, so even when it’s freezing outside, places just a few inches underground can stay above freezing. A blanket of snow can help hold in the Earth’s heat—kind of like a sweater does for us. So, many frogs spend the winter in an underground burrow. Or beneath a log or pile of leaf litter.

Other frogs hole up in a pond. Thanks to the soil on the bottom, and the bits of dirt and plant matter in the pond water, the lower parts of a pond may not freeze. So, a frog can find a warm spot underwater or burrow into the bottom.

That works because frogs breathe through their skin as well as their lungs. Plus, frogs have a strategy to survive near-freezing temperatures: brumation. It’s the amphibian and reptile version of hibernation.

A brumating frog slows its body way down, like it’s in suspended animation or deep sleep.

“They don’t need to eat, so they’re just living off the fat they’ve stored,” Crespi said. “They barely need oxygen, so their breathing rate goes way down.”

When the temperature outside goes back up, frogs wake up. They start doing normal frog things—like breathing normally, eating and laying eggs.

But scientists like Crespi are worried that climate change is making it harder for frogs.

“Frogs are so used to having predictable annual cycles, and things are becoming less predictable,” she said.

If the temperature goes up unexpectedly in the middle of winter, frogs stop brumating too early. They start moving around and laying eggs. If the insects they usually eat aren’t around yet—because it isn’t spring—the frogs can use up the fat they stored for winter.

If the temperature goes back down, the frogs will brumate again. But they’ll have less fat to rely on, and the eggs they laid too early might not do as well.

That’s one reason scientists study frogs. Another is that some frogs do something super weird to survive the winter.

They freeze.

Crespi studies wood frogs like this frozen one. Photo: National Science Foundation/Jan Storey

A few kinds of frogs have evolved ways to decrease water, increase sugars and form special antifreeze proteins in their bodies. That protects the most important parts of the frog—like its heart and brain—from ice crystals.

The result is a frozen frog that simply unfreezes when winter is over.

It doesn’t get cooler than that.


Dr. Universe