Backyard Nature: Where do the frogs go when it’s cold? (Correction)

Alex Sanders, as program manager of the Potomac Appalachian Trail Club, spends a lot of time in parks and on local trails. And he is so tuned into frog calls, he spotted my error: The frogs pictured and heard in the article are not, in fact green tree frogs. They are green frogs. Green tree frogs do produce glucose, which works to prevent their blood from freezing, but green frogs do not. They instead burrow into leaf piles, which can reach up to 150 degrees Fahrenheit at the center. Providing a winter home for our amphibian friends is another argument for leaving a leaf pile here and there during the coldest months.

Here’s the original article:

A green tree frog soaking in a Linnean pond (summer 2022 photo)

by Marlene Berlin

Any child can tell you that bears hibernate in the winter, and many birds migrate to warmer climes. But the winter homes of local amphibians have long been a mystery to me.

The above pond in Linnean Park, by the new wooden footbridge, is a popular hangout for green frogs. At the height of summer, their croaks can be heard here and throughout the length of the stream.

As fall progresses, the frogs decrease in numbers and and song. And by the middle of November, their cacophony of mating croaks no longer greets those who walks through the area.

One day in mid December, I walked by, missing their croaking and wondering how these frogs make it through winter. And what happens to tadpoles?

As it turns out, they’re still here. The adults hibernate in a variety of places: the bottoms of ponds, in masses of leaves and aquatic vegetation, underground, or under leaf litter. These are not exactly warm and cozy spots for a winter’s slumber. Yet frogs survive colder winters than ours, and this Minneapolis-area parks blog explains how they do it: by producing their own anti-freeze.

[S]ome frogs accumulate glycerol in their bodily fluids during the fall when the temperature begins to cool. This protects the cells from rupturing when the frogs partially freeze.

When a frog begins to freeze, its liver converts the glycerol into glucose. The glucose is then circulated to the frog’s major organs so that ice crystals do not form in its organ tissues.

The green tree frogs that spend the coldest months underwater still need oxygen to survive, but they have very low requirements and breathe through special patches in their skin.

And those tadpoles? They can take a year to mature, and research indicates they stay in the water, where they remain active, feed, and grow.

In the meantime, I’m looking forward to hearing their croaks again, beginning in March.