The Spring Miracle of Thawing Wood Frogs

Now that’s it’s officially spring, it’s pretty safe to say that all over the northeastern part of the US, Alaska, & Canada a variety of amphibians are currently making a serious ruckus. Here in the mid-Atlantic, I hear the unmistakable sound of spring peepers every where I go in the evening. For those of you living in latitudes more north and altitudes higher, however, the wood frogs are finally thawing out and performing their annual show.

I always find it fun to quietly sneak up on a vernal pool (temporary, spring-time pond) in early March here in the mid-A just so I can listen and watch the activity of wood frogs. If you’ve ever been near them as they begin to mate, then you’ll know that their sound can be almost deafening to hear in person. Regardless, they fascinate me, and I loved the time I spent studying them every spring.

However, what amazes me most about these guys is their ability to survive such harsh temperatures in winter as well as their ability to all simultaneously know when to wake up to mate. Currently, herpetologists and naturalists have a good understanding on the basics, but we’re still a bit stumped on all the details. We agree, though, that both functions are nothing short of miraculous.

Wood Frog. The Spring Miracle of Thawing Wood Frogs

The North American wood frog has developed an impressive strategy for surviving cold winters. It doesn’t seek warmth as other animals do. The wood frog goes with the cold and actually freezes in the winter months. Come spring, it thaws out, ready for mating season.

How does it do that?

We don’t really know. While we don’t know exactly how it all works (yet) we can see what’s going on inside the frog. It all starts at the first sign of ice. When the frog touches an ice crystal its body is signaled that winter has arrived and its time to go into freeze-mode. It will bury itself under leaves and mud as this biochemical feature is triggered. Water inside the frog is pushed away from the center — away from the organs — and all that surrounding water turns to ice. The frog slowly stops breathing, its heart stops beating, and it stays that way all winter long!

How does a frog survive this process, let alone wake up in the spring without tissue damage?

Here’s where things get really cool: the North American wood frog produces antifreeze within its body. Much like the stuff you put in your car, the frog’s antifreeze lowers its freezing point. Water typically freezes at 32 degrees Fahrenheit. Mixing in some antifreeze will lower the water’s freezing point to say -32 degrees Fahrenheit (depending on how much antifreeze is added). That’s basically what the wood frog does. When it touched the first ice of the season its body knew to start the winterization process. That starts with producing a lot of urine and storing it in the blood. Meanwhile the liver produces glucose (sugar). The sugar mixes with the urine creating antifreeze within the cells. Then the frog enters a state of suspended animation.

Come spring — or when the temperatures regularly hit 40 degrees Fahrenheit, the frog begins to wake up. It takes about a day to thaw out, starting with the insides, then the use of its limbs returns. Then it’s back to normal for the frog. After a long overdue bathroom break, the frog makes its way to its natal vernal pool.

For more cool sounds of amphibians, check out:
The Calls of Frogs and Toads
The Frogs and Toads of North America: A Comprehensive Guide to Their Identification, Behavior, and Calls
The Frogs and Toads of North Carolina: Field Guide and Recorded Calls

See also: Throwback Thursday: Jewels of Spring

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