Not a Peep Out of the Woods When Winter's Freeze Comes

We’ve had some evenings recently that were so unseasonably warm that you may have heard the pleasing peeps of tree frogs coming from your local woods.

It’s unusual to be hearing tree frogs in December. (But, then again, who’s to say what is usual with the weather any more?)

Commonly, we don’t hear tree frogs in December – or January or February – because it’s a little difficult to make a “peep” when you’re frozen solid.

Yep, frozen solid.

Wood frogs (Rana sylvatica) and our local tree frogs – gray tree frogs (Hyla versicolor) and spring peepers (Pseudacris crucifer) – each have an amazing adaptation for surviving the winter: rather than try to fight off the cold and stay warm, they just freeze. Come spring, they thaw and off they go. (They can even thaw and re-freeze, thaw and re-freeze, through the temperature whims of winter.)

For animals that can’t migrate south, dealing with the cold is a critical matter. Deer grow winter coats. Raccoons, chipmunks and skunks curl up in their burrows and go into torpor. Seals have insulating blubber but also can restrict their blood flow to their “core” to prevent heat loss. Bullfrogs sit on the pond floor and hibernate. Turtles bury into the pond mud and also hibernate.

And if ice, snow or bitter cold air reaches under the leaf litter, wood frogs, gray tree frogs and spring peepers turn nearly as solid as a rock.

It’s a great strategy in two ways: the frogs don’t need to expend energy trying to stay warm; and the bugs that the frogs eat aren’t around in winter any way, so freezing solves a potential hunger issue.  But the catch: raccoons, skunks and other creatures that eat frogs sometimes do come out on warmer winter days, and there’s nothing better than digging up a tasty frozen frog-sicle.

Here’s how a frog freezes without dying: special proteins in its blood cause water in the blood to freeze first, and that resulting ice sucks most – but not all – of the water out of the frog’s cells.  At the same time, the frog generates large quantities of glucose from its liver and this sugar acts as a natural anti-freeze.

The frog enters a state of suspended animation. Its heart stops. Other organs shut down. It doesn’t breathe.

Here’s a good video that explains the process in wood frogs: www.wimp.com/woodfrogs/

Naturally, scientists find this very interesting and wonder how we might replicate and use this ability – perhaps when we’re under critical time constraints while transporting human organs for transplants.

At The Maritime Aquarium, the habitats in our “Frogs” exhibit aren’t seasonal so you can come see the gray (and green) tree frogs in a non-frozen state – even on the coldest, snowiest of winter days.