The Frog That Freezes

Part One · Getting Ready

Could a frog freeze like an ice cube and still be alive? In Alaska, one can. It is called the wood frog. Every winter it freezes solid. Its heart stops, and it stops breathing. Then, in spring, it thaws out and hops away.

The wood frog is not big. Even a grown one is only about two inches long. It has a dark stripe across each eye, like a mask. It is also the only amphibian that lives north of the Arctic Circle.

So how does a small frog get through an Arctic winter? First it settles into a shallow dip in the ground. A layer of dead leaves covers it. Then the snow piles on top, like a second blanket. Now the frog is ready to freeze.

When the air turns freezing, the cold starts at the frog's skin. Its eyes and legs begin to freeze first. But inside, the frog is busy. It pumps water out of its cells and into the spaces around them, so there is less water inside to freeze.

Next, the frog's liver makes a sugar called glucose, and the frog pumps that sugar into its cells. The glucose works like antifreeze. It makes a thick, syrupy juice inside each cell. That juice keeps the cells from freezing and from drying out.

Part Two · Frozen and Thawed

So the ice grows in the spaces around the cells, and not inside them. Within a few hours, ice fills the frog's belly and wraps around its organs. Flat crystals of ice slide in between its skin and its muscles, and its eyes turn white.

By now, more than half of the water in the frog's whole body has become ice. Its heart stops beating. It does not breathe, and it does not move. The frog stays in a kind of pause until spring, when it thaws.

How long can a wood frog stay frozen? For as long as seven months! Alaska's wood frogs freeze for longer than wood frogs anywhere else in North America. Imagine lying under the snow, as still as a stone, while winter goes on and on above you.

Then spring comes. In April and May, the ice begins to melt along the edges of Alaska's ponds. The frogs thaw out and come back to life. Their hearts and livers, which froze last, thaw first. Soon the rest of the frog warms up, and it can move again.

Within hours, the male wood frogs are on their way to the shallow ponds. They call out loudly to find mates, and their calls sound like ducks quacking. How does a frog that was frozen solid know when to wake up?