The freeze-thaw, freeze-thaw see-saw of this winter's temperatures may be a sign of global warming. But for now wood frogs are weathering the flux in style, according to an expert on the amphibians.

"They undergo freeze-thaw cycles all the time," said Kenneth Storey, a professor of biochemistry at Carleton University in Ottawa, Canada.

Some animals migrate to warmer climes for the winter and others burrow deep underground to sleep until spring. Wood frogs instead seek cover under leaves near the surface, where they actually freeze and thaw with their surroundings.

In his lab, Storey has taken the frogs through multiple consecutive freeze-thaw cycles and found no adverse effects. In nature the frogs consistently go through freeze-thaw cycles, he said.

"We have false springs here all the time where it gets really warm and all the snow melts and then suddenly—bam—the wind comes from the north and it's back down to minus 10, minus 15 [degrees Celsius], and they're fine," he said.

"So in nature and in the lab they have to do it, and they do."

Freezing Frogs

Storey studies the molecular mechanics that allow wood frogs to freeze and thaw. He hopes that doctors will one day be able to copy the technique to aid human organ transplants.

Currently doctors only have hours to get a donated organ into a living patient before the organ becomes too damaged. Freezing organs isn't an option, as the cells dehydrate.

Once the first ice crystals reach a wood frog, however, its skin freezes. The frog becomes hard and crunchy. "When you drop it, it goes 'clink,'" Storey said.

Special proteins in their blood, called nucleating proteins, cause the water in the blood to freeze first. This ice, in turn, sucks most of the water out of the frog's cells.

At the same time the frog's liver starts making large amounts of glucose—a type of sugar—which packs into cells and props them up.

The concentrated sugar solution helps prevent additional water from being pulled out of the frog's cells, which can destroy them.

"Inside the cells there's no ice," Storey explained. "It's just really, really, really dehydrated, all shrunk down osmotically and full of massive amounts of sugar."

Humans lack these nucleating proteins. So when our skin freezes, we get frostbite, which lethally sucks all the water out of our cells and causes them to collapse.

"Even if you take the ice away, it's way too late," Storey said. "All the cells are broken because you haven't made all that sugar."

The frogs, however, enter a state of suspended animation. Inside the cells there's thick sugary syrup, while outside the cells all the water is frozen.

"It can stay like that apparently, no beating heart or brain activity or anything, until you decide to thaw it," New York-based science writer Mariana Gosnell said Monday on a broadcast of the Pulse of the Planet radio program.

(This news story and Pulse of the Planet receive funding from the National Science Foundation.)

When temperatures warm and the ice melts, the frogs thaw. Water slowly flows back into the cells, blood starts flowing again, and the frog revives.

In the lab, Storey said, ice thaws in about 20 minutes and the heart takes another 20 or 30 minutes to start.

"Once the heart starts, it pumps the blood around the animal and the animal starts to revive, then it starts to gulp, then it starts to breathe, then it starts to hop away. So it takes a little while to reactivate after you've been frozen down," he said.

Too Warm?

According to Storey, the wood frogs can go through this cycle again and again. When spring finally arrives and decides to stay, the frogs hop around unharmed.

But warmer temperatures could ultimately prove troublesome if the amphibians never freeze in the winter.

"Freezing helps drop their metabolism and helps them survive over the long winter months when there's no food," he said.

"But our frogs would be fine if there was food out there, if the insects came out early and there was less winter. Yeah, they'd be laughing. Well, croaking actually."