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.
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. (Related: "Frog Extinctions Linked to Global Warming" [January 12, 2006].)
"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."
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