Frozen Frogs

December 8, 2014

Frozen Frogs

In winter, many animals burrow below the frost line to protect themselves from the Northeast’s bitter cold. But the wood frog has adapted a different strategy: It freezes to survive.

Wood frogs spend the winter under the snow, just a few inches down in the leaf litter or soil. That’s not deep enough to escape the frost, but they don’t need to: As the temperatures drop, up to 65 percent of the total body water in a wood frog can freeze and the frog can still survive. As ice forms in the frog’s arteries and veins, its heart and brain stop working and it becomes solid to the touch. It can spend two to three months in this frozen state, and when a thaw comes and temperatures rise, it can “melt” back into its normal condition over several hours, restart its heart, and hop away.

The amazing process works because “individual cells in the frog don’t freeze,” explains Angelena Ross, a biologist with the New York State Department of Environmental Conservation. Wood frogs, which are found throughout the northeastern United States and Canada, eat extra bugs before winter hits, storing excess starch in their livers. When the first freeze arrives, their bodies convert the starch into glucose (sugar). The glucose enters the frogs’ cells in a very high concentration and acts like a natural antifreeze, lowering the freezing temperature of water within the cells, Ross says. This allows the cells to stay liquid even as ice fills the spaces around them.

“If you put wood frogs in a negative 80 degree freezer, they would probably die,” Ross says. “But their freeze tolerance is enough to get them through in our climate.” In fact, wood frogs survive even worse winters farther north: They’ve been found in much of Alaska and are the only species of frog known to live north of the Arctic Circle. Wood frogs also can tolerate freezing and thawing more than once each winter, which may be helpful as climate change affects weather patterns.

Remarkably, wood frogs are not the only creatures that can survive freezing. The spring peeper and gray tree frog undergo a similar process, and painted turtles in the northern United States and Canada can too, but only in their first winter while still in their nests, Ross says.

Medical researchers are studying how wood frogs and other animals survive freezing in hopes that understanding the process may help us preserve donated human organs.

Meanwhile, when out on a winter hike, you don’t need to worry about stepping on a frozen frog and damaging it. “They’re probably not on trails,” Ross says. “They tend to go under the leaf litter where it is deeper.”

You’re more likely to encounter wood frogs in the early spring, when they crowd vernal pools for a few days to breed. The frogs, which are between two and three inches long, have a distinctive dark eye mask and are usually brown, tan, or rust-colored.

Last year the wood frog was officially named the New York State amphibian, after a campaign by schoolchildren. The title does not bring extra protections. “They’re not endangered or threatened,” Ross says. “They’re just a really cool animal.”

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Heather Stephenson

AMC Outdoors, the magazine of the Appalachian Mountain Club, inspires readers to get outside and get engaged. Learn more.