Wood Frogs Sweeten Up to Survive Alaska’s Winters

With snow now covering the Anchorage landscape, temperatures dipping into single digits earlier this month, and daylight dropping below eight hours, my thoughts have turned toward the wild creatures who have evolved an assortment of ways to survive Alaska’s longest and harshest season. Among the most amazing adaptations are those utilized by Rana sylvatica—the wood frog.

The only North American amphibian to inhabit the Arctic, the wood frog is a true marvel. Yet from Angoon to Anaktuvuk Pass—and yes, here in Anchorage, too—the critter is virtually unnoticed as it hops, swims and mates among us humans. Partly that’s because wood frogs spend most of their lives on land, showing a preference (as their name suggests) for wooded areas, where they can easily hide. And except for a short period in spring, when males engage in grand singing competitions, they don’t make much of a commotion or otherwise draw attention to themselves.

Adult frogs leave ponds and lakes and other pools of water soon after mating and go their own separate ways, followed later in summer by their young, after the offspring have shifted shape and physiology from tadpole to frog (these “metamorphs” being yet another small wonder).

Their hunt for insects and other small invertebrates may take adult frogs several miles from where they bred. But as the air begins to cool, daylight hours shorten, and meals diminish, they retrace their hopping steps and find a convenient hiding place close to water (though whether they return to their breeding pond or the nearest available pool remains a mystery). There the frogs burrow beneath moss or decaying leaves or dig their way into sandier soils.

And this is how they’ll spend their winters: huddled alone in shallow, earth- and snow-covered shelters, somehow surviving even if the ground around them drops to 0° Fahrenheit or below. Which brings us to another impressive fact about Alaska’s wood frogs: they not only survive colder temperatures than their Lower 48 counterparts, but do so for much longer periods—up to seven or eight months in some locales.

As air and ground temperatures drop toward freezing and below, the frogs’ still and slowly hardening bodies internally remain hard at work for a while, with one organ kicking into overdrive.

While their eyes freeze into a whitened stare and ice crystals form in their abdominal cavity, the liver of hibernating wood frogs produces unusual amounts of glucose, a syrupy, sugar-rich solution. Entering the bloodstream, the glucose is flushed into all of the body’s cells, which keeps them from icing up and also prevents dehydration or other damage. So freezing occurs only in spaces outside the cells.

Put another way, wood frogs “effectively become super sweet” as they freeze, says Brian Barnes, a researcher (and former director of UAF’s Institute of Arctic Biology) who has long studied the hibernating strategies and mechanisms of Alaska’s animals. And that super sweetness is what keeps them alive. But neither kicking nor breathing.

Barnes is largely responsible for what’s known about the wintering adaptations of far north frogs. Years ago he and some graduate students decided to follow the movements of wood frogs by attaching tiny radio transmitters to their bodies. Besides hopping up to a quarter mile or more from breeding waters, all six radio-tagged frogs survived a Fairbanks winter while huddled beneath snow and soil.

“All of them dug into the soil, but not deeply,” Barnes later reported. “They were about one inch deep in leaf litter, three to four inches deep in sphagnum moss, or just barely buried in sand.” The insulating abilities of snow kept ground temperatures at 20 to 23 degrees even as air temperatures sank to minus 40 or 50 below 0 (Fahrenheit).

In an experiment he didn’t intend, Barnes left several other frogs in an uninsulated crate outside his home and then forgot about his subjects. Temperatures fell to minus 10 degrees, freezing them solid. Barnes naturally figured the frogs were dead meat. But when moved inside his heated garage, the frozen bodies gradually thawed. As they defrosted, the frogs began to twitch, then hop around.

Once he got over his amazement, Barnes decided to investigate the mechanisms that enable wood frogs to survive being frozen, which led to the discovery of their “super sweet” survival strategy.

What’s more, it appears that Alaska’s frogs become even sweeter than those farther south. The increased glucose production helps to explain their greater tolerance to extreme cold—the more sugary a solution, the lower its freezing point—but it raises other questions that for now remain unanswered. For instance: how the heck do they do that?

By the time their winter preparations are complete, wood frogs from the Panhandle to the Brooks Range will have frozen eyes, frozen limbs, frozen lungs, frozen liver, and a frozen brain. In the end, even the heart freezes up.

In all, about two-thirds of a wood frog’s body will turn to ice. In essence, it has become something of a frogcicle. Barnes says that if you were to drop a frozen frog onto a table, it would sound much like an ice cube. And if bent too far, its limbs will simply snap. And yet the animal is alive, in what is truly a state of suspended animation.

Barnes emphasizes that this process is a gradual one. For all of their amazing adaptations, not even wood frogs are able to survive being put into a freezer and left overnight.

Besides learning more about wood frogs themselves, Barnes and other researchers believe that a better understanding of the amphibians’ cold-weather strategies could benefit humans. For instance, we might learn how transplanted organs might be frozen and thawed without being damaged. Or perhaps scientists will come to better understand, and manage, diabetes in humans.

Just as remarkable as their winter freeze-up, is the wood frogs’ springtime meltdown. The frogs thaw from inside out. First the heart begins to beat (though scientists don’t know exactly how it’s “jump started”), then the brain and other organs kick into gear, and glucose levels return to normal.

Once completely thawed, the frogs dig their way out of the ground, hop to the nearest pond or lake, and proceed with mating, going about their vernal business as if nothing astonishing had just occurred.

Anchorage nature writer and wildlife/wildlands advocate Bill Sherwonit is a widely published essayist and the author of more than a dozen books, including “Living with Wildness: An Alaskan Odyssey” and “Animal Stories: Encounters with Alaska’s Wildlife.” Readers wishing to send comments or questions directly to Bill may do so at akgriz@hotmail.com.