Frogsicles: Wood Frogs Freeze to Survive Harsh Alaskan Winter

By Michael Kuhne, AccuWeather.com Staff Writer
September 25, 2014; 3:50 AM ET
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Buried beneath the snow-covered ice of the wind-battered, subarctic earth, Alaskan wood frogs freeze solid each year in order to survive the harsh, unforgiving Alaskan winter.

These unusual amphibians, seemingly impervious to the subzero chill, can remain in this extreme state of lifeless, suspended animation far longer than researchers originally thought.

"We originally thought that the frogs could only survive for a few weeks to maybe a couple of months at most, but our study showed that they can survive much longer," Institute of Arctic Biology at the University of Alaska-Fairbanks Researcher Don Larson said.

(Photo/Institute of Arctic Biology/ Uwe Anders)

"We didn't see any mortality in our frogs," he said, referring to the specimens used to conduct research for a paper recently published in the Journal of Experimental Biology.

Larson's research, possibly the first of its kind to study the frogs under natural conditions, has shown that the animals can remain in a suspended state for about seven months, demonstrating their extreme tolerance for the subarctic chill during the overwintering process.

AccuWeather.com Meteorologist Brian Edwards said while the average high is zero in January across the Fairbanks area, it is not unusual to see temperatures dip down to minus 40 F in the winter.

"They can go days without getting temperatures above minus 30 F," Edwards said.

In addition, the research demonstrated that the wood frogs will continually thaw and refreeze, which allows them to produce the means of their cryoprotection, or freeze-protection, through glucose.

"We found they can survive far longer than you can see something stay in your freezer," Larson said, adding the frog's unique biology prevents its cells from dehydrating during freezing and they do not become susceptible to freezer-burn.

Larson, originally from Wisconsin, has been studying frogs for nearly a decade, stating his interest in amphibians was sparked by the parasites afflicting frogs in Wisconsin.

Since he began his research in Interior Alaska, he has taken an interest in one of the only amphibians found in the most northern part of the world, the Alaska wood frog.

(Photo/Institute of Arctic Biology/Uwe Anders)

The process of cryoprotection comes from two separate systems acting together, the production of urea, a substance found in urine, and glucose, a blood sugar.

By thawing slightly during the day in the autumn months, which can begin in September, and repeatedly freezing at night, the process fuels the frogs to release more glucose. Over time, the sugar accumulates, giving the frog more protection against freezing.

"Glucose isn't made until the freezing starts," Larson said. "They only have so much time to make sugar."

The two substances stabilize the cell membrane and prevent dehydration during freezing, but the frog's body shuts down completely once freezing occurs.

"Their organs aren't functioning in any way," Larson said, adding that there is no activity in the heart or brains of the frogs while frozen. "As an organism, they are [essentially] dead."

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It is still not fully understood how the thawing process resurrects the amphibians from their icy tombs, he added, but a very small trace of metabolic activity remains in the animal's cells during freezing, which utilizes very low amounts of energy.

"They are actually hard to the touch when they're frozen," he said.

The amount of glucose in a single wood frog while in a frozen state is comparable to the amount of glucose found in the blood of a human child, he said.

The massive amount of sugar produced by the frogs would make them sweet tasting if one would venture to eat one.

(Photo/Institute of Arctic Biology/ Uwe Anders)

"They would be sweeter than soda," he said.

Wood frogs begin their annual deep freeze by digging into the earth, and covering themselves in leaf litter.

They remain within a few meters of their breeding ponds, and thaw in late April and May, which is their primary breeding season.

During the warmer months, the amphibians will feed before preparing for their annual return to the frozen earth.

While a few other amphibians and reptiles are known for their ability to withstand extremely low temperatures, the wood frog seems to be unique in its ability to remain in a frozen state and survive far longer than any other known species.

"None of them have the extreme tolerance of the wood frogs," Larson said.

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