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How Do Alaskan Wood Frogs Freeze And Thaw?

Alaskan wood frogs can freeze and thaw due to sugar buildup in their bodies.

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Did you know that Alaskan wood frogs [Lithobates (Rana) sylvaticus] can be frozen and thawed and survive? Researchers have known for years that the frogs have this capability based on laboratory experiments. But two researchers with the University of Alaska at Fairbanks wanted to conduct experiments on the frog in their natural environment and the following is what they found. 

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Researchers Don Larson and Brian Barnes with the University of Alaska at Fairbanks have studied the frog in their natural habitat rather than in a laboratory environment and have found that the frog overwinters on the ground under duff and leaf litter creating what is called a hibernacula, or a tent for wintering. Temperatures in the hibernacula can reach below freezing for more than six months with minimum temperatures of minus 4 degrees.

How does the frog freeze and thaw all over again? And how long could the frogs survive in their natural habitat? That is one of the duo's key questions and their findings are interesting. 

According to the researchers, the frogs pack their cells with glucose that helps to reduce the drying of their cells, stabilizing them in the process, which in scientific terms is called cryoprotection, sort of like what you see in the movies.

“Concentrating sugar inside the cell helps balance the concentration of salts outside the cell that occurs as ice forms,” said Barnes. “Less water leaves the cell than if sugar was not present and sugar and other cryoprotectants are thought to "hold" water inside the cell.”


In previous lab studies, researchers found that the wood frog in their natural setting acquired higher concentrations of glucose in their tissues than frogs that froze in the lab. The outside frogs retained glucose concentrations that were 13-fold higher in muscle tissue, 10-fold higher in heart tissue and 3.3-fold higher in liver tissue when compared to frogs frozen in the lab, he researchers said. This enabled the frogs to survive colder temperatures and for a longer duration than previously thought.

“In the field in early Autumn it’s freezing during the night, thawing slightly during the day, and these repeated freezing episodes stimulate the frogs to release more and more glucose,” Larson said. “It’s not warm enough for long enough for the frog to reclaim much of that glucose and over time it accumulates giving the frog more protection against cell damage.”

Frogs frozen in the lab were kept frozen at a constant temperature. Without the freeze thaw cycle that Larson noted in the wild, the frogs generated glucose only when they froze the first time. Lab-frozen frogs are held at a constant temperature and without the freeze-thaw cycles Larson observed in the wild and so the frogs made glucose only when they initially froze and that was that.

Larson believes that if science can figure out how to freeze human organs without them becoming damaged, it would enable more time for organs to be reached by people who need them to live. 


John B. Virata keeps a ball python, two corn snakes, a king snake, and two leopard geckos. His first snake, a California kingsnake, was purchased at the Pet Place in Westminster, CA for $5. Follow him on Twitter @johnvirata