How This Frog Survives Being Frozen

When you think about hardy arctic creatures bracing for a cold winter, you’re probably thinking of something huge and furry and possibly antlered. Odds are you’re not thinking of wood frogs.

But it may be time to cast that amphibian prejudice aside in light of a study in The Journal of Experimental Biology that examines the astounding ability of Rana sylvatica to freeze—and live through it.

R. sylvatica is a species of wood frog with a range that stretches from the state of Georgia all the way through Canada and into the Arctic Circle.

Deep-Frozen Frogs
There are a number of creatures, from reptiles and insects to marine life, that possess some level of freeze tolerance, but few can perform the trick quite like Rana sylvatica. The tiny amphibians can survive for weeks with an incredible two-thirds of their body water completely frozen—to the point where they are essentially solid frogsicles.

Even more incredible is the fact that the wood frogs stop breathing and their hearts stop beating entirely for days to weeks at a time. In fact, during its period of frozen winter hibernation, the frogs’ physical processes—from metabolic activity to waste production—grind to a near halt. What’s more, the frogs are likely to endure multiple freeze/thaw episodes over the course of a winter.

To test the freeze tolerance of Alaskan populations of Rana sylvatica specifically, the researchers compared them with frogs of the same species taken from the local Ohio population. While both exhibited impressive freeze tolerance, there were some obvious differences as well. While the Ohioan wood frogs could be frozen at -4 degrees Celsius and revived, the Alaskan wood frog was frozen at temperatures as low as -16 degrees Celsius before being thawed out and returning to its normal healthy state.

And Costanzo is convinced that the small but sturdy amphibians can survive even colder temperatures.

Avoiding Death
The way wood frogs avoid freezing to death is due to so-called cryoprotectants—solutes that lower the freezing temperature of the animal’s tissues. These include glucose (blood sugar) and urea and have been found in much higher concentrations in the Alaskan wood frogs than in their southern counterparts.

Increased levels of cryoprotectants help the frogs’ cells survive. In most animals, prolonged exposure to subzero temperatures causes cellular shrinkage—a process in which the formation of ice in the tissues pulls water from the body’s cells, essentially sucking them dry and eventually killing the cell. 

But cryoprotectants help the cells resist that shrinkage.

“The solutes tend to depress the freezing point [of tissue],” said Costanzo. “It limits the amount of ice that actually forms in the body at any part. The more of that cryoprotective solute you can accumulate, the less ice will form and therefore the less stress there is on cells and tissues.”

Costanzo and his team also detected the presence of an additional mystery solute in the northern wood frogs, not shared by the local Ohio frogs, and are planning future research to determine its exact nature.

Applications for Medicine
Beyond being fascinating science, the ability to freeze and unfreeze living organs and tissues without damaging them has potentially profound implications for areas such as organ transplantation.

“There’s an obvious parallel between what these frogs are doing to preserve all of their tissues simultaneously and our need to be able to cryopreserve human organs for tissue-matching purposes,” said Costanzo, noting that attempts to successfully freeze human organs for transplants have so far proved unsuccessful, perhaps due to their relative size and cellular complexity.

“If you could freeze human organs even for a short period of time, that would be a major breakthrough because then these organs could be shipped around the world, which would greatly improve the donor-matching process,” noted Costanzo.

And that, ironically, is a heartwarming thought.

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