Isabella Bautista | November 18th, 2024

Animals that live far from the equator have evolved a vast array of adaptations to help them survive the frigid, unforgiving winters: bears hibernate to slow their metabolism and conserve energy, penguins huddle together for warmth, and wolves grow thicker fur coats. But one animal in particular, the wood frog, utilizes a particularly puzzling and bizarre mechanism to endure the coldest season — it freezes completely solid.

Native to Canada, Alaska, and the northernmost parts of the continental United States, the cold-blooded wood frog must contest with the bitterest of winters. While most frogs hibernate underwater in the winter, wood frogs instead begin producing large amounts of both ice and antifreeze once they bury themselves under the leaves of their native forests. Sharp ice crystals between the frog’s bodily tissues pose the risk of puncturing the cells, so the antifreeze — glucose and urea — serves to lower the freezing point of the frog’s cellular fluids, preventing ice from forming inside the cells. (In fact, wood frogs can tolerate up to 100 times the blood sugar content of a normal human being.) Since these solutes are not increased outside the cells, ice crystals are still permitted to form there. 

If you were to pick up a wood frog when it is in this frozen state, you might think you were holding an amphibious zombie. Its eyes would be glazed over because of its frozen lenses, its muscles would be completely still, and it would have ceased its breathing and heartbeat. It can remain in this state for up to eight months. In the spring, the frog begins to thaw, restarting its circulation and — in a seemingly magical feat — leaping back to life in as little as two days. No one quite seems to know what initiates the thawing process, but that only means there is so much more to discover about these astounding little creatures.

Credit: Paplanus, P. (2016). Wood Frog (Rana sylvatica) [Photograph]. Wikimedia Commons. https://commons.wikimedia.org/wiki/File:Wood_Frog_%28Rana_sylvatica%29_%2825234151669%29.jpg. Licensed under CC-BY-2.0.

References

Roach, J. (2007, February 20). Antifreeze-Like Blood Lets Frogs Freeze and Thaw With Winter’s Whims. National Geographic. https://www.nationalgeographic.com/animals/article/frog-antifreeze-blood-winter-adaptation

U.S. Department of the Interior. (2022, October 6). Biological Miracle. National Park Service. https://www.nps.gov/gaar/learn/nature/wood-frog-page-2.htm