The Rip Van Winkles of the Animal World
By Jen Knight
Anyone who has accidentally left a can of soda in the freezer knows all too well what will happen: the can will burst. Water, unlike most substances, expands as it cools. This expansion, resulting in lower density, is what keeps ice cubes and icebergs afloat. It’s the reason why most living things—with the exception of aquatic life, which flourishes in subzero temperatures—cannot survive long in the freezing cold. Ice crystals in living tissue burst cell walls, puncture blood vessels and cause other fatal tissue damage—unless you are a wood frog.
Wood frogs are a widespread North American species with a range extending from northern Georgia to Alaska. Unlike most ranids, or “true frogs,” which spend most of their lives near or in water, wood frogs are forest floor dwellers. “But it is their specialized adaptation to extreme cold that truly sets them apart,” says Savannah Trantham, wildlife rehabilitator and co-founder of Appalachian Wild. “These cold-blooded creatures burrow below the freeze line and enter a state of dormancy when the weather gets cold. Wood frogs survive sub-arctic Alaskan winters by allowing their bodies to freeze.”
How does a small, thin-skinned frog pull off such an astonishing feat? “The same way your car radiator survives the winter: with the help of a little antifreeze,” says Trantham. “Wood frogs have adapted specialized metabolic processes that stockpile glucose and urea, a component of urine, in their blood and organs as the temperatures drop in the fall. These agents mix with water and lower the freezing temperature inside the cells, protecting them from damage.”
Some water is forced out into the interstitial spaces between cells where it can safely crystallize around mineral and bacterial “seeds” collected in the frog’s body for this purpose. This selective freezing covers 65 percent of the body and includes the skin, body cavity and eyes. The frog’s heart and lungs stop, blood does not flow and the remaining unfrozen cells enter a dormant state. The resulting “frogsicle” is an incredible example of cryobiosis—the metabolic ability to freeze and thaw to survive adverse conditions.
The benefits of the wood frog’s icy adaptations are not limited to winter survival; there is a substantial reproductive boost as well. Wood frogs can return to normal body function within 24 hours of thawing and are active as soon as temperatures rise above freezing. This makes them among the first ectotherms to “wake” in the spring, giving them early access to vernal pools for breeding. These small ponds are collections of melted snow or spring rain that tend to drain or evaporate by summer. Their temporary nature prevents populations of fish or other predators from taking up residence, making them ideal nurseries for vulnerable tadpoles, and early rising wood frogs have almost no competition for this critical resource.
Wood frog adaptations may hold benefits for humans as well. Researchers trying to refine transplant organ preservation are looking to wood frogs for clues to minimizing frozen tissue damage. Medical researchers are also studying how wood frog tissues withstand suspended blood flow in the hopes of treating stroke damage. The mechanism that signals the frogs’ hearts and brains to reactivate upon thawing is still unknown, making these unassuming amphibians among the most intriguing studies in the animal kingdom.
Jen Knight serves on the development committee of Appalachian Wildlife Refuge (AppalachianWild.org) and is the co-education director and senior naturalist at the Balsam Mountain Trust (BalsamMountainTrust.org).