for National Geographic News
The common wood frog displays a rare trait called freeze tolerance. When the mercury falls, the animal becomes, to the eye and touch, a frog- shaped ice cube. The way it does this may eventually be copied to aid human organ transplants.
"Two-thirds of their body water, or more, freezes," explained Jack Layne, a biologist at Slippery Rock University of Pennsylvania. "The heart stops, the breathing stops. For all practical purposes you'd assume that it was dead."
In reality, the frog's metabolism slows to a crawl, and its body temperature drops to between 21° and 30° Fahrenheit (6° and 1° Celsius). The amphibian's heart and brain cease to function.
Frozen frog experts, such as biochemists Ken and Janet Storey of Carleton University in Ottawa, Canada, believe the animals acquired their ability to withstand a deep freeze about 15,000 years ago, during ice age evolution.
Freeze tolerance allows common wood frogs (Rana sylvatica) to live in harsh climates as far north as the Arctic Circlethe only frogs to do so. But they can also be found as far south in the United States as Georgia.
The amphibians cannot survive if their body temperature drops below about 20° Fahrenheit (6° Celsius). But snow pack and other natural insulators can keep the frogs sufficiently warm during their winter hibernation.
A key to their survival is a natural antifreeze that prevents the amphibians' cells from dehydrating excessively during the freezing process.
Frozen Brains, Hearts
During this process, about two-thirds of the frog's body water freezes. The remainder, including water inside cells, remains liquid. Glucose produced by the amphibian's liver lowers the tissue freezing point (in the same way that ammonia lowers the freezing temperature of a car's windshield wiper fluid, which is mostly water.)
The glucose limits ice formation in the body and binds water molecules within the frog's cells. This curbs the damage caused by cell shrinkage, which is common with freezing.
"Normally under those freezing conditions, without glucose, the cells would dehydrate completely," said Boris Rubinsky, an engineer at the University of California at Berkeley.
In a recent issue of Discover magazine Rubinsky published images of temperature scanning electron micrographs (a sort of heat-based CT scan) of frozen common wood frogs. A cross section of a frog's liver illustrates how water remains in the cells.
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