As the last leaves tumble slowly to the ground and snow begins to blanket much of the country, many animals have prepared for the winter's scarcity of food by falling into the long slumber called hibernation. Secure in their burrows, some of the animals undergo remarkable physiological changes, including dramatic reductions of body temperature and heart rate.
Take the extraordinary case of a hibernating arctic ground squirrel. Its heart beats only a few times a minute, and its body temperature drops below the freezing point of water.
"It's hard to detect any kind of heartbeat in them. It's really difficult to tell if they're dead or alive. They're just cold little balls," says Kelly L. Drew of the Institute of Arctic Biology at the
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