From National Geographic http://news.nationalgeographic.com/news/2011/02/110217-bears-hibernation-warm-sleep-animals-science/:
Photograph from All Canada Photos/Alamy
by Christine Dell’Amore
Hibernating black bears can dramatically lower their metabolism with only a moderate drop in body temperature, a surprising new study says.
The North American mammals generally slumber about five to seven months without eating, drinking, urinating, or defecating, and then emerge from their dens in the spring none the worse for wear.
Scientists have long known that to survive this lengthy fast, the bears drop their metabolism, the chemical process that converts food to energy.
But it was thought that, like most animals, the bears would have to drop their body temperatures to put the brakes on metabolism—each 18-degree Fahrenheit (10-degree Celsius) drop in temperature should equal a 50-percent reduction in the chemical activity.
Not so, according to the new study. A black bear in Alaska can lower its temperature—generally about 91 degrees Fahrenheit (33 degrees Celsius)—by only about 9 to 11 degrees Fahrenheit (5 or 6 degrees Celsius), yet bring its metabolism almost to a grinding halt, at 25 percent of the normal rate.