Q. Besides bears, what other animals hibernate?

Belmont

A. This might come as a surprise. According to a number of experts, bears don't really hibernate. They are more like nappers -- just ask anybody who's had the misfortune of stumbling upon a bear's den during a winter hike.

Some scientists say the only true hibernators are deep sleepers that can take an hour or more to wake to a normal level of activity. In New England these are the woodchuck, little and big brown bats, and the jumping mouse (so named because it looks like a miniature kangaroo).

In all mammals that go into some level of dormancy, environmental conditions trigger the brain's hypothalamus to release hormones that signal the metabolism to slow.

Dr. Edgar Folk, professor of physiology at the University of Iowa, has been studying hibernation since the 1930's. ``It's a marvelous symphony in various organs all working together to change the body's temperature,'' he says, and the hypothalamus is the conductor that orchestrates those changes.

The first change is a drop in temperature. In deep sleepers, it drops from the high 90s Fahrenheit to just above freezing. The breathing rate drops to just one or two breaths per minute. The heart drops to just four or five beats per minute. Blood pressure falls way off, too.

Here's another surprise. Even the truest hibernators don't stay asleep all winter. Folk says they wake up every four days on average to eat and drink (except for the woodchuck, which feeds on fat stored in its body) from supplies stored in their burrows. They defecate, urinate, and go back to sleep.

The lighter sleepers, the nappers, include bears, raccoons, skunks, and chipmunks. But if the definition of hibernation includes some slowing of the metabolism, Folk says skunks and raccoons may not qualify: ``We were disappointed to find in many experiments we did that skunks and raccoons show no sign of metabolic depression,'' he says. They just sleep.

Bears are in the middle. They're not deep sleepers, but they're not plain old nappers either. They prepare for hibernation by increasing their calorie intake from 6,000 to 20,000 a day. Then, weeks before they enter the den, they stop eating and don't take another bite until they emerge in the spring.

Once bedded down, a bear's body temperature drops about 10 degrees. Its heart rate goes from 50 beats a minute in summer sleep to just 8 per minute. A hibernating bear's breathing rate has never been measured, but Folk says they use 50 percent less oxygen.

``Bears really do hibernate,'' Folk says. ``They're just better hibernators than the smaller animals. Evolution has given them a form of hibernation that's not as deep, so they can protect themselves from predators, especially other bears, which can be quite nasty.''

But hibernating bears neither eat, drink, defecate, nor urinate all winter. Dr. Ralph Nelson of the Carle Hospital's Research Center in Urbana, Ill., discovered that bears have proteins that let them convert the poisonous ammonia in urine back into usable proteins. Their food comes from stored body fat -- remember those 20,000 calories a day? -- a process that also creates water. Bears lose 15 to 25 percent of their body fat over the winter, but remarkably, their lean body mass actually increases.

Outside New England, deep hibernators include marmots, hamsters, and dormice (named from the French ``dormir'' -- to sleep). Hamsters can drop from 300 breaths a minute to one every two minutes. Beatrix Potter's Mrs. Tiggywinkle, the European hedgehog, is a true hibernator too. Hedgehogs can survive on one breath every twenty minutes!