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How and Why

Many species use hibernation to survive the rigors of winter

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By Ben Harder
Monday, December 6, 2010; 4:41 PM

If the cold days and long, dark nights of winter fill you with a primal urge to hibernate, rest assured you're not alone. Species in virtually every nook and cranny of the animal kingdom hunker down for the winter, each in its own manner.

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A chipmunk's heart rate drops from about 200 beats per minute in summer to a mere five beats per minute while hibernating. Wood frogs freeze almost as solid as ice cubes, only to thaw back to life in spring. Bears "hold it" for months on end without urinating. And some turtles cease to breath while hibernating in underwater mud. So don't curse your wintertime weakness for hitting the snooze button or lingering under the shower head: In the scheme of things, you're barely letting winter slow you down.

It's tempting to dismiss hibernators as lazy beasts that nap their way through the harshest stretch of the year. But hibernating is actually pretty tough biologically, in part because it requires organs to dramatically reduce energy usage. (It's better than starving, though, which could be the alternative if animals spent all winter searching for too-scarce food supplies.) And hibernators don't nap through anything: Hibernating and sleeping aren't the same.

Sleep is merely a state of lowered consciousness, a temporary muting of the mind. Hibernation, on the other hand, requires almost suspending metabolism; it's a near shutdown not only of the brain but of the entire body. In this subdued metabolic state, called torpor, hibernators ratchet down their inner thermostats, precipitously lower their heart and respiratory rates, and tune out nearly all external stimuli. Many hibernators are oblivious to loud noises, to light and even to being touched or moved.

Some will periodically rouse themselves from the chill of torpor, though. One of the first things they do? Take a nice warm nap right where they are. (They might also pee, or snack on provisions they'd laid in their burrows.) Then they go back into hibernation.

Hibernators are often bone-chillingly cold for most of the winter, with body temperatures sometimes falling below water's freezing point.

The wood frog takes things to an extreme: After burrowing into leaves on the forest floor, it lets most of the blood and other fluids in its body freeze solid. Ice crystals would normally destroy its cells, but the wood frog's tissues survive thanks to elevated concentrations of glucose and urea, which together act as a sort of antifreeze.

When spring comes, wood frogs reap the reward for their trouble: They warm up more quickly than frogs that hibernate underwater in partially frozen ponds, so they get a jump on finding the most desirable breeding grounds.

Bears don't cool off as much as other species in winter, and many experts say this disqualifies them from being considered true hibernators. But these big mammals show off some pretty cool adaptations when they take to their winter dens.

Going all winter without urinating, as bears can do, would be lethal to other species. They'd develop toxic levels of urea, a metabolic waste product that animals eliminate in urine. But bears have developed the ability to recycle urea into useful amino acids, the all-important building blocks of proteins.

Hibernating bears also maintain bone strength and muscle tone all winter long, despite having no real exercise. A similarly inactive person would quickly shed bone and muscle mass, becoming brittle and weak by spring.

For many animals, a major motivation for hibernating is a seasonal shortage of food, not just cold temperatures. Lowering their metabolism enables them to more economically use the energy they've stored as fat.

Big brown bats, for example, range across most of North America in the spring, summer and fall. But the insects they feed on are scarce in winter, so by early December, most of the bats gather in wintertime roosts and huddle together until their hunting prospects improve.

A hibernator uses not only less fuel from food or fat, but also less oxygen.

Most hibernating animals continue to inhale and exhale, just at a much-reduced rate. But a few reptiles, including aquatic turtles, completely stop breathing. They bury themselves in mud at the bottom of ponds and streams and get all the oxygen they need by absorbing it from air bubbles trapped in the mud.

The fact that hibernators are as numerous and as varied as they are - the club includes some ground squirrels and rodents, at least one bird, various snakes and the echidna (which is the platypus's closest living relative), among other species - suggests that the biological machinery that's needed for hibernation is both ancient and widespread in the animal kingdom.

A few years ago, scientists working in Madagascar discovered that at least one primate species, the fat-tailed dwarf lemur, hibernates.

If one of our animal relatives can do it, could we humans make use of hibernation, too?

Science-fiction writers seem to think so. The hibernation-like state called suspended animation has been a sci-fi plot device for decades.

Lately, medical researchers have also warmed to the idea. They figure that being able to induce metabolic suppression on demand - thereby lowering the oxygen and energy demands of critical organs - could be a lifesaver in battlefield medicine, transplant surgery and cardiovascular care.

Researchers recently succeeded in taking a first step when they temporarily (and harmlessly) suppressed respiration in lab animals using hydrogen sulfide, the gas - poisonous at higher concentrations - that gives rotten eggs their smell.

At least one human population seems naturally capable of metabolic suppression: babies. Anecdotes abound of young children who have fallen into frigid waters and been pulled out - blue, not breathing and seemingly dead - only to be revived after an hour or more without oxygen.

Scientists haven't been able to study the phenomenon in depth, for obvious ethical reasons. But they've determined that infants, much more than adults, exhibit a metabolism-lowering reflex triggered by the cold water. That ability may be an artifact of the now-dormant machinery that helped our ancestors cope with winter's chill.

Harder is general manager of health and science at U.S. News & World Report.


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