Hibernation: Deeeeeeep sleeeeeeeep

POSTED 11 OCT 2003

1. Chillin' out

2. Cold trance or transplant?

3. Stroke of luck

4. Waking, meeting, sleeping, mating

The 13-lined ground squirrel is a hibernator that lives across much of North America. This critter forms a spherical shape to minimize heat loss.

Hibernating animals are practically frozen. How come they don't get hurt when they  warm up?

Scientific purists say the black bear is not a true hibernator, but it does enter a state of extremely slow metabolism and heart rate during the winter. Photo: Washington Department of Fish and Wildlife.

Cold comfort
Whether you want to imitate Snow White or simply long for a longer snooze, you've probably wondered about hibernation -- the state of almost suspended animation that bears, groundhogs and ground squirrels use to survive gruesome winters.

During hibernation ("torpor" in scientific argot), heartbeat and breathing both slow to a few percent of normal. Body temperature is only slightly above the surroundings, and brain activity nearly ceases. Because hibernators get their energy from body fat, they can skip meals.

Hibernation is a drastic measure to survive winter: At first glance you might think a hibernating animal is dead. But during periodic "arousals," which last a few hours or even a day, hibernators briefly emerge from torpor (for purposes unknown). Then, in spring, with their fat largely burned away, they start the business of eating, mating, and storing energy for next winter.

squirrel curled up asleep in the palm of a human hand

Although hibernation is sometimes called "sleep," the conditions are different in terms of heart rate, duration, body temperature and brain activity. Plus, most mammals need to snooze more than once a year....

We Why Filers sure do...

All things considered, unless you have a furious fervor for freezing temperatures (think skiers, skaters and ice-cream makers), spending the winter in an insensate sphere sounds like a decent lifestyle to us....

Getting started
Plenty of questions remain about the biology of hibernation. What, for example, triggers torpor in the first place? "There's great diversity in the hibernating world," says Hannah Carey, a professor of comparative biosciences at the University of Wisconsin-Madison School of Veterinary Medicine. Hamsters and some others, she says, respond to cues from temperature, light and food. To induce hibernation, you just cover the grub dish and stash the animal in the cold and dark.

black bear's head and shoulders standing up above green brush and ferns In contrast, the hibernator that Carey studies, the 13-lined ground squirrel, features an onboard clock. Even if the food dish is full, the cage is lighted for half the day, and the thermostat is stuck at 70 degrees Fahrenheit, the little mammal starts to look kinda droopy in late August or early September, she says. "If they have ... gained a lot of fat, they get sluggish and act like they want to hibernate." Lab animals can't dig a burrow, but when Carey eases them into a cold, dark cage with lots of nesting material, they are "happy as clams, they can finally do what they want."

Before we move on, let's deal with some quibbles on lingo. Some scientific purists say bears don't hibernate, because their body temperature doesn't fall below 20 degrees Celsius. Bears get torpid enough to pass the stringent scientific standards here at The Why Files, but we feel obligated to draw the line somewhere, and we'll draw it at reptiles. While some snakes and lizards do go dormant in winter, their physiology is different from mammals, so we'll have to leave the reptiles out in the cold.

What does hibernation have to do with organ transplants?


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