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How Come? How animals survive hibernation

Why do some animals, like bears, hibernate in winter? asks reader Matthew Lawton.Spring is arriving in a few days. After an endless winter, we can look forward to the first blooming daffodils, hopping robins . . . and stirring bears.

When spring comes in the North, bears come out of hibernation. We think of hibernation as a kind of extra-long sleep, but it's actually more like the suspended animation of a science-fiction movie. A bear can go seven cold months without eating a bite, drinking an ounce or taking a bathroom break.

Hibernation is one answer to the question "How can I survive when nearly all the food is gone?"

For most animals, frigid winter is a dangerous time. Days are short and cold; nights are long and colder. Plants and their fruits are in short supply. Small animals that were the prey of larger animals may be hibernating themselves. Searching for what food remains can burn more calories than an animal gets from eating it.

Some, like migrating birds, go south for the winter. Others tough it out, and many die. The bodies of warm-blooded animals must keep their body temperature at its normal level -- say, 98 degrees. But among those that hibernate, body temperature and metabolism both drop, allowing an animal to get by on far fewer calories.

Besides bears, other hibernators include rodents like ground squirrels and prairie dogs, and three kinds of primates, all dwarf lemurs. A small hibernator's heart rate may drop from 300 beats per minute to 7; body temperature may drop from 100 degrees F. to near freezing (32 F.).

Hibernating bears lower their temperature less drastically, from around 91 degrees F. to about 80 F. However, a bear's heart rate drops from about 55 to 9 beats a minute. And his metabolism slows to just 25 percent of normal.

To prepare for their Big Sleep, bears spend the late summer pigging out. Each day a bear may gorge on 20,000 to 50,000 calories of food, from berries and nuts to birds' eggs, insects, field corn, and sometimes mice or other small animals. The bear may gain 4 to 16 pounds in 24 hours, building an enormous layer of fat to live on.

During hibernation, a bear's kidneys mostly shut down; toxin builds up in his bloodstream and the kidneys themselves are damaged. Somehow, bears survive, their kidneys healing and resuming their work in the spring.

As hibernation ends, a bear's body restarts its metabolism, raising his heart rate and temperature, and prompting the much-skinnier bear to sniff the spring breeze and lumber out of his den.

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