How do animals like bears sleep away the winter?

Emily Dunne, 6, a first-grader at William L. Thompson School in Lake Villa, asked, "How do animals like bears sleep all winter long? How do they stay alive when they don't eat anything? How do they know when it's spring and time to get up?

Heavy winter snows and cold temperatures make foraging for food very difficult for some animals, so they sleep away the time until the spring brings fresh berries and new grasses to eat. A few animals avoid winter by hibernating. It is true that bears are prone to dozing during the cold months, but bears are not considered true hibernators.

Scientists are still discovering the mystery of hibernation, the process in which animals sleep through the winter and wake in spring.

Woodchucks, ground squirrels, rodents, Western European hedgehogs, bats, some animals that eat only insects, some marsupials, and some rattlesnakes are true hibernators. One things all these creatures have in common is that they are small and weigh only a few pounds, especially when compared to a 500-pound bear.

Hibernators gain weight during the summer and fall, adding what scientists call brown fat around the parts of their bodies that protect their organs -- their shoulders and back.

They also have a special chemical in their blood -- hibernating inducement trigger or HIT. HIT is present during hibernation and makes an animal sleepy. Their heart rates and breathing rates plummet and their body temperatures drop 30 degrees or more. They sleep for a few weeks at a time, then wake up to go to the bathroom, take a few bites of food, and then resume their sleep.

Spring triggers the waking state although scientists don't know exactly what brings these animals out of their very long naps.

Bears gain weight in the summer like a true hibernator, adding the brown fat. They also sleep for a good part of the winter. As they curl up for their long naps, their heart rates slow down and body temperatures drop a few degrees. They don't eat from September through April, instead living off the stored fat.

One amazing thing bears do that keeps them from being true hibernators is that they give birth to one or two cubs during the late winter months. The mother cleans and nurses her babies while in a semi-sleep state, and then she wakes in the spring to forage for food.

In Wisconsin, where there are an estimated 13,000 black bears, most bears are now in their dens sleeping. Experts call a bear's sleep state "denning."

Keith Warnke, big game specialist for the Wisconsin Department of Natural Resources, said, "Dens can be any sort of place -- fallen trees, big logs, in the side of a hill, anywhere there's any easy place to excavate -- cracks in rocks, a pile of leaves, cornfields, under houses."

During warm spells in winter, bears will wake up and look for food. "When it warms up, they can feel and smell the warmer air. They are energy stressed and they need food," Warnke said.

Look for more information about bears and hibernation on the Wisconsin Department of Natural Resources Web site at http://dnr.wi.gov/org/caer/ce/eek/nature/snugsnow.htm.