During this fall feeding frenzy, grizzlies can eat up to 20, calories —and put on up to three pounds of weight—each day. This helps Mama Bear to conserve energy until hibernation, and may be a way to control the population if food is scarce.
If she has not accumulated enough fat by the time she settles into her den, the egg will spontaneously abort. Yellowstone bears typically dig their dens on slopes at high elevation. The den entrance is just large enough for the bear to squeeze through so it will cover quickly with insulating snow.
Bears in zoos will not hibernate if food is available, though they will slow down and sleep more than usual. Some zoo bears are fed year round, and do not hibernate. Since they do not undergo a winter weight loss like wild bears, some zoo bears can get very overweight, which is very unhealthy. Fortunately more zoos are allowing their bears hibernate during winter. Though it makes the animals unavailable for viewing, it helps the bears stay leaner and healthier. Bears do not need snow to make a den.
In fact, some bears may not dig a den at all. In one famous Pennsylvania bear study the researcher found bears denning in road culverts, underneath home porches or simply curled up on a nest of leaves. But if the weather is unpleasant bears will make a shelter by digging, crawling into the root structure of overturned trees, or using rock caves. In Alaska and elsewhere that might have large cottonwood trees that have hollow centers, black bears love to crawl up the outside of the tree, then back down into the hollow center.
When temperatures warm up and food is available in the form of winter-killed ungulates or early spring vegetation, bears emerge from their dens. Males, subadults, solitary females, and females with yearlings or two-year-olds usually leave the vicinity of their den within a week of emergence while females with new-born cubs remain in the general vicinity of the den for several more weeks Lindzey and Meslow , Haroldson et al.
Several physiological processes bears undergo during hibernation are of interest to medical researchers. When bears are hibernating and metabolizing body fat, their cholesterol levels are twice as high as during the summer and twice as high as the cholesterol levels of most humans Baggett Bears, however, do not suffer from hardening of the arteries arteriosclerosis or gallstones, conditions which result from high levels of cholesterol in humans.
The bear's liver secretes a substance that dissolves gallstones in humans without surgery. Another mystery of hibernation is that bears do not lose bone mass during hibernation. All other mammals which maintain non-weight bearing positions for an extended period of time suffer from osteoporosis, or a weakening of the bones Wickelgren When the substance responsible for this phenomenon is discovered it may help people who suffer from weak bones. Beecham, J. Reynolds, and M.
Black bear denning activities and den characteristics in west-central Idaho. Bear Res. Brown, G. The Great bear almanac. Lyons and Burford, publishers. New York, N. Craighead, F. Grizzly bear prehibernation and denning activities as determined by radiotracking. Haroldson, M. Ternent, K. Gunther, and C. Grizzly bear denning chronology and movements in the Greater Yellowstone Ecosystem. Ursus Jonkel, C. Black, brown grizzly , and polar bears.
Stackpole Books, Harrisburg, PA. Judd, S. Knight, and B. Denning of grizzly bears in the Yellowstone National Park area. Kolenosky, G. Hibernation can last days, weeks, or months depending upon the species. Chipmunks reduce their heart rate from the usual beats per minute to an almost undetectable 4 beats per minute during hibernation. Chipmunks do not sleep through the entire winter however, they wake every few days to feed on stored food and to urinate and defecate.
Hibernation is triggered by decreasing day length and hormonal changes in an animal that dictate the need to conserve energy. Before hibernating, animals generally store fat to help them survive the winter.
Many, like the chipmunk, wake up for brief periods, but for the most part, true hibernators remain in this low-energy state through the winter. Waking takes time and uses up an animal's energy reserve. Many animals once thought to hibernate, including bears, really only enter a lighter sleep-state called torpor. Like hibernation, torpor is a survival tactic used by animals to survive the winter months, and is triggered by colder temperatures and decreased food availability.
Torpor also involves decreased breathing and heart rates, and lower metabolic rate.
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