There is fresh snow on the high mountain ridges, a bone chilling wind from the west rattles the tamarack, scattering golden needles across the forest floor. Tendrils of silver fog accent the hush that lies over the valley floor; winter is fast approaching and all of nature prepares.

Much of the wildlife that inhabits these north woods is getting ready for hibernation. Many mammals have the ability to sleep through the cold, gray days ahead. Chipmunks, skunks and squirrels as well as raccoons, rodents and bats simply bed down and hardly stir until they arise again in the spring. As true hibernators, their body temperatures drop to near ambient and heart rates slow dramatically as they experience a deep comatose sleep state of torpidity. However, these animals will periodically rouse themselves from their deep slumber to eat stored food, quench their thirst and to urinate or defecate. Rodents sleep very deeply while others slumber more lightly.

Bears are a bit different. Both black bears and grizzly bears go into a physiological state often called hibernation or winter sleep, yet it is not true hibernation but rather a winter dormancy. During a bear’s winter sleep their heart rate decreases only slightly and body temperature drops only a few degrees from normal. Usually the bear will not awaken during this extended deep sleep and therefore they do not drink, eat, exercise or defecate and will remain in their den for the entire winter. Hibernating bears are quite an amazing biological wonder. Although they are completely inactive all winter, they do not suffer muscle atrophy or bone loss as a human would. Bears' bones continue to grow during their winter sleep and their reserve of fat meets all their nutritional needs.

Left undisturbed, bears will sleep all winter long, sometimes without even changing position once they are comfortably settled in. Although deeply asleep, bears are easily aroused and can, if necessary, respond and be active very quickly. Like humans, a disturbed bear can be a very grumpy bear.

Females give birth to their cubs during their winter sleep and together they snuggle until spring. Scientist tell us that bears do not enter into deep hibernation because they require a higher body temperature to meet the demands of pregnancy, birth and nursing their young. Actual births are rarely observed in the wild; however, it is believed that bear cubs are born between early December and late January or early February. Pregnant females are the first to retire to their dens, followed by mothers and their cubs. Last to settle down are the big males. Female bears have been known to hibernate from the end of September until May and even early June.

In Montana, most bears enjoy their winter sleep for as many as five to six months or more, usually entering the den in the latter part of November and not awakening until mid April. The length of hibernation depends on location, climate and the sex, age and reproductive status of the individual bear. While temperature and the amount of daylight hours are important motivators in dictating when bears hibernate, the most important factor is whether bears have eaten sufficient amounts to keep them going throughout their long rest, A malnourished bear with an inadequate reserve of fat may not hibernate or will do so only for a short time.

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