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Bears hibernate during the winter months when their food supplies are functionally nonexistent. Hibernation is a state of minimal activity when the body slows down; heart rate and breathing slow and body temperature and metabolic rate lower. 


Bears generally enter the den during fall and emerge the following spring, although the timing varies with reproductive status and between years. For example, pregnant females often enter the den earlier and emerge later than other bears. Female bears give birth in the dens, where the cubs nurse while the mother sleeps and show normal physical and physiological activity, i.e., they do not hibernate alongside their mother. Females with yearling also leave the den later than other bears, except for females with newborns, and rarely leave the den before snowmelt. The timing of the first snowfall appears to have the greatest influence on when bears go into their den, although bears will enter their den before the first snow if the first snowfall comes unusually late in the year.


 Illustration: Juliana D. Spahr,

The most common type of dens in Scandinavia’s southern bear population are made from excavated abandoned anthills. Old anthills are common in the Scandinavian forests, and their tufts effectively insulate against the cold and are usually covered with a layer of berry plants that help stabilize the den. Bears also use dens in rock crevices or caves, and between or under boulders. In rare cases, they use open dens that are made from branches, plants, and moss piled to form something resembling a large nest. Open dens are mainly used by large male bears, which end up spending almost 1 month less in the open den compared to other males. In all den types, bears commonly make a bed of tree branches, twigs, moss, and berries.


Bears commonly use different dens each year and often visit the next fall’s denning area several times during the summer. This suggests they choose their den site in advance. Dens are often centrally located in the bear's home range, and they will commonly choose a site somewhat near their previous den. It doesn’t take very long for a bear to dig and arrange their den, and they often enter the den a few days after their den has been prepared.


Hibernation can last up to 6 to 7 months for bears in Scandinavia, during which time they do not eat but live off the fat reserves they built up over the summer and fall. As you can imagine, this is a difficult time for the animal. They do not eat, drink, or pass waste for large blocks of time and lose weight at a rate of ~300-400 g per day. However, bears that leave the den periodically during winter lose 

more weight than bears that stay in the den, suggesting that leaving the den results in a greater loss of energy. Leaving the den periodically can also affect their physical condition when they finally emerge in the spring. For example, 60% of pregnant females that changed dens during winter emerged in the spring without cubs, while only 6% of those that stayed in their original den emerged without cubs.

Hibernation and Human Medicine

Hibernating bears are a physiological wonder, and there is much to be learned from how their bodies adapt to long periods of inactivity. Bears and human have relatively similar physiology, yet prolonged periods of rest cause a host of health problems for humans, while bears emerge from hibernation relatively unscathed. For example, several physiological adaptations prevent the bear from being poisoned by the waste products produced and accumulated in the body during hibernation. In addition, bears have adaptations that prevent brittle bones, blood clots, and muscle atrophy from occurring. Understanding the mechanisms behind these adaptations may provide insight into the treatment of people with similar disorders.

Image by Usman Yousaf

Hibernation and Human Disturbance

Any disturbance that causes a bear to wake up and change dens can be costly, as they must move through the snowy landscape in search of a new site and then prepare a new den. Thus, bears usually choose den sites at least 2 km from the nearest infrastructure with regular human activity, such as open roads and buildings, but are more tolerant to irregularly used infrastructure. Most bears will leave their den if there is human activity within 200 meters.


On average, about 22% of bears in central Sweden change dens during winter. At least 67% of those bears abandoned their den because of human disturbance. This includes hunting, especially hunting dogs, forestry activities, skiing, or a dog or human near their den for unknown reasons. Indeed, human activity may have caused a higher proportion of den abandonment, but it is often difficult to document. Most bears, about 95%, abandoned their den during winter and found a new den. Bears that abandon their den early in spring will generally not den again until fall.


It is difficult to change the level of human activity around den sites, as specific den sites are unpredictable and generally change from year to year. However, bears often return to a general area each year, and knowing which areas to avoid could help reduce conflict.

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