Brown bears usually enter a winter resting period between October and December.
Many bears find or dig a den which they may use for several consecutive years. Before winter sets in, bears cushion their dens with grass, leaves, ferns, moss and lichen.
During hibernation, bears reduce their metabolic rate, their body temperature drops, their heartbeat dips to an average eight beats per minute, and they may breathe only once every 45 seconds or so. Due to this inactivity, they may also lose up to 40% of their body weight.
The scientific community is divided about how to describe the bears' resting condition: as winter sleep, winter rest, torpor, hibernation – all these terms are in circulation. But it is always about saving energy during a time of scarce food.
Brown bears are not 'true' hibernators like, for example, hedgehogs. Although the frequency of their heart beat and breathing slows down, the bears are easily woken and are able to defend themselves in the case of attack.
In general, Asiatic black bears do hibernate within the the northern latitudes (Russia, Korea, Japan and northeastern China) of their range. Further south, where food is available year round, hibernation is not necessary.
Bears are extraordinary individualists! Different bears will sleep for different lengths of time, put on different amounts of weight before hand, and their bodies drop to different temperatures. A high degree of flexibility is involved in hibernation, which is part of what allows bears to thrive in different conditions.
All our sanctuaries around the world provide the appropriate materials and landscape for bears to hibernate. Their health is closely monitored by veterinarians and sanctuary staff, and the bears are given plenty of undisturbed time to hibernate in peace.
The natural instinct to hibernate is so ingrained in a bear’s DNA that although a bear may have been born in captivity, knowing nothing but concrete underfoot, they can in essence (re-)adopt this natural behavior, given the proper circumstances.
At our sanctuaries and partner projects, almost all of the bears hibernate in winter. Some even have an extra long first sleep, but some bears take longer to mentally recover and thus it can take some years before they first hibernate.