Between December and March bears are in a state of hibernation, or lethargy. This is due not so much to the winter cold as to the lack of food. During the winter months, a bear has to survive on its accumulated fat.

So the bear sets about preparing its winter den, cleaning out the debris, expanding it and making itself a comfortable bed of leaves, grass, moss and twigs. Bear lairs have been discovered in the oddest of places: in caves and rock crevasses, under the roots of fallen trees, in decommissioned mine shafts, in holes under mountain chalets or bridges. The entrance is always narrow and hard to spot.

Before retiring to bed, a bear must fatten itself by accumulating up to 10-15 centimeters of subcutaneous fat, growing in weight by up to 18 kilograms per week, because the long winter sleep may well cut its body weight by half. Its gut seals itself up by developing something like a natural stopper at the end, until all nutrients in its digestive system are fully utilized. The huge body size and thick fur minimize body cooling in the cold months, resulting in further energy savings.

Hibernation is a form of numbness where the heartbeat and breathing slow down considerably, while body temperature slightly decreases, like in normal sleep. This allows the bear’s brain to remain fully operational, especially in circumstances like giving birth, nursing and guarding the cubs inside the den.

A wakeful bear’s heart maintains a rate of 40 to 90 beats per minute, and when the bear is asleep it slows down to 8-10 beats. The breathing rate drops from 10-45 for a wakeful animal to a bare 1 or 2 cycles per minute for a sleeping one. Metabolism is also reduced by half.

Although hibernating, a bear remains fully alert and every now and then wakes up. If disturbed, it reacts instantly.