The term that describes a bear’s winter hibernation denning period is denning. It refers both to the bear’s use of a sheltered den and to the seasonal period spent there during winter.
Bears usually enter dens when cold weather and reduced food supplies make remaining active costly. Black and brown bears may excavate earth dens or use hollow trees, rock crevices, caves, or spaces beneath roots. Before denning, they go through hyperphagia, a period of intense feeding that builds the fat reserves needed for winter.
Denning is often called hibernation, but bear dormancy is physiologically different from the deep hibernation of many small mammals. Bears reduce their heart rate, breathing, metabolism, and body temperature, yet their temperature falls only modestly and they can awaken if disturbed. Pregnant females may give birth and nurse cubs inside the den.
A common mix-up is torpor: torpor is a broad term for a temporary reduction in activity and metabolism, while denning specifically describes the bear’s winter life in a den.