Bears enter a state of winter dormancy to survive periods when food is scarce. This state is often mistakenly called hibernation. True hibernation involves a dramatic drop in body temperature, but bears maintain a relatively high core temperature, allowing them to remain responsive and easily roused. The den they construct or find must satisfy specific requirements for security and insulation, adapting directly to this unique survival strategy.
Selecting the Den Site
Den entry is driven primarily by the lack of available food, not just cold temperatures, and site selection begins long before snowfall. Bears seek maximum seclusion to minimize disturbance from predators or human activity. This drive for security often leads them to rugged terrain with lower visibility and dense canopy cover for concealment.
Den entry typically occurs in late fall or early winter, correlating with lower temperatures and diminishing food resources. Bears prefer steep slopes, which naturally aid in drainage and prevent flooding. Steep slopes also allow excavated material to be easily deposited downhill, concealing the entrance. They often select sites that accumulate a deep, insulating snowpack, providing additional thermal protection.
Architecture of Bear Dens
Bear dens are highly variable, reflecting the diverse landscapes and available resources. The den is a secure cavity, or den chamber, providing shelter from the elements. The most common type is an excavated earth den, dug into a hillside, often under the stable ceiling of a tree root mass or a rock overhang.
Excavated dens feature a small, restrictive entrance leading into a short tunnel and then opening into a larger chamber. The small entrance minimizes heat loss and serves as a defensive measure against intruders. For instance, a large den chamber may be over two meters wide, but the entrance remains only slightly larger than the bear’s body.
Bears also utilize existing natural structures, such as caves, rock crevices, or the hollow base of large trees and stumps. The use of hollow logs and stumps is common, particularly in coastal areas where large, decaying trees provide ready-made, well-insulated cavities. In milder climates or areas with insufficient cover, bears may create a surface den, which is a shallow scrape made in thick brush or under windfalls. Regardless of the structure, bears line the den floor with materials like grass, leaves, or moss to improve thermal insulation and comfort.
The Physiological State of Winter Dormancy
While denning, the bear enters a specialized state of winter dormancy, distinct from the deep torpor of true hibernators. This profound metabolic suppression allows the bear to survive for months solely on stored fat reserves. The heart rate slows dramatically, dropping from a summer average of 40–50 beats per minute to 8–12 beats per minute.
Despite metabolic reduction, the core body temperature only decreases slightly, stabilizing around 33 to 34 degrees Celsius. This high temperature allows the bear to remain moderately alert and capable of quick arousal, unlike true hibernators. The metabolism slows by up to 75% without causing the muscle atrophy or bone loss seen in other confined mammals.
A remarkable adaptation is the complete cessation of urination and defecation during the denning period. The nitrogenous waste product urea is recycled into new proteins by the bear’s body. This process conserves water and protein, allowing the bear to maintain kidney function and muscle mass while fasting. Females also use the den as a maternity chamber, giving birth to and nursing cubs in this quiet, well-insulated environment.