The grizzly bear, Ursus arctos horribilis, is one of North America’s largest terrestrial mammals. Its annual retreat into an underground den has led to the assumption that it spends the winter in a deep, traditional sleep. However, the bear’s winter state is a complex biological adaptation. This mechanism differs significantly from true hibernation, allowing the bear to survive months of resource scarcity and extreme cold.
Defining Bear Winter Sleep
Grizzly bears do not engage in the deepest form of winter rest, known as true hibernation, seen in smaller mammals like ground squirrels or bats. True hibernators drop their body temperature to near-freezing levels (2 to 10 degrees Celsius), requiring a long, energy-intensive process to wake up. Instead, the grizzly bear enters a state often described as “winter sleep,” “denning,” or “carnivore torpor.” This dormancy involves only a mild reduction in core body temperature, dropping about 5 to 12 degrees Celsius from normal. Because the temperature remains high, the bear can be roused quickly by a threat or disturbance. This ability to rapidly awaken is necessary for a large predator, and maintaining a near-normal temperature prevents the massive energy expenditure needed to rewarm a large body.
Preparing for Winter Denning
The shift to winter dormancy is preceded by an intense autumn phase known as hyperphagia, a period of excessive eating. Grizzlies must consume tremendous amounts of high-calorie food to accumulate the fat reserves necessary to sustain them through the five to seven months of winter sleep. A bear may spend up to 20 hours a day foraging and can gain 3 to 4 pounds of fat daily, which serves as the sole energy source for the entire denning period. Once sufficient reserves are built, the bear seeks or constructs a suitable winter den, typically an excavated hole. Dens are often located on steep slopes, under tree root systems, or beneath large boulders. They often select north-facing slopes, where snow accumulation provides an insulating layer that stabilizes the den’s temperature.
The Physiology of Winter Torpor
Once inside the den, the grizzly bear’s body undergoes metabolic changes to conserve energy over the winter fast. The heart rate slows dramatically from a typical active rate of 40–50 beats per minute down to 8–19 beats per minute, and the overall metabolic rate is suppressed by 50 to 60 percent. The most astounding adaptation is the bear’s ability to recycle metabolic waste, allowing it to remain anuric (not urinating) and not defecate for the entire denning period. The bear recycles the nitrogen component of urea back into amino acids, preventing toxic waste buildup and preserving muscle mass and bone density. By the time the bear emerges in the spring, it will have lost between 15 and 30 percent of its total body weight, almost entirely as fat.
Birthing During Denning
The grizzly bear’s reproductive cycle further highlights the unique nature of its winter sleep, as pregnant females give birth while dormant. Mating occurs in late spring or early summer, but the fertilized egg does not implant until the female enters her den in the fall, a process known as delayed implantation. If the female has not stored enough fat during hyperphagia, the pregnancy will not proceed. If successful, she gives birth to one to three tiny, blind cubs, typically in late January or early February. The cubs are born weighing only about one pound. The mother partially rouses herself to assist with the birth and nursing, but remains largely torpid, using only her fat reserves to produce milk for her young throughout the winter.