Hibernation is a survival strategy for bears, enabling them to endure cold months when food is scarce. During this period, their bodies undergo physiological changes that conserve energy. This adaptation helps ensure their survival until resources become available again, especially in regions with harsh winters.
When Bears Begin Hibernating
Bears begin their hibernation or denning period as winter approaches, though the timing varies. For many species, including black and brown bears, this occurs between October and December. Male black bears often enter dens around mid-December, while females, especially those expecting cubs, may den earlier, sometimes by late November. Brown bears, including grizzlies, go into their dens between mid-October and early December. This winter sleep can range from a few weeks to eight months, depending on the bear species and its environment.
Key Factors Influencing Hibernation Start
Several environmental and biological factors influence when a bear starts to hibernate. Geographical location plays an important role; bears in colder, northern climates hibernate longer, often for six to eight months, compared to those in warmer, southern regions where hibernation can be shorter or even skipped. For example, black bears in northern areas may begin denning in September or October, while those in eastern North America may delay until late November or December if food remains available.
Food availability is another important determinant; a scarcity of high-calorie food sources triggers the onset of hibernation. Bears without sufficient fat reserves may emerge earlier or interrupt their hibernation to forage. Individual characteristics also contribute to the timing, with pregnant females entering dens earlier and remaining there longer than males or other bears. This extended denning period allows them to give birth and nurse their cubs in a sheltered environment.
Preparing for Winter Slumber
Before entering their winter dens, bears engage in a period of preparation, both behaviorally and physiologically. This phase, known as hyperphagia, is characterized by excessive eating and drinking to build substantial fat reserves. Bears can consume up to 20,000 calories per day, packing on hundreds of pounds to serve as their primary energy source during months without food or water.
Bears seek out or construct suitable dens. These can be hollow logs, rock crevices, burrows, or large nests made from branches and leaves. The den provides shelter from harsh weather, conserves body heat, and offers a secure location for the long period of inactivity, particularly for pregnant females who will give birth there. This preparation ensures they have resources and protection to survive the winter.
The Physiological State of Hibernation
During hibernation, a bear’s body undergoes physiological changes, differentiating it from typical sleep. While their body temperature drops, it remains relatively high compared to “true” hibernators like rodents, decreasing from around 100°F to 88-90°F. This allows them to remain somewhat responsive and able to defend themselves if disturbed.
A hibernating bear’s heart rate slows, from a summer rate of 40-50 beats per minute to as low as 8-19 beats per minute. Respiration also decreases, with bears taking as little as one breath every 45 seconds. Their metabolic rate can drop by 75%, allowing them to conserve energy efficiently and live off fat reserves for months without eating, drinking, urinating, or defecating. Bears also recycle urea into protein, maintaining muscle mass and organ health despite prolonged inactivity.
Waking from Winter Sleep
As spring approaches, bears gradually emerge from their winter dens between March and April. The primary triggers for this awakening are rising temperatures and increasing food availability. Male bears emerge earlier than females, especially those with newborn cubs, who remain in the den longer, sometimes until mid-April or May.
Upon emergence, bears are hungry and immediately begin searching for food to replenish weight lost during hibernation. They may appear somewhat sluggish at first as their bodies transition back to an active metabolic state. This period is important for them to regain strength and begin accumulating fat reserves for the following winter.