The annual winter sleep of bears is a survival mechanism that allows them to endure the cold and scarcity of food across Canada’s diverse landscape. The two primary species, the American Black Bear (Ursus americanus) and the Grizzly Bear (Ursus arctos), enter this state of dormancy to conserve energy until spring. This prolonged period of reduced metabolic activity is a direct response to the lack of available forage, as Canada’s harsh winters cover their food sources in snow and ice. Polar Bears (Ursus maritimus) are an exception; only pregnant females den for the winter, while others remain active hunting seals on the sea ice.
Timing Variation Across Canada
The moment a bear enters its den is a variable event influenced by species, environmental conditions, and geography. Generally, bears in Canada begin their winter sleep between mid-October and early December, with the exact timing dictated by local factors. The most significant variables are latitude and altitude, which directly affect the onset of cold weather and the disappearance of food sources.
Bears inhabiting the far northern regions of Canada enter their dens weeks before those in the southern, milder parts of the country. Similarly, bears living at higher altitudes in mountain ranges like the Rockies must den earlier than their counterparts in valley bottoms because snowpack and freezing temperatures cut off their food supply sooner. An earlier snowstorm or a sudden drop in temperature can signal the end of the foraging season, prompting a swift move into the den.
Grizzly Bears often begin denning earlier than Black Bears, especially in the western mountains, with some pregnant females starting as early as September. Males and non-pregnant females typically follow throughout October and November. Black Bears generally start later, around November, and their denning period can be shorter in areas where food remains available longer. The decision to den is less about the calendar and more about the immediate ecological pressure of food depletion.
Prepping for the Den
The behavioral phase preceding the winter sleep is a period of intense feeding known as hyperphagia, or “excessive eating.” During this time, a bear’s sole focus is to consume massive quantities of food to build up the necessary fat reserves that will sustain it for months. Bears can consume up to 20,000 calories per day and gain 20 to 30% or more of their total body weight in fat before denning.
Stored lipids serve as the bear’s only source of energy and water throughout the denning period. For a female bear, achieving this fat threshold is important because the implantation of a fertilized embryo is delayed until this point. This ensures she only carries a pregnancy if she has sufficient resources to survive winter and nurse the cubs. The hormone leptin, which normally signals fullness, is suppressed during hyperphagia, allowing the bear to eat almost continuously.
Once the bear has accumulated sufficient fat, it selects and prepares a den site, which is chosen for its security and insulation. Dens are typically located in hidden, sheltered spots, such as under the root mass of a large tree, in a rock cave, or excavated into a hillside. Grizzlies often dig dens on steep, north-facing slopes at higher elevations. Black Bears frequently use hollow logs, small caves, or brush piles, sometimes lining the space with leaves and twigs for insulation.
Physiological Changes During Winter Sleep
The winter sleep of bears differs significantly from the deep hibernation of smaller mammals like ground squirrels. Bears undergo a profound metabolic slowdown while maintaining a relatively high body temperature. This adaptation allows them to remain responsive to disturbances and gives them the ability to wake up quickly if necessary.
A bear’s body temperature drops only slightly, typically by about 3 to 5 degrees Celsius, maintaining it between 33 and 35 degrees Celsius. The heart rate sees a dramatic reduction, slowing from a summer rate of 40 to 50 beats per minute to a low of 8 to 12 beats per minute. This decrease in metabolic activity allows the bear to conserve energy without the massive temperature drop that requires periodic, costly arousals for smaller hibernators.
Bears do not eat, drink, urinate, or defecate for the entire duration of dormancy. They survive by metabolizing stored fat, which produces water as a byproduct, eliminating the need to drink. They recycle metabolic waste products, converting urea—a toxic waste from protein breakdown—back into nitrogen compounds used to build new proteins. This process prevents the accumulation of toxic waste, maintains muscle mass, and prevents bone density loss, allowing the bear to emerge in the spring without severe physical degradation.
The Spring Awakening
The emergence from the den, or the spring awakening, is dependent on local environmental cues. This event typically occurs between March and May, driven by rising ambient temperatures and the melting of the snowpack, which signal the renewed availability of food. The process of leaving the den follows a sequence determined by a bear’s sex and reproductive status.
Adult males, having the greatest energy reserves, are typically the first to emerge, often leaving their dens in early to mid-March. Solitary females and subadult bears follow several weeks later, waiting for more snow to melt and for early spring vegetation to appear. Females that gave birth during the winter are the last to leave, usually in late April or May, ensuring their newborn cubs are brought out when the weather is milder and forage is abundant.