Bears are often associated with the deep sleep of winter, but their dormancy is a unique physiological state distinct from the “true” hibernation of smaller mammals. While animals like groundhogs and bats allow their body temperature to drop dramatically, bears enter a state of deep seasonal torpor, sometimes called carnivoran lethargy. This allows their body temperature to decrease only slightly, about 8 to 12 degrees Fahrenheit below their active temperature. This slight drop lets them remain responsive and wake quickly if disturbed. This adaptation means they can stay in their den for months without eating, drinking, or eliminating waste, relying entirely on stored fat reserves for energy.
Environmental Signals That Dictate Timing
The decision for a bear to enter its den is triggered by environmental signals, primarily the scarcity of food resources. As autumn progresses, the natural food supply, such as berries and nuts, diminishes. This signals that the bear’s energy intake can no longer sustain its active lifestyle. Bears are considered facultative hibernators, meaning they can delay or even skip denning if food remains abundant, confirming that resource depletion is the primary driving factor.
Another cue is the photoperiod, or the progressive decrease in daylight hours after the summer solstice. This acts as a reliable annual calendar for the bear’s internal biological clock. This change initiates hormonal shifts that prepare the bear for dormancy, regardless of short-term weather fluctuations. While dropping temperatures and the first heavy snowfall are noticeable changes, they are secondary signals that confirm winter’s approach, rather than the initial prompt for den entry. However, a significant early snowfall can prompt a bear to seek shelter sooner, often to conceal the den entrance.
How Species and Location Influence the Start Date
The precise timing of den entry varies based on the bear species, sex, reproductive status, and habitat latitude. Bears living in northern regions, such as Alaska or the Canadian Yukon, must enter their dens earlier and stay longer due to the prolonged winters. In these colder climates, bears may begin denning as early as October and remain until April or May, sometimes spending up to seven and a half months in torpor.
Conversely, bears in southern or coastal regions with milder climates and consistent food availability often begin denning later, sometimes as late as December or January. They may only remain dormant for two to five months. For instance, Mexican Black Bears may not den at all, or only for a few weeks, because a warm environment does not necessitate prolonged energy conservation. Within a population, pregnant females consistently enter dens earliest, often in late November, to ensure a safe environment for giving birth in mid-winter. Adult males typically den later, around mid-December.
The Pre-Denning Phase
The period immediately preceding den entry is a preparatory stage known as hyperphagia, characterized by a strong drive to eat. During this time, which spans late summer and autumn, a bear may spend up to 22 hours per day foraging. This intense consumption builds the necessary fat reserves that will fuel its winter dormancy. This allows the bear to gain up to 2 to 4 pounds of body weight per day, often increasing its body mass by up to 40%.
Bears must consume high-calorie foods, sometimes reaching 20,000 calories daily, to accumulate fat. This fat serves as the metabolic fuel source and provides water, as bears do not drink while in the den. Following hyperphagia, a short pre-denning phase begins, lasting one to two weeks. During this time, the bear’s activity levels decrease as it focuses on selecting and preparing its den site. The den, which may be a rock crevice, a hollow tree, or a dug-out hole, is often lined with soft material like leaves and moss to create an insulated chamber before torpor begins.
When the Bears Emerge
Dormancy concludes when rising environmental cues signal the return of a reliable food supply. The primary factors triggering awakening are increasing ambient temperatures and the lengthening daylight hours of late winter and early spring. The earliest bears to emerge are typically adult males, who often leave their dens between late February and mid-March.
Females with newborn cubs remain in the den the longest, delaying their emergence until April or even early May to allow their young to develop. Once awake, a bear will remain near its den for a few days, groggy and slow. The bear then ventures out to begin the search for the first available spring vegetation. This final phase marks the end of the torpor cycle and the beginning of the annual effort to replenish body condition.