Bears enter a period of dormancy during colder months, a remarkable adaptation that allows them to survive when food is scarce and environmental conditions are challenging. This seasonal behavior enables them to conserve energy and endure periods of limited resources. Their bodies undergo significant physiological changes, preparing for extended inactivity.
Timing of Hibernation
Bears typically begin their dormant period in late fall or early winter, usually between October and December, and emerge in early spring, often around March or April. The exact timing can vary depending on the bear species, its geographic location, and the availability of food. For instance, bears in colder, northern regions might den for longer periods, sometimes up to eight months. Those in warmer coastal areas might only undergo dormancy for two to five months, or even not at all if food remains plentiful. Pregnant female bears often den earlier and remain in their dens longer than males or non-pregnant females.
The duration of this dormant state is influenced by local conditions. Bears in areas with abundant fall food sources might delay entering their dens. Conversely, a lack of food can prompt an earlier entry into dormancy. While temperatures do play a role, the primary driver for the timing of denning is largely tied to food availability.
What Triggers Hibernation
The onset of this dormant period in bears is primarily governed by environmental and physiological cues. Decreasing daylight hours, known as photoperiod, act as a significant trigger, initiating hormonal changes within the bear’s body. While dropping ambient temperatures are also a factor, they are typically secondary to the diminishing availability of food resources.
Before entering their dens, bears undergo a phase called hyperphagia, characterized by excessive eating. During this time, they consume large quantities of food, sometimes up to 20,000 calories daily, to accumulate substantial fat reserves. This stored fat is crucial, as it serves as their primary energy source throughout the months of inactivity, allowing them to lose up to a third of their body weight.
Bear Hibernation Versus True Hibernation
Bears do not undergo what is considered “true” hibernation, a state observed in smaller mammals like groundhogs or marmots. Instead, their dormant state is more accurately described as torpor or winter lethargy. A key distinction lies in their body temperature; while true hibernators can drop their body temperature to near-freezing levels, bears only experience a moderate decrease. Their body temperature typically falls from around 100°F (37.7°C) to between 88-95°F (30-35°C).
Unlike true hibernators, bears can be roused relatively easily from their torpid state. Their metabolic rate, while significantly reduced by up to 75%, is not as profoundly low as in true hibernators. A bear’s heart rate slows from 40-70 beats per minute to about 8-12 beats per minute, but they remain responsive to external stimuli. This ability to quickly respond allows them to defend themselves or their cubs if disturbed.
Life in the Den
Once settled in their dens, bears typically remain there for five to eight months, though this duration varies by region and species. Dens can range from simple shelters like rock crevices, hollow logs, or brush piles to excavated burrows under tree roots or hillsides. Pregnant females often choose more protective den sites and may reuse them in subsequent years.
During this period, bears do not eat, drink, urinate, or defecate. They metabolize their stored fat for energy and water. Remarkably, their bodies recycle waste products, converting urea into proteins to maintain muscle mass and organ tissues, preventing atrophy. Female bears also give birth in the den during mid-winter, typically in January or February. Cubs are born tiny, blind, and helpless, relying entirely on their mother’s milk and body heat for survival and growth within the den.