Bears undergo a physiological transformation to navigate harsh winter conditions. This process, known as hibernation, allows them to conserve energy when food is scarce and temperatures plummet.
Defining Bear Hibernation
Bears engage in a unique dormancy, often called “winter lethargy” or “denning,” which differs from “true hibernation” in smaller mammals like ground squirrels or bats. While true hibernators experience drastic body temperature drops, sometimes near freezing, bears maintain a relatively higher temperature. This allows them to be more easily roused from their sleep, a significant advantage, particularly for females giving birth. Biologists consider bear hibernation a specialized, seasonal reduction of metabolism, not solely defined by a dramatic temperature drop.
Preparing for Winter Slumber
Before entering their winter dens, bears undergo hyperphagia, meaning “excessive eating.” In late summer and fall, they consume vast amounts of food to accumulate substantial fat reserves. As opportunistic omnivores, they forage for berries, nuts, fish, insects, and carrion, sometimes eating for up to 22 hours a day. This intense feeding allows some bears to gain up to 3 pounds per day, potentially doubling their body weight with fat that serves as their primary winter energy source. Bears seek secure, insulated dens in hollow trees, rock crevices, or self-dug excavations.
Physiological Adaptations During Hibernation
Once in their dens, bears exhibit physiological changes. Their heart rate decreases from 40-50 bpm to as low as 8-12 bpm, with some recordings as low as 4-5 bpm. Respiration also slows, sometimes to just one breath every 45 seconds. Despite these reductions, body temperature only drops a few degrees Celsius, typically remaining within 5-12°F (3-7°C) of their active temperature. This moderate temperature reduction couples with a metabolic rate decrease, slowing to about 25% of normal active rates.
Bears sustain themselves by metabolizing stored fat, which provides energy and water, eliminating the need to eat or drink for months. A key adaptation is their ability to recycle waste products. Instead of urinating, bears convert urea, a toxic waste product, back into nitrogen. This nitrogen synthesizes new proteins, maintaining muscle mass and organ tissue despite prolonged inactivity. This process prevents muscle atrophy and bone loss that would typically occur in other mammals during disuse. Bears also exhibit insulin resistance, preventing high blood sugar levels, a mechanism studied for human applications.
Emergence and Post-Hibernation
As spring approaches, bears gradually emerge from their dens, typically around March or April. They are initially groggy, dehydrated, and have lost 15-30% of their body weight. Their first priority is to find water, then food to replenish energy stores.
Pregnant females often give birth to two or three cubs in the den during mid-winter. These cubs nurse and grow, protected by their mother, until they emerge with her in spring. The timing of emergence coincides with new food sources for recovery.