Grizzly bears, formidable inhabitants of North America’s wild landscapes, use hibernation, an extended period of dormancy, to endure harsh winter conditions. This adaptation allows these large mammals to survive seasonal changes in food availability and temperature.
The Core Motivation for Hibernation
Grizzly bears enter hibernation due to food scarcity in colder months. Berries, fish, and vegetation, which constitute a large portion of their diet, become unavailable or difficult to find as winter progresses. This seasonal lack of sustenance makes maintaining active metabolism unfeasible.
Hibernation conserves energy, preventing starvation when food sources are depleted. Instead of expending energy in a futile search for food, bears reduce their metabolic demands significantly. Environmental cues like diminishing food supply, dropping temperatures, and increasing snow cover trigger hibernation. Retreating to a den also helps bears avoid extreme cold and deep snow, which would pose survival challenges.
Physiological Transformations During Hibernation
During hibernation, grizzly bears undergo physiological changes for prolonged dormancy. Their metabolic rate can drop by approximately 75%, conserving energy. Body temperature decreases, though not as drastically as in smaller hibernators. A grizzly bear’s body temperature falls from its active summer average of 100-101°F (37.7-38.3°C) to between 86-95°F (30-35°C). This modest drop allows bears to remain responsive and wake up quickly, unlike some “true” hibernators that enter a much deeper state of torpor.
The bear’s heart rate slows from 40-50 beats per minute (bpm) during active periods to as low as 8-19 bpm during hibernation. Respiratory rates also decrease significantly, with bears sometimes taking only one breath every 45 seconds. Bears rely almost entirely on their extensive fat reserves for energy and water during this period, metabolizing fat without significant muscle or bone loss. They recycle waste products like urea, preventing kidney damage and converting it into protein to maintain muscle mass. This means they do not need to urinate or defecate while in the den.
Preparing for the Long Sleep
Before entering their dens, grizzly bears engage in hyperphagia, a period of intense feeding. This involves consuming vast quantities of food to accumulate substantial fat reserves, which are essential for sustaining them through the winter. During hyperphagia, bears may consume up to ten times their normal daily caloric intake, gaining as much as four pounds per day. Their diet during this time includes a wide range of high-calorie foods such as berries, nuts, fish, and roots.
As hibernation approaches, bears become less active and begin selecting or constructing dens. Dens can be natural features like caves or rock crevices, or excavated into hillsides. These dens provide insulation and safety; some bears move up to a ton of material to create their winter homes. Pregnant females often seek more secure, insulated dens, sometimes at higher elevations, as they will spend longer inside and give birth there.
Life in the Den and Emergence
Once settled in their dens, grizzly bears enter a state of minimal activity, relying on stored fat. While inactive, they are not in a deep, unresponsive coma like some smaller hibernators, and can be aroused if disturbed. Female grizzly bears often give birth to cubs during this denning period, typically in late January or early February. The cubs, born small and helpless, nurse and grow within the den until spring.
As spring approaches and temperatures rise, grizzlies gradually stir. Males emerge first, often in March, while females with cubs appear later, in April or May. Upon emergence, bears are typically hungry and begin searching for food to replenish the weight lost during hibernation, which can be between 15-30% of their body mass. Their initial activities are cautious as they reacclimate and seek early spring food sources.