When winter arrives, many animals adapt to the cold and scarcity of food by entering a state of reduced activity. While often said to hibernate, grizzly bears employ a unique strategy for surviving winter, distinct from the deeper sleep of true hibernators like rodents.
Winter Dormancy in Grizzly Bears
Grizzly bears do not undergo “true” hibernation like some smaller mammals. True hibernators experience a drastic body temperature drop, making them difficult to rouse. Grizzly bears, however, enter winter dormancy or torpor, a lighter deep sleep. Their body temperature only decreases by a few degrees Celsius (typically 30-34°C), allowing them to be easily awakened if disturbed – a survival advantage for reacting to threats.
Unlike true hibernators, female grizzly bears give birth to cubs during this period, typically in late January or early February. The mother nurses her cubs safely in the den until spring. This physiological difference ensures the bear’s survival and successful rearing of young.
Body Changes During Winter Sleep
During their winter dormancy, grizzly bears undergo physiological adaptations to conserve energy. Their heart rate slows significantly, dropping from a typical 40-50 beats per minute during active periods to a much lower rate of 8-19 beats per minute. This reduction in heart rate, along with a decreased metabolic rate, allows them to sustain themselves on stored energy reserves for months.
Grizzly bears primarily rely on large fat reserves accumulated in the fall as their energy source. Their bodies efficiently break down this fat, producing both energy and water, which helps meet their hydration needs without needing to eat or drink. Despite prolonged inactivity, bears experience minimal muscle and bone loss due to their unique ability to recycle nitrogen (a byproduct of muscle breakdown) back into protein, preventing severe weakening common in other large mammals during extended immobility.
Preparing for and Leaving the Den
As autumn progresses, grizzly bears prepare for winter by entering hyperphagia, a phase of excessive eating. They consume vast quantities of food, often gaining hundreds of pounds and up to 20,000 calories daily, to build fat reserves for winter sleep.
Bears then select or excavate a den, often choosing north-facing slopes for insulation and snow accumulation. Den construction can involve moving up to a ton of material over several days; dens are often lined with bedding like tree boughs for warmth. Den entry typically occurs between late September and early November, influenced by food availability and weather. Pregnant females often enter their dens earlier and emerge later than males. Cubs are born in the den, and the family remains there until spring, usually emerging between March and May. While groggy upon emergence, bears quickly regain full function, ready to resume foraging as food becomes available.