Many wonder if brown bears truly hibernate, entering a deep, prolonged sleep during winter. While the term “hibernation” is often broadly used, its scientific definition is quite specific, leading to distinctions in how different species cope with winter.
Understanding Hibernation
True hibernation involves a profound physiological transformation, where an animal’s body temperature drops dramatically, often nearing the ambient temperature of its environment. For instance, a ground squirrel’s heart rate can decrease from hundreds of beats per minute to just a few, and its body temperature can fall close to freezing. Such animals become difficult to rouse, requiring considerable time and energy to return to their normal active state. This deep dormancy is an adaptation for surviving periods of extreme cold and severe food scarcity.
Brown Bear Winter Behavior
Brown bears do not undergo true hibernation. Instead, they enter “winter lethargy” or “denning.” During this period, a brown bear’s body temperature drops moderately, typically from around 38°C (100°F) to about 31-34°C (88-93°F), a reduction of only 5-7°C. Their heart rate decreases significantly, from a normal 40-70 beats per minute down to 8-19 beats per minute, but this is still considerably higher than the near-single-digit rates of true hibernators.
Brown bears also maintain higher respiratory rates compared to true hibernators. While in their dens, they do not eat, drink, defecate, or urinate for several months, relying entirely on their stored fat reserves. Despite this prolonged inactivity, bears lose remarkably little muscle mass, a unique physiological adaptation. Unlike true hibernators, brown bears can be aroused relatively easily from their winter lethargy and are capable of defending themselves or responding to disturbances.
Survival Benefits of Winter Lethargy
The winter strategy of brown bears offers several advantages for their survival. This period of reduced activity allows them to conserve energy during the winter months when food resources become scarce or unavailable. By retreating to dens, they also avoid the harsh weather conditions that prevail during winter. For female bears, denning provides a secure and insulated environment for birthing and nursing cubs, which typically occurs mid-winter. Pregnant females usually enter their dens earlier and emerge later than other bears, allowing ample time for cub development and early care.
Preparing for Winter
Before entering their winter dens, brown bears engage in a crucial preparatory phase known as hyperphagia. This involves excessive eating to build substantial fat reserves. For instance, a brown bear can gain up to four pounds per day during hyperphagia, consuming over 20,000 calories daily. During this time, the hormone leptin, which normally signals fullness, is suppressed, allowing bears to eat continuously without feeling satiated.
As autumn progresses, bears also select or construct suitable dens, often in protected spots like natural caves, rock fissures, or self-dug excavations. The timing of den entry and emergence can vary based on factors like food availability and weather conditions, with pregnant females generally denning first.