Bears undergo a remarkable transformation as they prepare for their long winter dormancy, a complex physiological state. This preparatory period involves significant changes in their behavior, diet, and internal body chemistry, all geared toward ensuring survival through months of inactivity and food scarcity.
Intensive Eating and Weight Gain
As autumn approaches, bears enter a period of intense eating known as hyperphagia, a biological imperative to accumulate sufficient fat reserves for winter. During this time, they focus on consuming vast quantities of food, often foraging for up to 20 hours a day. Bears can ingest between 15,000 to 20,000 calories daily, significantly more than their summer intake.
Their diet during hyperphagia is diverse and high-calorie, including berries, nuts, roots, insects, fish, small mammals, and carrion. This voracious consumption leads to rapid weight gain, with some bears gaining up to four pounds per day. Over weeks, they can increase their body weight by hundreds of pounds, storing fat as their primary energy source for hibernation. Their bodies become exceptionally efficient at converting consumed food into fat.
Physiological Transformations
Beyond visible weight gain, bears undergo profound internal physiological changes to prepare for their dormant state. Their metabolism shifts to prioritize fat storage and usage, and hormonal levels adjust to support this transition. For instance, bears develop a temporary insulin resistance, allowing them to process large amounts of sugar and store fat without the negative health consequences seen in other mammals.
During hibernation, bears exhibit unique adaptations that prevent muscle atrophy and maintain bone density, issues common in other species during prolonged inactivity. They can recycle nitrogen from waste products like urea to build new proteins, largely preventing muscle loss. Their bodies also suppress bone remodeling to maintain skeletal integrity, ensuring they emerge from their dens with strong bones despite months of immobility. This intricate internal machinery allows them to remain healthy while relying solely on stored fat reserves.
Finding and Preparing a Den
Well before winter, bears begin selecting and preparing a suitable den. They investigate potential sites throughout the summer, seeking locations that offer shelter, security, and insulation. Common den types include hollow trees or logs, rock crevices, caves, and excavated burrows dug under tree roots or in hillsides.
The chosen den must provide protection from weather and predators. Bears often select sites in rugged terrain, on steep slopes, or with high concealment. Black bear den chambers are typically at least 60 centimeters across, with entrances around 25 centimeters wide, while larger grizzly bears require more spacious dens. To enhance comfort and insulation, bears may line their dens with materials such as leaves, branches, or shredded bark.
Reducing Activity and Entering Den
As colder temperatures arrive and food sources diminish, bears exhibit a noticeable reduction in their activity levels. They become increasingly sluggish, conserving energy and spending more time resting near their chosen den site.
Bears typically wait for snowfall before entering their dens, as the snow provides insulation and helps conceal the entrance. Once inside, they settle in for the winter, allowing their metabolic rate to drop significantly. Their heart rate can decrease from an active 80-100 beats per minute to as low as 8-21 beats per minute. This physiological slowing initiates their winter torpor, allowing them to survive months without eating, drinking, or eliminating waste.