Bears employ a sophisticated series of behavioral and physiological preparations to survive the winter months. This preparation begins long before the first snowfall, allowing the animal to endure a prolonged period of fasting and reduced activity. The transition to deep dormancy, often called torpor, involves a dramatic shift in diet, careful selection of shelter, and profound internal metabolic adjustments. The weeks leading up to den entry are focused on accumulating the resources necessary to survive until spring.
The Race to Gain Weight
The most visible phase of preparation is a period of excessive feeding known as hyperphagia. During the late summer and fall, the bear’s biological imperative is to consume as many calories as possible to build substantial fat reserves. This focus drives the animal to eat for up to 20 hours a day, often consuming 15,000 to 20,000 kilocalories daily.
The diet shifts dramatically to high-energy sources, such as berries, nuts, and salmon. This allows bears in some regions to gain up to four pounds of body weight per day. This rapid weight gain results in the accumulation of white adipose tissue, the primary form of energy storage that fuels the bear’s metabolism throughout the winter. These substantial fat stores allow the bear to lose between 20 to 40 percent of its body weight during dormancy.
A specialized tissue called brown adipose tissue is also involved, particularly in mothers who give birth in the den. Unlike white fat, which stores energy, brown fat generates heat through nonshivering thermogenesis. This mechanism is especially important for newborn cubs, which rely on it to maintain their body temperature in the cold den environment. The mother’s ability to carry a pregnancy to term is directly tied to her fat reserves, regulated by delayed implantation where the embryo only implants if her body condition is sufficient.
Selecting and Preparing the Den
Once the bear has amassed sufficient fat stores, attention turns to securing a safe, well-insulated winter shelter. Den selection focuses on locations that offer protection from predators and the elements, ensuring a stable microclimate. Bears often choose dens that are difficult to access, favoring rugged terrain, steep slopes, and areas offering high concealment.
The physical structure of the den varies widely, including hollow logs, rock crevices, caves, or dens excavated into hillsides or under large tree root systems. Bears generally use a new den site each year, modifying or lining the chosen space. They gather materials such as leaves, moss, and pine boughs to create a thick, insulating bed that helps conserve body heat during the cold months.
The Internal Transformation
The final weeks before den entry are marked by profound internal changes that prepare the body for extended fasting and immobility. The bear enters the “fall transition,” where activity decreases and the body begins to slow its systems. The heart rate, normally 80 to 100 beats per minute, begins to drop, eventually settling into a range as low as 8 to 21 beats per minute during torpor.
Respiration also slows dramatically, sometimes to just a single breath every 15 to 45 seconds. The overall metabolic rate is suppressed to about 25% of the summer rate. This reduction occurs without a major drop in core body temperature, which typically remains between 86 to 95 degrees Fahrenheit. This relatively high body temperature allows the bear to remain responsive and alert, which is why their state is described as torpor rather than true hibernation.
A remarkable physiological adaptation is the management of waste, allowing the bear to avoid urinating or defecating for months. The body recycles the nitrogenous waste product urea, normally excreted through urine. By breaking down urea and reabsorbing the nitrogen, the bear synthesizes new proteins, effectively preventing muscle atrophy and maintaining bone density despite prolonged inactivity.
Furthermore, a dense mass forms in the lower intestine, commonly known as a fecal plug or tamping. This plug is not formed by purposefully eating indigestible material. It results from the accumulation of intestinal secretions, sloughed-off cells, hair from grooming, and bedding material, which hardens as the intestinal walls absorb all the fluid.
When the Time is Right
The precise moment a bear enters its den is dictated by internal readiness and external environmental signals. The bear’s internal clock is keyed into the shortening photoperiod, which signals the changing season. However, the ultimate decision to den is closely linked to the availability of food and weather conditions.
A depletion of local food sources, coupled with significant temperature drops and the arrival of snow, serves as the final trigger. Brown bears often enter their dens when the ambient temperature reaches the freezing point. The timing varies significantly by geography and individual; pregnant females often enter their dens earlier than others to secure a safe place to give birth during winter.