Bears do not eat when they are in their winter dens. During this period of denning, the bear’s body enters a state so profoundly altered that it does not need to drink, urinate, or defecate for months. This extraordinary abstinence is possible because the bear undergoes a unique physiological transformation, allowing it to survive solely on its stored body reserves. This biological process conserves energy and sustains life during the harshest part of the year when food is scarce.
Defining Bear Winter Sleep
What bears experience in their den is technically not the same as the “true hibernation” observed in smaller mammals like ground squirrels or bats. True hibernators drop their body temperature dramatically, sometimes close to freezing, and must periodically wake up to warm their bodies, eat, and eliminate waste. Bears, however, maintain a much higher body temperature, dropping only slightly to a range of 86°F to 95°F, which is only a few degrees below their active temperature of about 100°F.
This state is more accurately described as “winter lethargy” or “walking hibernation” because the bear remains relatively alert. The minimal drop in core temperature allows the bear to rouse quickly if disturbed or threatened in its den. This higher metabolic rate means the bear is not a deep sleeper but maintains a state of readiness, which is likely an evolutionary adaptation due to its large body size.
Preparing for the Fast
The bear’s extended winter fast depends entirely on intense preparation in the months leading up to denning. This preparation involves a driven eating period known as “hyperphagia.” Starting in the late summer and fall, a bear’s biological imperative is to consume massive amounts of calories to build up a thick layer of fat.
During hyperphagia, a bear may consume over 20,000 calories every single day, often spending up to 20 hours foraging for food. This incredible caloric intake allows the bear to gain hundreds of pounds, sometimes up to 400 pounds, of body mass before winter sets in. These fat reserves are the sole fuel source that powers the bear’s body, including its brain and organs, throughout the winter.
The Physiological Stop
Once in the den, the bear initiates a metabolic shift that enables its body to survive without external input for months. The primary fuel source becomes stored fat, broken down in a process called lipolysis. This fat metabolism is highly efficient, providing necessary energy and generating metabolic water, eliminating the need to drink.
Simultaneously, the bear’s body enters a state of metabolic depression, where its overall metabolic rate can drop by as much as 75%. The heart rate and respiration slow significantly to conserve energy, allowing the bear to stretch its fat reserves over the winter. The most remarkable adaptation is how the bear handles its internal waste products.
Bears have the unique ability to recycle the urea produced as a byproduct of metabolism, preventing the buildup of toxins. While other mammals flush this nitrogenous waste through urination, the bear’s kidneys greatly reduce their filtration rate. Instead, the urea is shunted back into the bear’s gut, where microbes break it down.
The resulting nitrogen is then reabsorbed and used by the bear to synthesize new proteins and amino acids. This process, known as urea-nitrogen salvage, allows the bear to maintain its muscle mass and organ tissues without suffering from muscle wasting. Any solid waste in the intestines compacts into a “fecal plug” that is retained until the bear wakes and leaves the den in the spring.