Inside the Den: The Habitats Where Bears Hibernate

The average person sleeps for 9 hours a day, the average dog for 11 hours, and the average cat for 16 hours. Bears can hibernate for several months, with the duration varying among species and environmental conditions. Hibernation, however, is very different from sleeping. During hibernation, an animal’s metabolic rate, heart rate, and respiratory rate slow down greatly.

Hibernation is well studied in bear species, but scientists debate whether this behavior in bears is “true” hibernation. This article will explore hibernation in bears and whether or not it is “true” hibernation. It will also answer that all-important question: where do bears hibernate?

What is Hibernation?

In short, hibernation is a state in which an animal uses less energy for extended periods.

During hibernation, an animal’s metabolic rate decreases greatly. Metabolic rate describes the amount of energy being used by all the cells of an individual and is usually measured in terms of oxygen consumption over time. A decrease in metabolic rate is therefore tied hand-in-hand with a decrease in respiratory rate and heart rate. An endotherm is an animal that regulates its body temperature with metabolic processes. Decreasing metabolic rate conserves energy and causes body temperature to drop.

The initial definition of “true” hibernation was a decrease in metabolic processes that result in a substantial decrease in body temperature. Bears’ body temperatures only drop 3-5ºC, which is less than the significant drops seen in some small hibernating mammals. Scientists were therefore unsure if bears were actually hibernating. Recent research has established that many bears do hibernate and do not enter some other form of dormant state.

Why Do Bears Hibernate?

Not all bears hibernate. For example, polar bears are active all year and have a unique ability to fast for months at times when food is scarce. Panda bears also do not hibernate because they are able to move to higher and lower elevations as ambient temperatures change. Similarly, the sun bear does not hibernate because its food resources are available during all seasons.

Bears that do hibernate do so because seasonal changes to food availability make them unable to provide for themselves for long periods. Even the omnivorous brown bear, which subsists on both meat and plant-based resources, hibernates during winter months. Many plant food resources lose their fruit and foliage in environments with harsh winters. Prey also becomes more elusive and may hibernate as well. The lack of available food requires certain species to enter an inactive state where their metabolisms will slow. This is so they can stretch what resources they do have for longer periods.

When Do Bears Hibernate?

Bears that do hibernate include the Andean bear (also known as the spectacled bear), the American black bear, some Asiatic black bears (also known as moon bears), and brown bears. These species enter hibernation based on environmental cues. A 2016 study followed the hibernation cycles of 14 brown bears over three winters to better learn how this happens. It was found that bears enter their dens for hibernation at first sight of snow and when temperatures reached 0ºC.  Their body temperature, activity levels, and heart rate begin to slow weeks before, however.

In the period before hibernation, these bear species are hyperphagic to build their nutrient and energy reserves. An American black bear eats up to 90 pounds of food a day in preparation for hibernation. Brown bears also weigh their most in late fall before the onset of winter. Himalayan black bears usually weigh between 200 and 265 pounds but can weigh as much as 400 pounds when engorging themselves before winter.

Females of some bear species, including American black bears, will enter hibernation for the duration of their pregnancy. Nearly all pregnant Asiatic black bears hibernate as well. The mothers will greatly increase their body mass before hibernating and gestation will occur while they are dormant. The offspring are born either during or shortly after the period of hibernation. The health and weight of the offspring are a direct reflection of how much weight the mother could amass before hibernating.

Where Do Bears Hibernate?

Bears hibernate in dens that they find or construct to protect themselves from the elements while they are dormant. American black bears typically dig out dens in the ground. They may also shelter themselves in hollowed-out tree cavities, under logs, or in caves. Female black bears have been observed being pickier in choosing their dens than males.

Many Asiatic black bears remain active all year, but some northern subspecies hibernate seasonally. They usually prepare dens similar to American black bears. Brown bears also craft dens like black bears, preferring remote, forested locales.

Although polar bears do not hibernate, they do create maternity dens and enter a dormant state that has some similarities to hibernation. The pregnant female digs a maternity den in a snow drift consisting of an entrance tunnel and three chambers. She then enters a dormant state similar to hibernation. This state, however, is not continuous sleep, and her body temperature does not decrease.