Bears are True Hibernators
Northern hemisphere bear species (except polar bears) such as brown bears, American black bears, and Asian black bears are known to hibernate throughout the winter months when the growth of their largely vegetarian food resources lay dormant. Southern hemisphere bears such as sloth bears and Andean bears are known to hunker down in nests during rainy seasons. Even polar bears that live in the southern portions of their range and experience seasonality are thought to go through a ‘walking hibernation’ during their summers on the tundra. For years the biological question as to whether or not bears should be considered ‘true hibernators’ hinged in large part on a narrow definition of hibernation generated through the study of rodents and small mammals that suggested that hibernation was linked to (what was thought to be) an interdependent decrease in body temperature and metabolic depression. In addition, the bears’ uncanny behavior of waking quickly, being able to run, and on occasion eating in the middle of hibernation, had lead much of the captive community in North America to believe that allowing bears to den up was optional and not important to their well-being.
In 2011, scientist Øivind Tøien found that bears could resume normal body temperature while maintaining metabolic depression and indeed did so for up to three weeks afte denning. Tøien 2011 This phenomenon, ‘walking hibernation’, has led to a redefinition of the process. Today hibernation is considered a process of changes in metabolism and ensuing behavior to allow an animal to survive under extreme environmental changes such as temperature and/or food abundance. Biologists now agree that bears do in fact hibernate.
Research over the past ten years has shown the process to be complex and not well understood. Some important biological processes that occur during denning are sloughing and regeneration of foot pads, and wound healing without scarring. To prepare for hibernation, bears become very hungry (hyperphagic) increasing their food intake up to twice as much as their summer intake. They become hyperinsulinemic and insulin resistant. In the den, the body temperature drops slightly to 30-36˚C with occasional muscle-shivering to generate a temperature increase if it drops too low. Bears become bradycardic with a heart rate of 8-10 b.p.m., down from 40 b.p.m. Oxygen consumption can decrease to 50% of normal, and the basal metabolic decreases to 40%. Remarkably they do not suffer from bone loss or muscle atrophy, while living off of their fat tissue which is metabolized to water to keep them hydrated. Stenvinkel 2013
Hibernation is a serious biological process that does not appear to be driven purely by food availability and has some benefit to the animal’s physical well-being, most of which is as of yet undiscovered. The principles of behavior-based husbandry dictate that captive bears be given the opportunity to choose to den if the bear deems necessary. Wild bears will even den up for periods in warmer climates such as Spain and Florida, USA. The need to den appears to be personal relating to age, sex, familial status, body condition, and/or ability to compete locally.