Hibernating Bears (Ursidae)

Metabolic Magicians of Definite Interest for the Nephrologist

Peter Stenvinkel; Alkesh H Jani; Richard J Johnson


Kidney Int. 2013;83(2):207-212. 

In This Article

Survival and Hibernation in Bears

Black and brown bears, similar to many mammals, undergo hibernation during the winter as a means to protect against periods of food shortage. Whereas true hibernators, such as the hedgehog or ground squirrel, tend to be small mammals (<7–8 kg) that reduce their metabolic rate by >90% of the basal metabolism at a body temperature near 0 °C,[3,4] bears undergo only mild hypothermic hibernation (or dormancy), reduce their metabolism only by 20–50%, and arouse easily if awaked.

To prepare for this long period of caloric deprivation, bears become hyperphagic during autumn to build up their fat reserves. Their caloric intake increases significantly to 15–20,000 kcal/day, which is more than twice their summer intake. They also become hyperinsulinemic and develop characteristics of insulin resistance.[5] Subsequently, the bears retreat into their winter dens, where they remain for ~5–7 months. During this time, they are physically inactive and sleep with slightly depressed body temperatures (30–35 °C). Bears maintain this body temperature by undergoing periodic muscular shivering,[6] and do not appear to use brown fat–mediated nonshivering thermogenesis.[7] As part of a reduction of their metabolic rate, they become bradycardic, with a heart rate of 8–10 b.p.m. compared with a rate of 40 b.p.m while being active. Their basal metabolic rate decreases by 40%, and their oxygen consumption ~50% of normal.[8] Importantly, throughout the hibernation period they do not move, eat, drink, urinate, or defecate. Survival during this period is provided almost entirely from their fat stores, which are metabolized to water to keep the animal hydrated.