Hibernation | Can’t Sleep? Discover Nature’s Longest Nap

A female black bear enters her den in winter weighing several hundred pounds, carrying cubs she will give birth to while in a state of deep torpor. She does not eat, drink, urinate, or defecate for months. She will lose 15 to 27 percent of her body weight, but she will not lose bone mass. She will not suffer from muscle atrophy. When she emerges in spring, she will have recycled her own urea into protein, used her fat stores to generate metabolic water, and given birth to offspring that nursed while she slept. This is hibernation — but it looks almost nothing like the hibernation of a ground squirrel, whose body temperature can plunge to below freezing.

For decades, researchers debated whether bears were true hibernators at all. The classic definition came from rodents: body temperature drops to near ambient, heart rate slows to a few beats per minute, and the animal cycles through periods of deep torpor interrupted by brief, unexplained arousals back to normal body temperature. Bears only lower their body temperature by three to five degrees Celsius. That seemed too modest to count. But a 1977 study comparing EKG readings of typical small hibernators and three bear species found that the reduced QT interval — a measure of the heart's electrical relaxation phase — was identical. A 2011 study on captive black bears and a 2016 field study on brown bears settled the question. Bea