Are bears “true” hibernators? For years the answer to this question was a resounding “no,” as bears do not experience the dramatic decrease in body temperature associated with true, or deep, hibernation. Recently this view has shifted. Bears are what are known as “super hibernators.”
In Yellowstone, most black and grizzly bears hibernate four to five months during winter. During that time, the bear’s heart rate drops from a summer rate of 40 to 50 beats per minute to as low as 8 to 12 beats per minute. Respirations come at a rate of one breath every 45 seconds. Body temperature, however, remains within 12°F of the bear’s summertime body temperature of 100-101°F.
By maintaining a high body temperature throughout hibernation, bears gain advantages—and face challenges—unknown to deep hibernators. Bears can react to danger immediately, whereas chipmunks and ground squirrels—with hibernating body temperatures as low as 40°F—must warm up before they can move quickly.
Throughout hibernation, a bear must metabolize fat to maintain its high body temperature. Fat metabolism causes the bear’s cholesterol level to skyrocket to twice that of summer (and twice that of a healthy human).
So why don’t bears suffer the same adverse health effects we humans do? Somehow a bear prevents its arteries from hardening, while the bear’s liver secretes a substance that dissolves gallstones. Perhaps even more remarkably, in contrast to deep hibernators that must arouse periodically to urinate and defecate, bears avoid eliminating metabolic wastes by instead recycling them.
Urea, a toxic waste found in urine, is a prime example. A bear breaks down urea and uses the resulting nitrogen to build protein, which is then used to maintain organs and muscles. In other words, bears lose fat and may actually gain lean muscle mass while hibernating!
This article was originally published in Yellowstone Quarterly.