Winter World_ The Ingenuity of Animal Survival - Bernd Heinrich [104]
Three-month-old nonhibernating cub climbing on hibernating mother.
Overwintering bears have many physiological and behavioral characteristics, but they were for a long time not considered to hibernate, simply because their body temperature showed only modest drops and hibernation was defined in terms of low body temperature. A bear’s body temperature during hibernation remains near 35°C, only slightly lower than the 37° to 38°C or so when active. At a body temperature of 35°C a bear may be slightly sluggish, but it is by no means unresponsive to disturbance, especially from human researchers who would dare to enter its lair to take its temperature with a rectal thermometer, or stick it with a syringe to draw its blood in trying to track down the marvels of the bear’s hibernation physiology. It turns out, however, that the key to a bear’s hibernation is not to be found in the temperature of its rectum. Instead, diagnostic characteristics are discerned through the bear’s appetite physiology, waste metabolism, water balance, and bone retention despite lack of exercise. Indeed, the marvels of hibernation concern many medical matters of acute practical relevance to humans, especially as regards aging, space flight, and osteoporosis.
One of the first issues of hibernating bears to be studied was how, despite maintaining a high metabolic rate (high body temperature) the bear still does not need to drink or urinate all winter. We can, like bears, also go without food for a long time, provided we have body fat. But we can’t get along without water. If we were to spend considerable time in a bear’s den in winter we would, even without sweating, quickly dehydrate due to urination. But if we shut off our kidneys, then our metabolic waste, principally urea, would pile up in our blood until it poisoned us. Urea is our vehicle for getting rid of nitrogen, which becomes a waste product after we digest protein or nucleic acids. A bear does not urinate all winter. Thus the question is: Does urea not poison the bears or don’t they produce urea? To find out, physicians Ralph A. Nelson and Dianne L. Steiger from the Carle Foundation Hospital at the University of Illinois teamed up with game biologist Thomas I. Beck from the Division of Wildlife in Colorado to try to examine the urea content in the blood of hibernating bears. But how to get the blood? Bears in their winter dens are alert enough to be intolerant of people with hypodermic syringes. In part to get more compliant subjects for their project, the researchers did the next most difficult thing, they live-trapped bears in the fall and equipped them with radio transmitters that could be used to track down their subjects later when they were denned up. There, the bears were tranquilized with chemicals (Rompun) from a dart gun that made then more tractable and the task of taking their blood easier. A total of 76 blood samples from 48 bears were collected and analyzed.
Since hibernating bears metabolize mostly fat, they do not accumulate huge amounts of urea in their blood. What small amounts that they do produce they convert into creatine, which is nontoxic. Additionally, instead of becoming a toxic waste, the nitrogen wastes in hibernating bears are biochemically recycled back into protein; hence no loss of muscle mass is experienced even as they don’t exercise. Thus a hibernating bear never needs to get up to take a drink or go take a leak all winter. Water is conserved because none is needed to flush out toxic wastes, and the animals stay in shape. But that alone does not make the bear the ultimate enviable couch potato that it is. Other physiological wonders of its fitness continue to be elucidated.
We require mechanical stress of exercise on our skeleton to maintain bone structure and function, as was dramatically illustrated during weightlessness experience in space (Johnson 1998). Bone mineral content loss, depending on specific bones, was 3.4 percent