Researchers Verify Strength of Hibernating Bears
(Contact information: Jim Kearns, (307) 766-2670, [email protected])
Inside Rocky Mountain caves during the coldest part of winter, hibernating black bears are providing researchers with information that eventually could help lessen the debilitating effects of muscle atrophy or help astronauts to maintain their strength during long space flights.
In an article published in the Feb. 22 issue of "Nature," University of Wyoming researcher Henry Harlow and others report that hibernating bears preserve as much as 80 percent of their muscle strength in winter. The article, "Muscle Strength in Overwintering Bears," culminates five years of research in hundreds of mountain caves to determine why bears are strong and active when they emerge from their winter dens.
"If we find out how bears can preserve their strength while lying there for four or five months, the next step will be to see if we can apply some of these finding to humans," Harlow says. "Bed-ridden patients loose 0.7 percent of their strength per day, resulting in problems such as severe atrophy. This just doesn't happen in bears, and we've been working to find out why."
The "Nature" article, by Harlow, a professor in the Department of Zoology and Physiology, and Paul Iaizzo, a professor at the University of Minnesota, presents the most recent findings describing how bears can maintain so much of their strength, even though they are inactive and go without eating or drinking during the long winter hibernation periods.
In earlier studies, Harlow and other scientists examined bears' muscle and blood samples, and found that hibernating bears do not loose much protein. Harlow says that such protein conservation is accomplished by urea recycling, in which the nitrogen in their urine is converted into new amino acids, preserving protein and contributing to new muscle tissue. University of Washington researchers confirmed that bears can recycle nearly 100 percent of their urinary waste, while other hibernating animals recycle only about 20 percent.
The findings published in "Nature" build upon the earlier protein conservation studies. The researchers found a non-invasive way to measure strength of the hibernating bears. Iaizzo has done much work with humans who have muscle disorders such as dystrophy. He developed a way to measure strength by stimulating a nerve that causes a muscle reaction, which can be measured as a visual display of strength.
UW scientists modified Iaizzo's system to develop a leg brace that fits over a bear's knee. Researchers enter the bear's den and test its strength twice during the winter. When the nerve is stimulated, the bear's muscular reaction can be displayed on a computer screen, providing a reading that shows a maximal force of strength. These tests confirmed that hibernating bears loose only about 20 percent of their strength while confined to their den over winter. For comparison, humans that are similarly inactive often loose 80 percent of their strength as their muscles continue to wither away.
The ability to conserve protein is only one part of maintaining strength. Harlow notes that exercise is also required. He speculates that hibernating bears undergo massive shivering episodes, during which many of their muscles contract in a sort of involuntary isometric exercise. Instruments that log the bears' temperatures showed spikes (temperature increases) occurring about four times a day, which Harlow believes is a measure of such violent shivering episodes.
"It's almost like a human getting on an exercise bike. We think these massive muscle contractions are helping the bears to maintain strength and muscle tone through the winter," Harlow says. He thinks bears developed these physiological responses as a survival mechanism.
"In dens, the bears are vulnerable to cougars and other predators. These phenomenal physiological responses likely evolved as a flight or fight defense," he says. "The bears must be able to arouse quickly enough to avoid or fend off the intruders."
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