Hibernating Bears Have Healthy Hearts

PULLMAN, Wash. – Grizzly bears love pastries, can be 50 percent body fat and spend nearly half the year sleeping.

Yet the hibernating bears don't suffer heart attacks, have no hardening of the arteries, no fatty deposits or any circulation issues, said Charles Robbins, director of the Washington State University bear research center.

Robbins, who founded the center in 1986, and other WSU scientists are seeking to learn how bear hearts stay so healthy, and whether the answers can be applied to humans.

"The changes in the heart that occur in hibernation are things you and I couldn't survive," said Lynne Nelson, a veterinary cardiologist who has spent seven years studying the bears. "Yet bear hearts are very healthy."

Scientists have found that grizzly hearts drop from more than 80 beats per minute when bears are awake to less than 20 beats, and sometimes into single digits, when bears are hibernating. Nelson said echocardiograms show blood starting to pool in the heart, but the bears do not suffer clots.

So far, Nelson and Robbins have only theories to explain how the bears, which share 95 percent of human DNA, maintain healthy hearts. They have published papers saying that protein changes make bear hearts stiffer during hibernation, allowing them to maintain their shape and perhaps avoid the formation of clots. Bears also shut down two of the four heart chambers without suffering damage.

The scientists are also studying beta receptors and beta blockers in bears, which control heart rate.

Washington State's Bear Center is the only university facility in the nation that has adult grizzly bears, Nelson said. Grizzlies are listed as a threatened species, although numbers have rebounded since the 1970s.

Learning how bears cope with being asleep for so long could have implications for long manned space flights, and for humans who are bedridden, scientists say.

Harry Reynolds, a past president of the International Association for Bear Research and Management, said bears make excellent stand-ins for humans in medical research because they share many biological similarities.

"If there is any model species for human medicine and health studies, it's got to be bears," said Reynolds, retired from the Alaska Department of Fish and Game, but still actively involved in bear field studies in Mongolia.

The U.S. Fish and Wildlife Service is in charge of preserving grizzlies and has provided some financial support for the WSU Bear Center. Christopher Servheen, Fish and Wildlife grizzly recovery coordinator in Missoula, Mont., said the studies are important for bears, as well as what they portend for humans.

"All the work that comes out of there brings added information about the physiology and food habits and survival of bears," Servheen said.

The WSU Bear Center is a modest building on the edge of campus. It includes a series of cinderblock dens where up to 10 bears live. The bears are free to move outside, where they are separated from humans by two layers of chain link fence and can be viewed by anyone who stops in the parking lot. On a recent day two children gawked and shouted at the bears.

The animals can also roam some hilly fields that are fenced off next to the building.

On a recent hot day, an 8-year-old adult named John frolicked in a steel tub of water, climbing in and out and shaking the water out of his thick fur.

Another named Frank stood on his hind legs, towering over his handlers, while Nelson fed him slices of apple through the fence.

The bears eat a mixture similar to dog food, plus salmon and road-killed deer, apples and day-old cakes and pies donated by Safeway, Robbins said.

"We ring a cowbell and they know there is cake and pie for them," Robbins said. "Sugar and grease, who doesn't love that?"

The bears are involved in a wide variety of research projects. Nelson is seven years into her heart research, but the process is slow because the WSU bears only hibernate from October to March, she said.

The bears give blood samples, have their blood pressure measured, are given ultrasounds and echocardiograms. Some of the bears are trained to stick a limb out of the cage to be tested, even as they hibernate, she said.

Other scientists are looking at aspects of bear hibernation.

Seth Donahue, a researcher at Michigan Tech, has written that black bears do not develop osteoporosis during long periods of hibernation because their cells continue to produce new bone-making material as they sleep. That could eventually be helpful in battling osteoporosis in humans.

Bears also manage to maintain muscle strength and mass, even though humans would lose some 70 percent of muscle strength in similar inactivity. One reason is that bears appear to exercise while they sleep by performing whole body, isometric contractions that start at the neck and move down several times a day.

"There are many, many interesting questions with hibernation and bears," Reynolds said.