Lessons Learned From a Long Winter's Nap

How do hibernating bears stay in good physical shape?

02-01-2003 // Steve Nadis

TRUDGING AROUND THE ROCKY MOUNTAINS in chest-high snow and subzero temperatures, rappelling down cliffs, hacking through ice and breaking into the dens of hibernating black bears—who don't take kindly to home invaders. It may sound like an episode of Fear Factor. But it's just a typical day at the office for Hank Harlow, a University of Wyoming physiologist who is trying to solve the mystery of how black bears maintain their enviable muscular strength after five months of dozing and fasting.

Harlow's research might someday help people suffering from muscle disorders, as well as patients confined to hospital beds and even astronauts on long space voyages. So far, he and his colleagues have found that black bears can stay dormant for almost half a year, yet lose only 20 percent of their strength. Sedentary, well-fed humans normally lose 90 percent of their strength in that time.

Each summer, Harlow and a team of graduate students and wildlife specialists begin their investigations by attaching radio collars to a dozen or so bears that will be tracked down during their winter slumber. Months later, when the cold and snowy weather sets in, the team takes off for their first round of den visits. When the researchers locate a bear, they tranquilize it and implant two tiny items in the abdominal cavity: a thermometer that records core body temperatures and a device that records nerve impulses to muscles. They also collect blood samples, pea-sized muscle biopsies and muscle measurements. The team moves swiftly and carefully, keeping their interactions with the bears minimal. And when their work is done, they tiptoe out of the dens, leaving the snoozing bears in peace.

Later that winter, the researchers repeat the measurements a second time to see how the animals have fared after months of rest. Then, when summer rolls around, they track down each bear one last time to remove the radio collars and other devices. "It's a good feeling, knowing the bear will go back to being a bear and not a research subject," says Wyoming graduate student Tom Lohuis.

So far, such studies have provided some crucial medical insights that may eventually prove beneficial to people. For example, unlike other mammals, hibernating bears prevent muscle protein loss by recycling 98 percent of the urea that would otherwise be discharged in urine. The captured urea is used in the synthesis of new proteins and amino acids to preserve muscle mass. By contrast, humans recycle only about 10 percent of their urea. One day, bed-ridden hospital patients might retain more muscle protein and therefore experience quicker recoveries if scientists can find a way to safely introduce urea-recycling microbes to the human digestive system.

That would be a start, Harlow says, "but you still need to do something to maintain strength." Bears keep fit during their prolonged snooze by shivering and performing isometric contractions about four times a day—comparable to four ten-minute workouts on an exercise bike. By monitoring the nerve-firing pattern that activates bears' muscle fibers, Harlow hopes to devise an electrical muscle stimulation regimen that could stave off atrophy in humans.

Of course, bears are not the only hibernators of biomedical interest. Arctic ground squirrels, for instance, adjust to frigid temperatures and the scarcity of food by instinctively lowering their core body temp-eratures, sometimes below freezing, and drastically cutting their metabolic rates. The squirrels, explains John Hallenbeck of the National Institutes of Health (NIH), "basically enter a state that would kill most other animals."

Hallenbeck is trying to find ways of making people more resistant to stroke—a condition characterized by low blood flow and oxygen delivery to the brain. If we can see how squirrels cope with low brain oxygen levels, he says, human applications might follow.

Although Hallenbeck has not yet figured out how to make people—and their brains—more tolerant of oxygen deprivation, he and his colleagues are shedding light on the challenge of preserving organs and tissues. In a recent study, they showed that when the squirrel's body temperature plummets, proteins and lipids (fats, such as cholesterol) redistribute themselves to preserve cell function. In particular, proteins are moved from lipid-filled regions that freeze to parts of the membrane that remain fluid. One of Hallenbeck's colleagues, NIH researcher Bechara Kachar, predicts that organs and tissues slated for transplant might last longer if kept in solutions rich in cholesterol.

Other investigators are looking beyond ground squirrels to wood frogs, box turtles, gallflies and Alaskan beetles—animals that endure extended bouts of freezing. Wood frogs, for example, freeze solid but revive rapidly upon thawing—a fact observed by Hudson Bay explorer Samuel Hearn, who accidentally uncovered the frozen creatures during the mid-1700s while digging latrines. More recently, Ken Storey of Carleton University in Ottawa, Canada, found that the frogs pump their cells full of glucose—nature's antifreeze—to control the formation of ice crystals in their bodies. Storey and Boris Rubinsky of the University of California at Berkeley subsequently learned that the animals can survive partial freezing as long as no more than two-thirds of the water in their bodies turns to ice. Using this as a guide, Rubinsky has frozen rat livers and hearts and reinserted them—intact and functioning—in the animals. The next step, he says, is to explore the clinical potential for extending the life of precious transplant organs that sometimes go to waste before they reach their intended recipients.

Still, many scientists believe we need to understand hibernation on a molecular level before we can reproduce aspects of the phenomenon in humans. "We need to find the genes responsible and figure out how the animal turns them on and off at the right time and place in the body," says University of Minnesota biologist Matthew Andrews. Although several genes and associated proteins have been identified to date, the system as a whole remains elusive. "There's reason for optimism because when you look at the genes and proteins involved, we haven't found anything you and I don't have," says Andrews. "But determining how it's all regulated may take decades."

In the meantime, Hank Harlow plans to continue investigating black bears and other animals that employ equally clever survival strategies. Among those are komodo dragons—the world's largest lizards. The voracious carnivores can devour a water buffalo—hooves and all—in minutes. They're also patient predators, often lying still for weeks at a time before attacking their victims with stunning quickness. How do they maintain their muscle tone while awaiting their prey? It's a question Harlow aims to answer over the next several years. For him, going from the wilds of the Rocky Mountains to the islands of Indonesia, where the creatures reside, is not a weird transition. From the standpoint of muscle physiology and animal adaptation, it's a small step from the bear's den to the dragon's lair.

Massachusetts writer Steve Nadis wrote about "getting the lead out" of fishing tackle in the August/September 2001 issue.

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