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Hibernating Bears Hold Lessons for Human Health

Estimated read time: 6-7 minutes

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The black bear may be unique in the animal world with it's extraordinary ability to emerge from months of hibernation with bones that are about as strong as when it went into its den for the long winter. A shorter period of bed confinement leaves humans with bones so weak they can snap like a dry potato chip, and an aging population has left millions of Americans suffering from osteoporosis.

How bears manage to avoid both those maladies is unknown, but Seth Donahue is determined to find out.

Donahue is an assistant professor of biomedical engineering at Michigan Technological University, and if he can figure out just what makes black bears so different he may be able to create a drug that will help people with osteoporosis. Half the women and a fourth of the men who are over 50 will suffer a bone fracture related to osteoporosis this year, according to the Osteoporosis Foundation.

Donahue and his fellow researchers have found that the bones of a black bear get stronger as it gets older, while ours get weaker. And a black bear is somehow able to recycle calcium and restore its bones, even while fasting and sleeping.

Calcium Keepers

Those major differences between humans and Ursus amaricanus makes it worthwhile to mess around with black bears, a greatly misunderstood animal that tends to be as fearful of humans as humans are of it. Yet they can be extremely dangerous, and although called "black bears," their color can range from light gray (also known as glacier bears) to cinnamon to dark brown. Occasionally they are as white as a polar bear, although much smaller.

Donahue says he and his colleagues still have a long way to go before they can figure out exactly how the bears cope with long periods of inactivity, but he has a theory and some evidence to suggest that he's on the right course.

"Bone loss is related to keeping blood levels of calcium at certain levels," Donahue says. Bears apparently have some "unique features" that allow them to keep calcium near the right level. Whereas other hibernators lose calcium through urinating and defecating, bears do neither during hibernation and apparently are able to recycle calcium that is released into the blood stream during bone deterioration.

It's those "unique features" that Donahue hopes to define. He set off on this course several years ago when he ran across a study showing that black bears do not lose much of their muscle mass during hibernation. Anyone who has had a limb in a cast for awhile knows how weak a human muscle becomes when it is not used for an extended period, but somehow bears manage to spend months in the sack and come out about as strong as when they went in.

Tapping Bear Blood

There's not a lot of bears hanging around his lab at Michigan Tech, but fortunately Donahue heard about Michael Vaughan, a professor of wildlife science at Virginia Tech, who has several captive bears he is studying. The two men joined forces, enhancing both their research projects.

Vaughan is studying bear physiology, like reproduction, and collecting blood samples every 10 days, so it wasn't hard to expand the research to include blood chemistry and bone strength. The bears at Virginia Tech go into a forced hibernation every year by being deprived of food, so the samples offer a continuous flow of data during all seasons of the bears' lives.

In addition, Vaughan and his associates go into the wilds to collect blood samples from hibernating bears, with due caution.

They anesthesize the bears before sucking out some of their blood.

"Obviously, you don't want the bear to wake up," says Donahue.

One major difference between humans and bears emerged quickly in the research. Animal bones are "continuously remodeled," Donahue says. Bone tissue is broken down by "resorption cells," he adds, and new bone is formed by "forming cells."

"During disuse, like when humans are confined to bed rest for medical reasons, or even in other hibernators like bats and ground squirrels, the bone resorption increases so more bone is being removed and bone formation decreases, so less bone is being made.

"That's normal in all mammals except for the black bear, which only increases the resorption. It doesn't decrease the formation. It continues to form new bone."

To do that they have to have a continuous supply of calcium, but they don't eat or drink while in hibernation, so the calcium is recycled. What's taken from the bone is put back in.

But how do they do that?

Donahue suspects it's all in the hormones.

More Mystery

One hormone, especially, is suspect. The parathyroid gland secretes a hormone called parathormone which is the subject of much research today. In various clinical trials the human version of that hormone is being given to patients with osteoporosis, and it appears to be helping, Donahue says.

But when he and his colleagues tried to determine the precise structure of that hormone in the blood taken from the bears, they were in for a surprise. The standard test, which is based on the human hormone, didn't work. Neither did the ones for rats and mice.

The bear hormone was clearly different. And therein could lie a valuable bit of information.

The researchers have begun the time consuming process of breaking down the bear hormone into various parts to see if they can determine what makes it different. If they can, they might be able to produce a synthetic version of the hormone and see if it helps humans with osteoporosis.

That would be a major achievement in itself, but it doesn't completely explain how bears work their magic.

Most animals that hibernate need at least twice the amount of time to recover from hibernation as the amount of time they spend on the couch. But the black bears in the Virginia Tech lab normally hibernate for six months, so they would still be only half way back to normal by the time they are supposed to return to the den. At least, if they were like other hibernators.

Surprisingly, the researchers have found that the black bears ramp up their rate of bone recovery toward the end of their hibernation, and they emerge from the den in nearly as good a shape as when they went in.

And wouldn't it be great if these folks could figure out how bears increase the strength of their bones as they get older? If it's all in the hormones, perhaps there's a pathway here to the elimination of osteoporosis.

But of course, as Donahue emphasizes, we don't really know yet how bears pull off this clever bit of physiology. So we don't know if their hormones will help us keep our bones strong.

Maybe they'll just make us grow black hair all over our bodies, and tend to walk on all fours.

Lee Dye's column appears weekly on A former science writer for the Los Angeles Times, he now lives in Juneau, Alaska.

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