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In Hibernation, Bears Surprisingly Preserve Bone Mass

Deep in the winter-dark woods, beneath the roots of a fallen tree, a mother black bear hibernates with her two yearling cubs. In the spring, they will wake up in a near-starvation condition, their fat reserves depleted. The mother bear’s bones, however, will be as strong and as thick as the day she lay down, and her young may even have added bone mass over the winter.

Bears are the only animals known to maintain their bone mass during prolonged periods of inactivity. To consider what a feat this is, consider humans’ susceptibility to bone loss: astronauts who spend six months in the weightless environment of space can lose nearly 10 percent of their bone mass, and people forced to spend several months in bed may experience similar declines.

So why are bears different? And what can we learn from their biochemical processes that may help us treat osteoporosis and other bone diseases?

Rita Seger, a medical doctor with a Ph.D in ecology and environmental science, has been working to unravel some of the mysteries of bear hibernation. In particular, she has examined data related to the breakdown and formation of bear bone mass over time.

“Skeleton is very dynamic tissue,” Seger explained. It’s so dynamic that in adult humans a whopping tenth of a skeleton is replaced every year. “So the skeleton is constantly being broken down, especially where there is wear and tear, and that bone gets reabsorbed. Then new bone forms, so you always have new bone forming.” How much bone is added back depends on how much weight the skeleton bears; less weight-bearing activity means less bone is reformed.

Seger initially considered several possibilities for how bears emerge from hibernation with strong bones. Perhaps bear bones were broken down at a high rate due to inactivity, but were replaced at a high rate, too? Or maybe bears — in contrast to other hibernating mammals — suspended bone breakdown over the winter?

To find out, Seger and a team of researchers from the University of Maine used radio collars to track 85 black bears, active and hibernating. Over a three year period, from 2005 to 2007, they took blood samples and X-rayed paws. Back at the lab, they studied the X-rays to quantify bone mass and analyzed the blood for substances associated with bone turnover.

What the team discovered was that a particular hormone, leptin, seems to be the key for bears’ bone maintenance. Leptin, also found in humans, is released by fat cells; its main function appears to be telling the body how much energy it has. More fat equals more leptin.

After leptin is released by fat cells, it crosses the blood-brain barrier and docks in the hypothalamus. There, in addition to its other roles, leptin affects the functioning of the sympathetic nervous system, which regulates bone turnover. Seger is not sure what, exactly, leptin is doing to the sympathetic nervous system — that process is extremely difficult to study. What her research did determine is that bears have strong bones in spring because somehow, through leptin, the skeleton is telling the body that it is bearing weight, even though it is not. Bone turnover continues throughout hibernation, but at a slower rate.

Seger hopes her research will be of use to those studying human bone diseases. “If we could give humans the same signal, then it would help them build bone,” she said.

Audrey Clark is a naturalist and writer who lives in Burlington. The illustration for this column was drawn by Adelaide Tyrol. The Outside Story is assigned and edited by “Northern Woodlands” magazine and sponsored by the Wellborn Ecology Fund of New Hampshire Charitable Foundation: wellborn@nhcf.org