Experiment of the month

Natural History, June, 2003 by Stephan Reebs

Use it or lose it: that's a rule that governs employee vacation days--and bones. In every bone there's a steady turnover of material, a continuous balancing act between bone formation and the resorption of bone tissue into the bloodstream. Those two processes are kept in healthy equilibrium by the near-constant compression and tension exerted on working bones. But if bones are not put to work, tissue formation slows down and resorption speeds up, and the bone structure weakens. Bedridden patients--and weightless astronauts in space--are prone to fractures simply because their bones aren't being used.

But that raises a question: What about hibernating bears? Are their bones compromised by five to seven months' rest? To find out, Seth W. Donahue, a biomedical engineer at Michigan Technological University in Houghton, along with several colleagues, analyzed blood from seventeen wild black bears. (Blood samples are easier to get and less invasive than bone samples, and levels of certain protein fragments in the blood reflect the rates of bone formation as well as resorption.) First, however, Michael R. Vaughan of Virginia Tech in Blacksburg had to fit the bears with radio collars so that the investigators could locate the animals in summer and in winter, dart them with an anesthetic (even during hibernation bears can move with reasonable alacrity), and collect a few drops of blood.

The blood samples, as expected, showed substantial bone resorption, but surprisingly, bone formation had not slowed. Furthermore, the investigators detected a spurt of bone formation in early summer--greater than the bone growth measured in any other healthy adult mammal--that canceled out the net bone loss caused by a winter of inactivity. The result offers some long-term hope for people who suffer from osteoporosis or other bone diseases: bears could serve as a useful animal model in the search for effective treatments. ("Serum markers of bone metabolism show bone loss in hibernating bears," Clinical Orthopaedics and Related Research 408:295-301, March 2003)

Stephan Reebs is a professor of biology at the University of Moncton in New Brunswick, Canada, and the author of Fish Behavior in the Aquarium and in the Wild (Cornell University Press).