Tucked into its den, the slumbering black bear hibernates, barely stirring throughout the winter months. Such sedentary behavior in humans would lead to weakened, brittle bones, but when the bear emerges in the spring, its skeleton is little worse for the little wear. How black bears (Ursus americanus) accomplish the feat has puzzled scientists for years. Now, a new study suggests that the creature’s ability to suppress and balance bone remodeling—the lifelong, two-step process by which mature bone tissue is removed from the skeleton (called resorption) and new tissue is created (called formation)—may provide the elusive explanation. For their research, scientists captured 13 black bears over four hibernation seasons and collected blood samples every 10 days before releasing the animals back into the wild. They found that the levels of a bone formation marker known as bone-specific alkaline phosphatase (BSALP) and a bone resorption marker known as tartrate-resistant acid phosphatase (TRACP) fell during hibernation, suggesting that overall bone turnover had decreased, the team reports online today in The Journal of Experimental Biology. The scientists also found that the bears’ levels of calcium—of which bone is made—in the blood did not change during hibernation, meaning that even though bone remodeling is suppressed, it remains balanced, which helps limit bone loss. A closer look at one of the hormones known to reduce bone resorption called the cocaine- and amphetamine-regulated transcript showed that it surged to levels 15 times higher than normal in the dozing bears, which suggests that it may play a role in regulating the ways bears have evolved to protect their skeleton from the long periods of stillness. Continued efforts to understand the protective measures could lead to new ways of treating degenerative bone problems or diseases in humans, the scientists say.