Bones weaken with inactivity in most mammals, but hibernating bears maintain theirs by suppressing bone turnover.
Seth Donahue at Colorado State University in Fort Collins and his colleagues analysed blood and bone samples from 13 black bears (Ursus americanus), which hibernate for up to 6 months every year. They found lower levels of key protein markers associated with bone formation and breakdown in hibernating bears than in active ones. The concentration of a hormone that reduces bone breakdown was 15 times higher during hibernation than during active periods.
Hibernating and active bears had the same level of calcium in their blood, suggesting that bears balance bone formation with breakdown during hibernation.