Decreased bone turnover with balanced resorption and formation prevent cortical bone loss during disuse (hibernation) in grizzly bears (Ursus arctos horribilis).

Disuse uncouples bone formation from resorption, leading to increased porosity, decreased bone geometrical properties, and decreased bone mineral content which compromises bone mechanical properties and increases fracture risk. However, black bear bone properties are not adversely affected by aging despite annual periods of disuse (i.e.

Black bear femoral geometry and cortical porosity are not adversely affected by ageing despite annual periods of disuse (hibernation).

Disuse (i.e. inactivity) causes bone loss, and a recovery period that is 2-3 times longer than the inactive period is usually required to recover lost bone.

Parathyroid hormone may maintain bone formation in hibernating black bears (Ursus americanus) to prevent disuse osteoporosis.

Mechanical unloading of bone causes an imbalance in bone formation and resorption leading to bone loss and increased fracture risk. Black bears (Ursus americanus) are inactive for up to six months during hibernation, yet bone mineral content and strength do not decrease with disuse or aging. To test whether hibernating bears have biological mechanisms to prevent disuse osteoporosis, we measured the serum concentrations of hormones and growth factors involved in bone metabolism and correlated them with the serum concentration of a bone formation marker (osteocalcin).

Hibernating bears as a model for preventing disuse osteoporosis.

The hibernating bear is an excellent model for disuse osteoporosis in humans because it is a naturally occurring large animal model. Furthermore, bears and humans have similar lower limb skeletal morphology, and bears walk plantigrade like humans. Black bears (Ursus americanus) may not develop disuse osteoporosis during long periods of disuse (i.