In humans and some animals, long periods of physical inactivity can lead to osteoporosis, a disease where bones break down and become weak. Osteoporosis is usually caused by either bone not being formed or an increase in bone resorption (where components of bone are reabsorbed into the body). But bears are inactive for 4-6 months of the year during their winter hibernation periods, yet they do not develop osteoperosis. New research, published in Scientific Reports, explores how black bears manage to escape this fate. 

During hibernation, black and grizzly bears (Ursus americanus and Ursus arctos horribilis) are known to decrease their metabolic rates by about 20-75 percent to save energy, relying solely on fat stores as their energy source. This new study shows that when bears reduce their metabolism, genes involved in bone resorption become less active. Additionally, there is no increase in the gene expression associated with bone formation or osteoblasts (cells that make bones). The researchers also found evidence of that genes involved with energy production from bone and bone marrow become more active during hibernation, which may provide the bears with an additional energy source during fasting.