Skeletal tissues of living animals are constantly being renewed and replaced as wear and tear creates stresses and microfractures. In order to know how much bone needs replacing, the skeleton must constantly know how much load it is bearing. If the skeleton detects that it has become “unloaded” due to hibernation or inactivity, it loses bone. This has happened to every animal that has been studied, with the most famous example occurring in astronauts, who lose bone while in space because they don’t need to support their weight in the gravity-free environment.
But a scientist at the University of Maine says that black bears don’t lose bone mass during hibernation, and she has identified a hormone that may help them avoid it.
“Small hibernators like bats and ground squirrels lose a profound amount of bone when they hibernate,” said Rita Seger, a medical doctor who studies bone pathology. “The physiology of small mammal hibernation is entirely different from bears, though. Small hibernators have brief periods of arousal during hibernation when they urinate and get rid of the calcium that builds up from their bone breaking down. Bears don’t urinate during hibernation, so if they were breaking down bone, their serum calcium would rise and wreak havoc physiologically.”
Seger hypothesized that bears either don’t break down bone during hibernation or they have a different mechanism for putting calcium back onto the skeleton. To find out, she collaborated with the Maine Department of Inland Fisheries and Wildlife, which has been monitoring bear populations in the state for 30 years. With their help, she collected data from active and hibernating female bears, including drawing blood and x-raying their paws to assess the thickness of the bones, a standard measure of bone density.
“We found that the bears were not experiencing unloading-induced bone loss,” Seger said. “It was as if the skeleton perceived it was loaded during hibernation when it wasn’t. Something was interrupting the signal telling them how much unloading was taking place.”
That something, Seger hypothesizes, is the sympathetic nervous system, which is suppressed during hibernation and is influenced by leptin, a hormone that helps regulate body weight. The researchers found greater amounts of leptin in hibernating bears than in active bears. In hibernating bears, leptin correlated with blood tests that measured bone turnover, leading the researchers to believe that the hormone’s effect on the sympathetic nervous system may help to prevent bone loss.
“If the signal from the sympathetic nervous system is interrupted between the brain and the bone, then the skeleton may not know that it’s unloaded,” Seger said. “Bears are the first animal measured that doesn’t experience unloading-induced bone loss.”
That wasn’t the only surprise. The researchers also found a difference between young bears and older bears – not only were young bears not losing bone mass, they were gaining it. “I wasn’t expecting that,” said Seger. “I expected they all would have lost bone or not. That left us scratching our heads for a bit.”
One conclusion she drew was that the young bears – those not yet eight years old – were still growing, so even though they were hibernating, they were still adding bone to their growth plates.
Seger hopes that a greater understanding of the role of leptin and the sympathetic nervous system in bone biology can contribute to better treatments for skeleton related diseases like osteoporosis.