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Polar bears avoid osteoporosis

Roxanne Fisher
14 March 2008

Category: News

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Polar bears have a unique ability to stop their bones degenerating. The discovery could provide an important insight into how to treat or prevent bone loss in both captive animals, and in bed-ridden people.

‘Wild pregnant polar bears are the first known animals to avoid significant bone loss altogether,’ say the researchers who made the discovery. ‘Understanding this physiological mechanism could have profound implications for a natural solution for the prevention of osteoporosis.’

All bears have an amazing ability to survive hibernation – curled up in a den they can survive six months or more without eating, drinking, urinating or defecating. However, polar bears are unique even among this group. When most bears hibernate, their physiology moves into ‘fasting mode’, and they stay this way for months. Polar bears can switch their physiology from this fasting mode into a ‘feeding mode’ within days, allowing them to hunt on the ice for prey at any opportunity.

Yet a new study by veterinarian Alanda Lennox and orthopaedic specialist Allen Goodship of the Royal Veterinary College in North Mymms, Hertfordshire, UK, and University College London, has shown that pregnant polar bears are even more special.

Female bears enter a den three months before giving birth, and they remain immobile in a state of hibernation until they give birth. Once the cubs are born, they continue hibernating for a further three months while they suckle their young.  Twenty-one wild adult polar bears were captured prior to and after hibernation near Churchill, Manitoba in the Hudson Bay area of Canada, as part of an ongoing research programme by the Canadian Wildlife Service. Blood samples were taken from each bear, and then sent to Lennox and Goodship for examination.

This analysis revealed that during hibernation, female bears do not lose any of the strength in their bones. In particular, the bears had raised levels of two biochemical markers for bone formation both immediately prior to, and after hibernation, suggesting they rachet up the creation of new bone. What’s more, there was no physiological indication that bone was lost into the body while the bears lay inactive for months on end.

This suggests that pregnant polar bears possess an advanced evolutionary mechanism for preserving bone, say the researchers.

Usually, animals and people lose bone if they are inactive for a long period of time. When in fasting mode, for example, hibernating black bears lose bone at up to four times the usual rate, though they also continue to produce small amounts of new bone. Overall, their bones still become significantly weaker while hibernating.

For people, it is even worse. If bed-ridden for long periods, they lose much more bone, and make much less; weakening their bones significantly. And it can take years for a skeleton weakened in this way to recover its former strength.

The fact that polar bears have a natural built-in physiological mechanism to prevent such bone loss, even when they are immobile for months, starving, yet still feeding their cubs with milk that uses up much of their calcium reserves, is an ‘unprecedented discovery’ that could provide vital insights into preventing or treating osteoporosis, the researchers write in the journal Comparative Biochemistry and Physiology, Part A (vol 149, p 203).

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