And here!
http://www.polarbearsalive.org/facts3.htm#anchor766763
Alone among bears, the polar bear is considered a marine mammal.
Polar bears are champion swimmers. They have been known to swim more than 60 miles without a rest. The polar bear's swimming limit is not known.
Mother polar bears can be so protective of their young that they have been known to rear up and leap at helicopters carrying research scientists. Studies have shown that darted and tagged female bears consistently produce smaller litters and lighter cubs. If tagged in the den area, many pregnant females abandon the site.
Biologists believe that starvation is the leading cause of death for subadult bears.
Those polar bears that manage to survive to adulthood have learned to master the challenges of arctic life. The annual mortality rate of adult bears is surprisingly low--as little as five percent a year.
Despite what our eyes tell us, a polar bear's fur is not white. Each hair shaft is pigment-free and transparent with a hollow core.