A polar bear watches a passing whaling crew near Barrow, Alaska, on the southern Beaufort Sea, in this 2006 photo. The amount of sea ice that remains during the summer has been declining. Some polar bears have responded by following the sea ice, while others spend more time on land. (Courtesy of Mary Sage, Joseph Napaaqtuq Sage/Associated Press)

A female polar bear swam the equivalent of 16 marathons back-to-back off the coast of Alaska, researchers report.

After swimming nearly 700 kilometres through the Beaufort Sea over nine days, she went on to swim and walk intermittently on the sea ice for an additional 1,800 kilometres over a period of about two months, according to a report published in the January issue of Polar Biology.

The study demonstrates that bears can adapt in some ways to the loss of sea ice in the Arctic as a result of climate change, said George Durner, a research zoologist with the U.S. Geological Survey who co-authored the paper.

"They can swim between ice floes that are far apart. They can swim between ice and land, even though that distance can be great," he said.

But it also shows that doing so can have a negative impact, he added. The bear lost 22 per cent of her body weight and her year-old cub over the course of her journey, and had obviously been food-deprived.

"I guess it shows the vulnerable side of polar bears to large changes in their environment," Durner said.

The researchers aren't sure how unusual the bear's behaviour is, but she did seem to travel much farther than any other bear studied before.

"What makes her particularly unique was the wealth of data that we have on her," Durner added.

The bear was fitted with a radio-tracking collar in August 2008 that recorded her GPS coordinates each hour until she was captured again in late October. The collar also collected data using temperature and motion sensors.

In recent years, more and more sea ice has been melting during the summer as the climate warms in the Arctic. Some polar bears deal with this by spending more time on land, but many follow the ice.

The study, a collaboration involving the geological survey, the University of Wyoming and the U.S. Fish and Wildlife Service, sought to figure out the costs and benefits to polar bears of spending more time on land or on the ice under the changing conditions.

The researchers' ultimate goal is to be able to improve their forecast of polar bear population trends and design strategies to manage the polar bear population.

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