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A World Wildlife Fund photograph taken along the western shore of Hudson Bay in November 2010 shows a female polar bear with two cubs near Churchill, Canada, in this file image released to Reuters on February 9, 2011.??[Photo/Agencies]

WASHINGTON - Polar bear cubs forced to swim long distances with their mothers as their icy Arctic habitat melts appear to have a higher mortality rate than cubs that didn't have to swim as far, a new study reports.

Polar bears hunt, feed and give birth on ice or on land, and are not naturally aquatic creatures. Previous reports have noted individual animals swimming hundreds of miles (kilmetres) to reach ice platforms or land, but this is one of the first to show these swims pose a greater risk to polar bear young.

"Climate change is pulling the sea ice out from under polar bears' feet, forcing some to swim longer distances to find food and habitat," said Geoff York of World Wildlife Fund, a co-author of the study.

York said this was the first time these long swims had been quantitatively measured, filling a gap in the historical background on this iconic Arctic species.

To gather data, researchers used satellites and tracked 68 polar bear females equipped with GPS collars over six years, from 2004 through 2009, to find occasions when these bears swam more than 30 miles (48.28 km) at a time.

There were 50 long-distance swims over those six years, involving 20 polar bears, ranging in distance up to 426 miles (685.6 km) and in duration up to 12.7 days, according to a paper for presentation on Tuesday at the International Bear Association Conference in Ottawa, Canada.

"A LOT LIKE US"

At the time the collars were put on, 11 of the polar bears that swam long distances had young cubs; five of those polar bear mothers lost their cubs during the swim, representing a 45 percent mortality rate, the study found.

Cubs that didn't have to swim long distances with their mothers had an 18 percent mortality rate, the study said.

"They're a lot like us," York said in a telephone interview. "They can't close off their nasal passages in rough waters. So for old bears or young bears alike, if they're out in open water and a storm hits, they're going to have a tough time surviving."

Two factors make it even harder for polar bear cubs to weather long periods in Arctic waters, said Steve Amstrup, a former scientist at the U.S. Geological Survey and now chief scientist at Polar Bears International, a conservation group.

"Young bears don't have very much fat and therefore they aren't very well insulated and cannot cope with being in cold water for very long," Amstrup said in the same telephone conversation.

Because they are leaner than their parents, Amstrup said, "they probably aren't as buoyant (as adult polar bears) so in rough water they'll have more difficulty keeping their heads above water.

The Bush administration listed polar bears as threatened under the Endangered Species Act because of the decrease in their Arctic ice habitat. That decision survived a legal challenge last month, and this month, Canada listed polar bears as a species at risk.

The Arctic is warming faster than lower latitudes due to the accumulation of greenhouse gases in the atmosphere, and the melting of sea ice in summer accelerates the warming effect.

Arctic sea ice extent -- the area covered by sea ice -- in June was the second lowest in the satellite record since 1979, according to the National Snow and Ice Data Center.