As the Arctic sea ice habitat retreats, polar bears have to swim longer distances to find stable ice or to reach land, and their cubs are suffering, according to a new study presented Tuesday at the International Bear Association Conference in Ottawa, Canada.
Polar bears, not naturally aquatic animals, hunt, eat and give birth on ice or on land, according to Reuters. Earlier studies have shown that the bears are swimming hundreds of miles to reach solid ice or land, but a new study demonstrates that the longer swims increase cub mortality, compared with cubs that didn't swim such long distances. According to Reuters:
"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.
Between 2004 and 2009, a team of researchers led by Anthony Pagano of the U.S. Geological Survey gathered data from 68 GPS collars that had been put on adult female polar bears, and looked at that data along with satellite imagery of sea ice, in order to isolate episodes where bears swam more than 30 miles at one time, according to a synopsis of the research on PRWeb. (The final report is not yet released but the full abstract is provided with the synopsis at PRWeb.) Researchers identified 50 long-distance swimming events, involving 20 polar bears, over the six years. The bears swam distances of as far as 426 miles, and as many as 12.7 days.
Eleven of the polar bears that swam the long distances had young cubs at the time that they were collared; five of those bears lost their cubs during the swims, which translated to a 45 percent morality rate. Only 18 percent of cubs died when they weren't swimming the long distances with their mothers.
A big difference.
The long-distance swims took a toll on the cubs partially because young polar bears don't have much fat and thus can't be in the cold water for long periods of time, Time said. Steve Amstrup, a former scientist at the U.S. Geological Survey and chief scientist at the conservation group Polar Bears International, said, according to Reuters:
"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.
Evidence that long-distance swimming takes a toll on polar bears was already presented in a study published earlier this year that tracked a polar bear mother that swam for 232 consecutive hours in search of pack ice, and lost 22 percent of her body mass as a result, while her cub lost its life, Discovery reported. And in 2006, researchers came to the conclusion that an apparent rise in the number of polar bears found drowned could be linked to the bears being forced to swim longer distances.
The Arctic is warming faster than elsewhere because of greenhouse gases accumulating in the atmosphere, and the melting of sea ice in the summer speeds up the warming effect, according to Reuters.
The extent of Arctic sea ice dropped to record low levels in July 2011; sea ice volume is now 47% lower than the levels in 1979, when satellite records began.