ANCHORAGE, Alaska — On a summer day two years ago, polar bear 20741 decided to leave a remote Beaufort Sea beach. The 7-year-old, nearly 500-pound bear walked north into frigid Arctic Ocean water east of Barrow in search of sea ice.
She swam. And swam. And swam.
She covered 426 miles — farther than researchers have recorded a polar bear swimming without a break. After nine days, she reached pack ice and walked or swam another 1,118 miles, eventually looping south back to Alaska soil a few miles from the Canada border.
Researchers recaptured her after two months and learned her journey had come at an extraordinary cost. Her body mass was reduced 22 percent and her internal temperature had dropped. Her yearling cub was gone, likely drowned.
A paper on the bear, published last month in the journal Polar Biology by U.S. Geological Survey and University of Wyoming researchers, concludes that polar bears can respond to a changing Arctic, said USGS research zoologist George Durner, but that there are limits to that ability as sea ice diminishes.
"If we continue to see declines in the extant of Arctic sea ice, it's hard to imagine that a bear would be capable of swimming much further than that," Durner said. "I'm not saying that's the limit, but it just boggles my mind."
Diminished Arctic sea ice is at the heart of the debate on whether ice-dependent marine mammals should be listed as endangered species, with all the ramifications that accompany that decision, including debate on drilling for offshore Arctic oil.
Polar bears use sea ice for hunting their main prey, ringed seals. Their most important feeding time is mid-spring to early summer, when ringed seal pups are born and weaned in snow lairs on sea ice. The pups grow fat and so do polar bears that collapse seal lairs and gorge.
"The Arctic turns into a big smorgasbord for polar bears," Durner said. "In a lot of the Arctic, that is the time for polar bears really to gain weight."
The National Snow and Ice Data Center has tracked a steady decline in sea ice. Some models see summer sea ice disappearing by 2030.
The summer low for sea ice, measured each September, averaged 2.7 million square miles from 1979 to 2000. Sea ice has fallen far below that in recent years, including a record low 1.65 million square miles in summer 2007. The 2010 low was 1.84 million square miles.
Diminishing sea ice led to the 2008 listing of polar bears as a threatened species. The listing is perceived as a threat to Alaska's oil and gas industry and the state of Alaska has sued to reverse it.
Durner is on the sidelines of the political debate but marvels at the bear. She was first captured years ago as a "subadult" with her identifying number tattooed on the inside of her upper lip.
She was captured again with a cub on Aug. 23, 2008. She weighed 497.2 pounds and her cub was 350 pounds, already the size of a North Slope grizzly.
Bear 20741 was fitted with a radio collar that recorded locations, ambient temperatures, and activity level. A device surgically implanted in her rump logged body temperature.
Three days after her capture, she started her epic swim. Recaptured and weighed Oct. 26, 2008, she had dropped 107.8 pounds to 389.4 pounds. The cub was not with her. Researchers assume the cub drowned but cannot say for sure.
Other polar bears have been seen in open ocean. A marine contractor hired in advance of Alaska offshore oil development spotted nine polar bears in 2008 in the Chukchi Sea. The bears were 15 to 65 miles off shore. Some were swimming toward pack ice.
Four polar bear carcasses were spotted after a storm in the Beaufort Sea in 2004. Near-shore winds had exceeded 33 mph, which can produce waves exceeding 13 feet.
Bear 20741 swam in waves of three to six feet. Durner knows of no other polar bear that has swam 426 miles.
"But really, what separates this bear besides the length of her swim was the detail of data that we were able to obtain from this animal," he said. The long swim, Durner said, demonstrates bears' fidelity for sea ice — or at least this bear's.
"Polar bears evolved to exploit the sea ice environment," he said. "Behaviorally, morphologically, they differ from their close relatives, brown bears. Those adaptations are for a sea ice environment . polar bears only occur where there's substantial sea ice during the course of the year because that's where they derive their sustenance."
Before 1995, even during the peak summer sea ice melt, polar bears could find remnant ice over the continental shelf, where seals thrive.
"If they decided to go swimming between ice and floes or between ice and land, they didn't have as far to go," he said. "They probably swam tens of kilometers instead of hundreds of kilometers like we're seeing with some of the bears now."Comment on this story
The second recapture wasn't the end of polar bear 20741. Researchers put another collar on the bear and tracked her until the collar stopped transmitting six months later. Durner hopes to see her again.
"I would be ecstatic," he said.
Her long-distance swim demonstrated a useful behavior but could compromise her ability to survive.
"They're mighty animals," he said, "but they're also vulnerable."