The predator made her epic journey in the Beaufort Sea (see map), where sea ice is shrinking due to global warming, forcing mother bears to swim greater and greater distances to reach land—to the peril of their cubs.
The cub of the record-setting bear, for instance, died at some point between starting the swim and when the researchers next observed the mother on land. She also lost 22 percent of her body weight.
"We're pretty sure that these animals didn't have to do these long swims before, because 687-kilometer stretches of open water didn't occur very often in the evolutionary history of the polar bear," said study co-author Steven Amstrup, chief scientist for the conservation group Polar Bears International. Amstrup is also the former project leader of polar bear research for the U.S. Geological Survey (USGS), which led the new study.
Another female bear in the study swam for more than 12 days, but appears to have found places to rest during her journey.
Long Swims Deadly for Polar Bear Cubs
Biologists collared 68 female polar bears between 2004 and 2009 to study their movements. Thanks to what study co-author and WWF polar bear biologist Geoff York calls an "accident of technology and design," the researchers noticed data gaps in the bears' whereabouts. The researchers were later able to link the gaps to periods when the bears were at sea. (See polar bear pictures.)
The scientists examined GPS data for more than 50 female polar bears' long-distance swimming events, defined as swims longer than 30 miles (50 kilometers). This data was then correlated to rates of cub survival.
"Bears that engaged in long-distance swimming were more likely to experience cub loss," said study co-author George Durner, a USGS research zoologist in Anchorage, Alaska.
Five of the 11 mothers that had cubs before they began their lengthy swims lost their young by the time the researchers observed them again on land, according to the research, presented July 19 at the International Bear Association Conference in Ottawa, Canada. The study is not yet published in a journal.
Sea Ice Loss to Continue
Until 1995, summer sea ice usually remained over along the continental shelf of the Beaufort Sea, a critical habitat for polar bears due to its rich seal population. Now the sea ice in the Beaufort and Chukchi Seas is retreating from the coast by hundreds of kilometers, Durner said.
In 2010, Arctic sea ice extent was the third lowest on record, part of a long-term trend of ice loss that will continue for decades to come, according to the National Snow and Ice Data Center in Boulder, Colorado.
"So the sort of conditions that contribute to long-distance swimming are likely going to persist in the future, and if cub mortality is directly related to this, then it would have a negative impact on the population," Durner said.
It's unknown whether the cubs are drowning at sea or whether the metabolically costly act of swimming long distances in nearly freezing water kills them after they reach land.