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,