Researchers in Alaska have tracked a female polar bear swimming for 232 consecutive hours, during which time she covered 687 kilometers (427 miles) until she finally reached the sea ice of the Arctic Ocean. The finding underlines the enormous capacity of polar bears to survive in the water, but also demonstrates the immense cost to them of having to do so for long periods. By the end of the ordeal, the bear had lost 22 percent of her body mass, and her yearling cub had apparently died.
Writing in the journal Polar Biology, George Durner of the United States Geological Survey and colleagues describe capturing an adult female bear and her cub on Alaska's Beaufort Sea coast in late August 2008. Around the adult's neck, the researchers placed a radio collar with GPS unit, satellite uplink, and an accelerometer to monitor the bear's activity rate independent of the GPS measurements. (She was one of 13 bears so equipped by the researchers that month.)
The bear weighed 226 kg (498 lbs), and the yearling weighed 159 kg (350 lbs). When the scientists found the bear again, two months later, she weighed just 177 kg (390 lbs) and was not lactating; the yearling was nowhere to be seen.
By analyzing the recovered data and overlaying the bear's movements with ice charts, Durner and colleagues deduced that on 25 August (two days after capture and release) she entered the water off the Beaufort Sea coast and swam north for nine days, before finally reaching sea ice. She then spent three days on the ice, another day swimming, and a further 49 days on the sea ice before being found again.
The researchers write that although polar bears are marine mammals, in that they derive their nutrition from the ocean (i.e. they eat seals), they are not aquatic mammals. They can swim reasonably well and survive for long periods in the water, but they pay a high energetic cost for doing so. (Durner and colleagues were unable to determine whether this particular bear had recovered some of her weight by eating seals after hauling out on to the ice, or whether her condition had continued to deteriorate, perhaps as a consequence of patrolling an area of ice that was not rich in hunting opportunities.)
The question might reasonably be asked: Why did the bear keep swimming? Why didn't she at some point turn around? The answer seems to be that, simply, late August is the period when polar bears in northern Alaska head north on to the sea ice, and that is what she was doing. It is what polar bears are programmed to do, and there is nothing in their mental toolkit that would lead them to believe that ice wouldn't be just around the figurative corner, as long as they headed in the right direction.
Which, for the great majority of the time that polar bears have been polar bears, has been a reasonable assumption. But in 2008, Arctic sea ice extent was the second-lowest on record, with losses particularly acute in the Beaufort Sea. At the time this bear and her cub took their plunge, the ice was well over 500 km (320 miles) north of the Alaska coast.
As Arctic sea ice continues to decline, situations such as that confronted by this female bear could become more common, with even more severe consequences, an observation previously made in a 2006 paper in Polar Biology. In that paper, Charles Monnett and Jeffrey Gleason reported that, during aerial observations from 1987 to 2003, they saw 315 polar bears, of which just 12 were in open water. All 315 bears were alive. But in 2004, they saw 55 bears, of which 51 were alive. Ten of those 51 were found in open water, as were all four of the dead bears. And while it could not be proven that those four bears had drowned, it could reasonably be inferred that they had died in the water.
Monnett and Gleason speculated that "mortalities due to offshore swimming during late-ice (or mild ice) years may be an important and unaccounted source of natural mortality," and suggested that "drowning-related deaths of polar bears may increase in the future if the observed trend of regression of pack ice and/or longer open water periods continues."
Top image: A healthy polar bear patrolling the sea ice in the Russian Arctic. Lower image: A skinny polar bear (not the one described in the story) that hauled itself onto a passing floe after being observed swimming in the open water off the coast of Alaska, far from the ice edge. Both photographs by Kieran Mulvaney.