At least in comparison to their bear brethren, polar bears are renowned as accomplished swimmers. Their massive front paws, ideal for swatting seals, also make powerful paddles. Their elongated necks allow them to keep their heads above water. And their blubber adds buoyancy and warmth.
But they are designed for brief dips, not aquatic marathons. In an ideal world, a polar bear would enter the water to travel a short distance between ice floes, or to sneak up on an unsuspecting resting seal. Although they can swim longer distances, they generally do so at a cost. In the book The Last Polar Bear, journalist Charles Wohlforth quotes biologist Craig George, who recounted an incident when a polar bear mother and two cubs, having swum ashore near Barrow, Alaska, had no energy remaining to move another inch:
Empirical evidence of the toll of long range swimming on polar bears has been harder to come by, although a study published earlier this year detailed a polar bear mother that swam for 232 consecutive hours in search of pack ice, as a result of which she lost 22 percent of her body mass and her cub lost its life.
As polar bears’ Arctic sea ice habitat shrinks, could such scenarios become more common? In 2006, two researchers concluded that an apparent increase in the number of polar bears found drowned could be attributed to bears being forced to swim ever-greater distances, and a new study adds support to the notion that less ice means more bears are spending more time in the water, to the particular detriment of their cubs.
The study, being presented today at the International Bear Association Conference in Ottawa, is, in the words of co-author Geoff York of WWF’s Arctic Program, “the first analysis to identify a significant multi-year trend of increased long-distance swimming by polar bears.” York and his three co-authors – Anthony Pagano, Kristin Simac and George Durner of the U.S. Geological Survey’s Alaska Science Center – analyzed data from 68 GPS collars fitted on adult female polar bears between 2004 and 2009 and overlaid it with satellite images of sea ice coverage. They found that, over the course of the study, 20 different bears swam distances longer than 50 km (31 miles) during 50 separate occasions. The shortest such journey was 53.7 km (33.3 miles), and the longest, lasting just shy of an almost unfathomable 13 days, was 687.1 km, or 426 miles.
Twelve of the 20 bears had yearling cubs with them at the time they were collared, 11 of which were recaptured or identified at a later date. Five of these 11 had lost their cubs by the time they were re-sighted; that’s a 45 percent mortality rate, compared to 18 percent among cubs of bears that did not make long-distance swims.
In most cases, the bears that swam long distances did so from scattered, or unconsolidated sea ice toward either land or the main pack ice. This implies, the authors write, “that bears are becoming stranded while using unconsolidated sea ice and are forced to swim to reach more stable habitats.” Furthermore, they may be doing so more frequently: 25 of the 50 swimming events took place in 2009 alone. The authors caution that the number of GPS collars, and the quality and quantity of data each collar could generate, also increased over that time, but the proportion of collared bears that underwent lengthy swims showed “an increasing and significant linear trend” from 25 percent in 2003 to 62 percent in 2009.
Polar bears are being forced “to swim longer distances to find food and habitat,” says York. “Climate change is pulling the sea ice out from under polar bears’ feet.”
Photograph by Eric Regehr/U.S. Fish & Wildlife Service