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The melting sea ice in the Arctic may be affecting polar bears more significantly than what scientists believed, says a new study published in the journal Science.
As the ice cover in the Arctic diminishes more and more each summer, polar bears have fewer opportunities to hunt seals, their main food source. The bears hunt seals on the surface of sea ice, most successfully from April to July.
It was believed that in the summer, when polar bears have less access to food and are required to work harder for it, they adopt a mechanism called "walking hibernation" that allows them to reduce their metabolic rate and expend less energy. However, researchers from the University of Wyoming found that the animals were unlikely to be able to compensate for the extended food deprivation through such physiological changes.
The researchers, who tracked and monitored the temperature and activities of more than two dozen polar bears, found that the summer activity and body temperature of the bears were typical of fasting, non-hibernating mammals, with little indication of "walking hibernation."
"We found that polar bears appear unable to meaningfully prolong their reliance on stored energy, confirming their vulnerability to lost hunting opportunities on the sea ice," John Whiteman, the doctoral student who led the project, said in a statement.
Whiteman and his colleagues captured the polar bears, put satellite collars on them, and implanted monitors to track the bears' movements and body temperature. The researchers accumulated data while the bears moved on shore and on ice in the Arctic Ocean's Beaufort Sea, north of Alaska and Canada, from 2008 to 2010.
While bears, both on shore and on ice, reduced their body temperatures and activities below those during an active hunting and feeding period, the reduced levels were not as low as what is observed during an energy-saving hibernation.
Arctic sea ice usually reaches its maximum cover in March, and then recedes to a minimum in September. According to the National Snow and Ice Data Center, the September ice cover has declined more than 11 percent per decade since 1979, when record keeping began. Climate scientists now expect the Arctic Ocean to be ice-free in summer before the end of century, with some predicting that might occur within the next couple of years.
An ice-free summer would dramatically alter the polar bears' habitat, as well as their hunting and feeding patterns.
"If you think of predictions of sea ice into the future, it raises a huge concern. The Beaufort Sea population of the bears is declining. They are unable to reproduce enough cubs to replace the older animals," Merav Ben-David, a professor at University of Wyoming's Zoology and Physiology department and a co-author on the study, told VICE News. "If the same sea ice melt pattern happens, we will see the same with other bear populations as well."
Polar bears were listed as a threatened species in the United States under the Endangered Species Act in May 2008, owing to the ongoing and potential loss of their sea ice habitat. According to data from International Union for Conservation of Nature, three subpopulations of these bears, out of total 19, are in decline.
The researchers, however, found one mechanism that polar bears use to shield themselves during long swims in ice cold waters. To maintain an interior body temperature that would allow them to survive longer in increasingly more frequent swims, the bears temporarily cool the outermost tissues of their core to form an insulating shell — a phenomenon called regional heterothermy.
"This regional heterothermy may represent an adaption to long-distance swims, although its limits remain unknown," the scientists wrote. One of the bears in the study survived a nine-day, 400-mile swim from shore to ice. When recaptured after seven weeks, the bear had lost 22 percent of her body mass, as well as her cub.
But while warmer temperatures continue to create havoc for these animals, scientists have not lost hope.
"The positive thing about sea-ice loss is that it is lineal. What usually happens when system change is that they don't show any impact for a long time and then suddenly crash … sea ice is reducing linearly, which means if we bring things back, it would come back. So we still have hope," Ben-David said.
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Follow Esha Dey on Twitter: @deyesha