Polar bears' struggle to survive the effects of climate change has been of a major concern to both scientists and the general public, and new research suggests polar bears cannot physiologically compensate for the loss of food associated with the loss of sea ice.

The loss of sea ice in the Arctic has exceeded expectations and caused polar bears to be listed as a globally threatened species under the U.S. Endangered Species Act in 2008, the University of Wyoming reported. Polar bears rely on the ability to hunt seals on the surface of sea ice, and they do so most successfully from April to July. The retreat of sea ice can reduce the polar bears' opportunities to hunt this food source, leading to a reduction in their overall nutrition intake.

"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 - even as they surprised us by also exhibiting an unusual ability to minimize heat loss while swimming in Arctic waters," says John Whiteman, the UW doctoral student who led the project.

Past research has suggested polar bears can partially compensate for this nutrition loss by reducing activity as well as their metabolic rate in what is referred to as a "walking hibernation." These newest findings suggest the struggling bears actually had activity levels and body temperatures typical of fasting, and not walking hibernation.

"This suggests that bears are unlikely to avoid deleterious declines in body condition and, ultimately, survival, that are expected with continued ice loss and lengthening of the ice-melt period," Whiteman said.

The researchers also observed the bears survive increasingly longer swims in the frigid Arctic waters by temporarily cooling the outermost tissues of their core to form an insulating shell in what is known as regional heterothermy. Through this method, one bear survived a nine-day, 400-mile swim from shore to ice in search of food. The bear lost 22 percent of her body mass in the subsequent seven weeks, and her cub passed away.

To make their findings, the researchers captured more than two dozen polar bears and outfitted them with temperature loggers. They also tracked the bears' movements on ice in the Arctic Ocean's Beaufort Sea between 2008 and 2010.

"Many colleagues - even some on our research team - doubted whether the study was possible, until we actually did it," said Merav Ben-David, the UW professor who developed the research plan along with Professor Hank Harlow, an eco-physiologist and colleague in the Department of Zoology and Physiology, and Steve Amstrup, previously with the USGS and currently the chief scientist at Polar Bears International. "This project was logistically so intense that it may never be replicated."

The findings were published in a recent edition of the journal Science.