Polar bears are unable to achieve a full hibernation, unlike most of their more southerly relatives. This could prove to be a glaring weakness in their struggle for survival in a warming climate, researchers report.

Scientists went into a multiyear study thinking polar bears would undergo a minihibernation in the summer when food was scarce, during which their body temperature and activity levels would drop in response to reduced food, what some biologists call a "walking hibernation."

They found, however, that did not happen. Instead, the animals exhibited a typical fasting response to food deprivation - they lost weight and their temperature declined a bit. "If you went into an extended fast, your body temperature would decline too," said Merav Ben-David, a wildlife ecologist at the University of Wyoming and a co-author of the study. "It's a normal mammalian response to fasting and losing metabolically active tissue - losing weight."

Global warming is likely to extend these periods of famine, researchers speculate. This could, in turn, endanger the animals, which are dependent on the winter chill.

"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," said John Whiteman, a doctoral student at the University of Washington.

Researchers captured more than two dozen polar bears, fitting the animals with body temperature recorders and watched them after they were released. Study of the bears continued from 2008 to 2010 as the animals roamed around the waters of the Beaufort Sea.

Polar bears cool the outer parts of their body core during long swims to maintain a proper body temperature, in a process called heterothermy. Animals in the study were seen to reduce this temperature by just around 1.25 degrees Fahrenheit. This change is too small to represent a walking hibernation but would be consistent with fasting, researchers concluded.

As global warming continues to raise temperatures at the poles, swimming will become more frequent and the animals will stay out for longer periods. One of the animals examined during the study undertook a 400-mile swim that took nine days to complete. Seven weeks later, researchers examined the bear, finding she had lost 22 percent of her body mass during the ordeal, and her pup had perished.

In 2008, polar bears were designated as an endangered species, but the move may have more to do with psychology than in effective conservation of the ice-loving creatures.

"In reality, we cannot do much under the Endangered Species Act to save polar bears. What we need to save polar bears is global action to reduce climate change," Merav Ben-David, a wildlife ecologist at the University of Wyoming, said.

Study of the polar bears and analysis of how the animals could be threatened by global warming was profiled in the journal Science.

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