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No 'walking hibernation'

Brigitte OsterathJuly 17, 2015

Nature has equipped polar bears well for their environment - but not for climate change, a new study suggests. Yes, they can adapt to hard times. But not through walking hibernation, as scientists had hoped.

Young polar bear Photo: Shawn Harper
Image: Shawn Harper

Winter means hard times for many animals - but for polar bears, the summer is worse. When the ice melts in summer, polar bears might get stuck on shore. Here, they cannot forage on ice and thus do not find as much food as they actually need.

Polar bears hunt most successfully on the surface of sea ice between April and July, when ringed seals rear their pups. Bears that pass the summer on shore miss out on this opportunity.

The loss of sea ice in the Arctic, which is outpacing predictions, has raised concerns about the future of polar bears. And it turns out: More polar bears will get stuck on shore as climate change progresses.

'Walking hibernation'

But there had been one hope for conservationists: Earlier research suggested that these "shore bears" decrease their metabolic rate and are less active. Scientists called it "walking hibernation."

Some scientists assumed that this energy-saving mode could compensate - at least partially - for the shortage of food polar bears experience as Arctic ice melt increases due to climate change.

Now this hope has been shattered, according to a study by the University of Wyoming: "Our findings suggest that bears are unlikely to avoid deleterious declines in body conditions that are expected with continued ice loss."

Polar bear research in the arctic Photo: Mike Lockhart
John Whiteman and Merav Ben-David inspect a temperature logger implantation site on a tranquilized polar bearImage: Mike Lockhart

Monitoring polar bears

John Whiteman, a University of Wyoming doctoral student, and his colleagues set out to investigate if this "walking hibernation" truly exists.

They captured more than two dozen polar bears in the Arctic Ocean's Beaufort Sea, north of Alaska and Canada. After implanting temperature loggers into the abdomens and the rumps of these bears, they tracked their movements on shore and on ice.

"Bears on shore and ice exhibited similar activity patterns," the authors wrote in the journal "Science." Also, their core body temperatures did not differ.

All data obtained from bears on shore and on ice were typical of fasting, non-hibernating mammals. There is little indication of "walking hibernation," the authors said.

Addressing the root problem

"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 said.

The animals do have an unusual response mechanism to survive in hard conditions. This shutting-down mode comes into action as they swim.

Polar bear swims Photo: Uli Deck dpa/lsw
Swimming in cold water for days? No problem!Image: picture-alliance/dpa

Whiteman and his team found that polar bears might temporarily cool the outermost tissues of their core to form an insulating shell - this way, they can survive swims of even several days long.

But that won't help them much when they can't find anything to eat in summer.

Co-author Steve Amstrup, chief scientist at Polar Bears International put it this way: "The key to polar bear conservation is arresting the decline of their sea ice habitat."