Polar bears’ ‘walking hibernation’ not much of an energy saver

Summertime dips in activity and body temperature may not be enough to protect bears from challenges of climate change

Polar bear

BUDGETING ENERGY  Polar bears, like this youngster on pack ice over the Arctic Ocean, have to learn to cope with lean times in summer. But their warm-weather energy-saving tactics may not give them much leeway as a changing climate extends hungry times.

Shawn Harper

Polar bears’ normal summertime energy-saver mode is called “walking hibernation.” But it may not be drastic enough to compensate for lengthening warm seasons — and accompanying food shortages — as climate changes, researchers say.

What happens to summertime bears is more like fasting than like winter hibernation, says ecologist John Whiteman of the University of Wyoming in Laramie.

Biologists suspected a general slowdown of the bears’ metabolism when the springtime seal-hunting bonanza ends as sea ice recedes, but measurements have been sparse. Using data-collecting collars and implants, Whiteman and colleagues showed that polar bears following the receding ice out to sea, as well as the left-behinds onshore, were less active in summer.

CHECK-UP In a project to collect rare data on what’s called “walking hibernation,” University of Wyoming researchers John Whiteman and Merav Ben-David inspect a temperature logger implantation site on a polar bear on offshore sea ice north of Prudhoe Bay, Alaska. Mike Lockhart

The percentage of time the bears spent moving around declined from a springtime peak of about 25 percent (already low for a bear species) to 12 to 22 percent between August and October, the researchers report in the July 17 Science. In contrast, polar bears hibernating in winter move as little as 1 or 2 percent of the time.

Summertime core body temperatures also gradually slipped downward less than a degree Celsius. The temperature of a hibernating bear that the researchers monitored “fell off a cliff,” Whiteman says, plunging about 2 degrees in just weeks.

Those differences mean that metabolic changes in summer are likely to be minor compared with real hibernation, Whiteman says. So if the hungry time in summer stretches longer in a warming world, the modest energy savings of walking hibernation may still leave polar bears vulnerable.

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