Polar bears hunt ringed seals on the surface of sea ice. But during the summertime melt, their hunting territories contract. With limited feeding opportunities, they might head for the shore and enter a low-energy mode called "walking hibernation." However, according to a new Science study, these energy savings aren’t enough to make up for food deprivation in the Arctic.

These bears are most successful at hunting seals between April and July, when the seals are on the ice surface of the continental shelf, rearing their pups and molting. The seals spend less time on the surface from August to October, and the ice retreats from shelf waters. (This period of ice retreat is getting longer and longer with rising temperatures.) As a result, some bears head for shore in the summer -- but food is pretty limited there too. It’s thought that polar bears compensate for the loss of on-ice foraging opportunities by reducing their energy expenditure. This state of lowered activity and reduced metabolism is known as "walking hibernation." However, recent work suggests that they actually expend significant energy in hot months: Their activity and body temperature were those of fasting, non-hibernating animals. 

To evaluate their summer metabolic rates, a team led by University of Wyoming’s John Whiteman attached telemetry transmitters and activity loggers to 26 polar bears and surgically implanted loggers into the abdomen of 10 bears and the rumps of 7. These continuously monitored polar bear summertime movements and core body temperatures on both the ice and the shore of Beaufort Sea north of Alaska and Canada in 2008 and 2009.

^{University of Wyoming researchers Hank Harlow, left, and John Whiteman collect a breath sample from a polar bear on pack ice in October 2009. Daniel Cox.}

They found that bears in both ice and shore habitats reduce their body temperatures and activity levels to below those of bears who are actively hunting and feeding. But these declines don’t drop as low as energy-saving hibernation levels. In fact, they resemble animals that are fasting, which is not a physiological response that offers much energy saving.

These findings suggest that polar bears aren’t able to reduce their metabolism in order to rely on their fat stores for longer. They have limited metabolic options to respond to the declining sea ice, the researchers conclude.

"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," Whiteman says in a statement. These remarkable bears avoid unsustainable heat loss by temporarily cooling the outermost tissues of their core to form an insulating shell.