Polar bears may not be able to adapt to food deprivation from sea ice loss
A new study has shown that when there is a lack of feeding opportunities during summers due to increased sea ice loss, polar bears may not be able to make up physiologically for the extended food deprivation.
In a first of its kind study, published in journal Science, researchers have found that though polar bears have the ability to compensate for the decrease in feeding opportunities by entering a low-energy state called “walking hibernation”; however, they reveal that this compensation is not enough and they spend significant energy, which is a characteristic of regular metabolism in hot months.
“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.
Recent studies have suggested there is an increased loss of sea ice in the Arctic, which have raised questions about polar bears ability to adapt and survive.
The bears depend on hunting seals on the surface of the sea ice over the continental shelf, most successfully from April to July. In parts of the polar bears’ range, the lengthening period of sea ice retreat from shelf waters — caused by increasing temperatures — can reduce their opportunities to hunt seals, leading to declines in bear nutritional condition.
Whiteman and his colleagues concluded in the Science publication: “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.”
Researchers arrived at their conclusion through an analysis of data captured from two dozen polar bears using implanted temperature loggers and tracking their subsequent movements on shore and on ice in the Arctic Ocean’s Beaufort Sea, north of Alaska and Canada, during 2008-2010.
Scientists found that polar bears use an unusual physiological response to avoid unsustainable heat loss while swimming in the cold Arctic waters. To maintain an interior body temperature that allows them to survive longer and, nowadays, more frequent swims, the bears temporarily cool the outermost tissues of their core to form an insulating shell — a phenomenon called regional heterothermy.
“This regional heterothermy may represent an adaption to long-distance swims, although its limits remain unknown,” wrote the scientists who, in an earlier publication — the journal Polar Biology — noted that one of the bears in the study survived a nine-day, 400-mile swim from shore to ice. When recaptured seven weeks later, the bear had lost 22 percent of her body mass, and her cub died.
By shedding light on potential mechanisms that facilitated that bear’s survival during her long swim, as well as the overall metabolism and activity of bears, the current study “profoundly contributes to understanding the value of summer habitats used by polar bears in terms of their energetics,” Harlow says. Amstrup adds, “It fills a gap in our otherwise extensive knowledge of polar bear ecology and corroborates previous findings that the key to polar bear conservation is arresting the decline of their sea ice habitat.”