Polar bears are unlikely to physiologically compensate for extended food deprivation associated with the ongoing loss of sea ice, according to one-of-its-kind research conducted by University of Wyoming scientists and others, and published today in the journal Science. "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.

The loss of sea ice in the Arctic, which is outpacing predictions, has raised concern about the future of polar bears, leading to their listing as a globally threatened species under the U.S. Endangered Species Act in 2008. 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.

Some earlier research suggested that polar bears could, at least partially, compensate for longer summer food deprivation by entering a state of lowered activity and reduced metabolic rate similar to winter hibernation -- a so-called "walking hibernation." But the new research shows that the summer activity and body temperature of bears on shore and on ice were typical of fasting, non-hibernating mammals, with little indication of "walking hibernation."

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."

The researchers reached that conclusion by capturing more than two dozen polar bears, implanting 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. The unprecedented effort, logistically supported by the U.S. Geological Survey (USGS) and funded by the National Science Foundation, USGS, U.S. Fish and Wildlife Service (USFWS), as well as the Environmental Protection Agency, required the assistance of numerous personnel, multiple helicopters and deployment of the U.S. Coast Guard ice-breaker, the Polar Sea.