According to a team of scientists led by Dr John Whiteman from the University of Wyoming, polar bears (Ursus maritimus) are unlikely to physiologically compensate for extended food deprivation associated with the ongoing loss of sea ice.
Polar bears (Ursus maritimus). Image credit: Steve Amstrup / USFWS.
Polar bears walk ice sheet surfaces looking for food. In summer, when the ice melts, their hunting territories dwindle and they may move on shore, where food is less plentiful, to forage.
Some biologists have suggested that the bears forced ashore can compensate by entering a hibernation-like state called ‘walking hibernation,’ and that this strategy could help the animals survive despite the loss of on-ice foraging opportunities caused by climate change.
But the new study – published in the journal Science – suggests polar bears actually expend significant energy, 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,” said study first author Dr Whiteman.
To evaluate the extent to which their metabolic rates decline in summer, Dr Whiteman and co-authors used satellite collars and surgically implanted loggers to continuously monitor polar bear’s summertime movements and core body temperatures on ice and shore.
Bears in both habitats reduced their body temperatures and activity levels below those of bears actively hunting and feeding, but not to levels as low as those observed during energy-saving hibernation.
Instead, the observed declines mirrored those of a fasting mammal, the response of which doesn’t offer significant energy savings.
“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 scientists said.
The team also 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 adaptation to long-distance swims, although its limits remain unknown,” the researchers said.
“Many colleagues doubted whether the study was possible, until we actually did it. This project was logistically so intense that it may never be replicated,” said study co-author Dr Merav Ben-David from the University of Wyoming.
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J. P. Whiteman et al. 2015. Summer declines in activity and body temperature offer polar bears limited energy savings. Science, vol. 349, no. 6245, pp. 295-298; doi: 10.1126/science.aaa8623
