Polar bears may be in more trouble than scientists thought. A study of polar bear metabolism contradicts a long-held idea that polar bears might be able to lower their metabolic rates to conserve energy during the summer, when their icy hunting grounds melt. Given the threat of climate change, the finding intensifies an already dire situation that led to the polar bear's 2008 listing as a globally threatened species under the US Endangered Species Act.
Polar bears "walk around on the surface of the sea ice, and wait to prey upon seals that come up to the surface," says John Whiteman, a polar bear ecologist at the University of Wyoming and a co-author of the study, published today in Science. That hunting method means that polar bears rely heavily on ice to survive — ice that has been melting at an increasingly rapid pace.
Scientists thought the bears experienced "walking hibernation."
The ice melt might seem like a pretty scary situation for the bears, but some researchers weren't so sure; they suggested that these predators might have a secret weapon that could help them weather the melt. "There had long been an idea that polar bears can reduce their metabolism during the summer to resemble that of winter hibernation," Whiteman says; doing that would reduce their need for food during the summer. Scientists called this "walking hibernation" — a state where the bears remain active while also minimizing the physiological stresses of fasting. But the evidence for this response was limited, Whiteman says. And "even though no one had studied polar bear metabolism in the way we tested it in this paper, the idea was perpetuated."
University of Wyoming researchers Hank Harlow, left, and John Whiteman collect a breath sample from a polar bear on pack ice in October 2009. Credit: Daniel Cox
To figure out what was actually going on with the bears in the summer, Whiteman and his team of scientists shot darts from helicopters to chemically immobilize over two dozen polar bears. Then, they measured and weighed the bears before surgically inserting an implant into their abdomens. The implant then recorded the bears' core body temperatures once per hour for a period of two years. Some of the bears also received a collar equipped with a radio transmitter — so they could be tracked via GPS — and an accelerometer, which was used to determine how active the bears were.
Thanks to these sensors, the researchers found that although bears lower their metabolic rate during the summer, it's a very gradual process that resembles what any mammal would experience in response to fasting; their reaction doesn't come close to what's seen in bears during winter hibernation. In short, whatever ability some researchers thought the polar beats might have to counteract the negative consequences of ice loss doesn't appear to exist — polar bears can't enter a "walking hibernation" state.
No secret defense mechanism
The data collected in the study wasn't perfect. The number of bears studied was small and they belonged to a very specific wild population that dwells in the Southern Beaufort Sea region. This means that they might not be representative of all polar bears. In addition, internal body temperature can vary depending on the tissues sampled, so it would have been better if the researchers had inserted numerous implants in different types of tissues, Whiteman says. But "for a variety of reasons, not the least of which is ethical, that's not possible."
University of Wyoming researchers John Whiteman and Merav Ben-David inspect a temperature logger implantation site on a polar bear on off-shore sea ice north of Prudhoe Bay, Alaska, in April 2009. Credit: Mike Lockhart
The study describes polar bear metabolism for the first time, which means that other groups will have to replicate the work to ensure that it's correct. But in the meantime, the study indicates that the bodies of polar bears won't adapt very well to melting ice.
"Currently, some subpopulations are doing poorly due to loss of their sea ice habitats, and other subpopulations appear to be doing well," says Eric Regher, a wildlife biologist at the US Fish and Wildlife Service and a co-author of the study. But "nearly all polar bears are likely to be negatively affected if sea ice loss continues as projected by scientists."