It seems like every time we turn around, polar bears are catching a tough break.
As climate change continues to heat up the planet and Arctic sea ice retreats further each year, conservationists are increasingly concerned that the bears — which use the sea ice as a hunting ground for catching seals — will have less access to the food they need to survive. It’s been an ongoing worry for years, and last week the U.S. Fish and Wildlife Service drove it home again with a new conservation management plan, which identifies climate change and sea ice loss as the primary threat to polar bears.
Despite all the doom and gloom, some research conducted in the early 1980s has helped conservationists maintain a glimmer of hope about the polar bear’s ability to survive long periods of time without food. This research found evidence in polar bear blood samples to suggest that the bears might go into a kind of “walking hibernation” when food is scarce, staying awake but significantly lowering their metabolism to use less energy. This would be a useful adaptation during the summer, when sea ice is at its lowest extent and hunting is most difficult.
It’s been a tempting theory for more than 30 years — but once again, we’re looking at bad news for the polar bear. A new study, published in Science, debunks the “walking hibernation” idea with data collected from more than two dozen captured polar bears in the Arctic’s Beaufort Sea, which the researchers spotted and tranquilized from helicopters.
The researchers, led by biologist John Whiteman at the University of Wyoming, outfitted bears with devices that collect and transmit data remotely to collect data on the bears’ movement and activity and their body temperature. Their sample included both “ice bears” and “shore bears” — that is, both bears who choose to chase the ice as it retreats north in the summer, looking for seals, and bears who choose to spend their summer on shore.
The researchers expected that if bears did indeed exhibit walking hibernation, their activity and temperature would drop down to the kinds of levels usually observed in other bears during true hibernation — that is, very low levels.
“If there was hibernation metabolism … you would see all of them have a very steep, abrupt decline in body temperature to about 35 degrees (Celsius) and then remain like that the whole period,” says senior author Merav Ben-David, a professor of wildlife ecology at the University of Wyoming. “But we don’t see that.”
Instead, the results showed that both ice bears and shore bears experience much more moderate declines in body temperatures during the summer, when food is scarce — just the kinds of declines you would expect to see in any mammal that wasn’t getting enough food. “If you went into an extended fast, your body temperature would decline too,” Ben-David says. “It’s a normal mammalian response to fasting and losing metabolically active tissue — losing weight.”
Unfortunately, this means that polar bears have no special protections against starvation as was previously thought. They simply exhibit a typical fasting response to food deprivation, and — given a long enough time without food — will starve.
Special adaptation
There are an estimated 20,000-25,000 polar bears, grouped into 19 sub-populations. While scientists don’t have adequate data for all the populations, we know at least some of them are declining, and the polar bear is currently protected under the Endangered Species Act. Scientists worry that their populations will suffer worse declines in the future as sea ice continues to melt.
It’s worth noting that while polar bears didn’t show any signs of walking hibernation, the study did produce evidence for another kind of special adaptation that may help bears survive under stressful conditions. The collected data suggest that when bears swim in the cold Arctic water for extended periods of time — longer than a few minutes, according to Ben-David — they can cool down the outer parts of their body cores so that less body heat is lost to the frigid water.
