Polar bears may be in even more trouble because of melting Arctic sea ice than previously thought. They aren’t coping with longer summers ashore without food by entering a hibernation-like state, found new research published in Science yesterday. That disputes an old hypothesis.
In the 1970s, researchers in the Hudson Bay had suggested that polar bears could slow their metabolism significantly to save energy when they couldn’t find food. The concept became known as "walking hibernation."
But the latest research — conducted over years in challenging conditions by a team from the University of Wyoming, the U.S. Geological Survey and the Fish and Wildlife Service — found evidence that makes that marked biological response less likely. The team used surgically implanted temperature loggers to measure core body temperature, an almost unprecedented technique.
Both polar bears that spent their summers on shore and those that summered on ice lost moderate amounts of body heat and gradually reduced their activity in the summer, they found. Those results don’t match up with a significant metabolic shift.
The polar bears acted more like fasting humans than like hibernating brown or black bears, said John Whiteman, a doctoral student at the University of Wyoming who led the research.
"People in the polar bear world will have to decide for themselves if walking hibernation is a term that shouldn’t be used anymore, or if saying it’s a fasting state is just not strong enough," Whiteman said. "If it were up to me, I think [walking hibernation] is too strong of a term."
The findings suggest polar bears might exhaust their stores of energy during periods without food or ocean swims made longer by sea ice loss. It wasn’t the outcome some of the scientists on the team were hoping for.
"When we filled out the grant, we still had hopes that indeed polar bears do have the ability to reduce their metabolism and activity and conserve energy," said Merav Ben-David, an animal behavioralist at the University of Wyoming and one of the authors. "It’s very discouraging that we didn’t find that. It was a unique concept and would have been wonderful if polar bears could do it, but the likelihood of such an adaption is low. Other bears don’t have it."
Andrew Derocher, a polar bear researcher at the University of Alberta, said he wouldn’t write off the idea of a metabolic shift. Instead, he says, this latest study "refines" the community’s understanding of how polar bears save and don’t save energy.
Not enough food on land
"Bears can do some amazing things, but it’s not nearly enough," he said.
Polar bears hunt seals from surface sea ice. But that ice is melting at a rapid rate because carbon emissions cause global warming. All of the lowest extents of summer sea ice have occurred in the last eight years, according to the latest data from the National Snow and Ice Data Center. The summer melting season forces bears ashore, where they face starvation and exhaustion.
"Bears eat seal blubber for a living, so when you take that away, there’s nothing," said Derocher. "There’s just no energy on land that will sustain a population of several thousands of bears."
As of now, groups of polar bears are reacting differently in different places because of a mix of regional conditions, said Jeffrey Bromaghin, a research statistician with the USGS’s Alaska Science Center. For example, the Chuckchi Sea still offers abundant prey. Some bear populations are increasing or stable, according to the International Union for Conservation of Nature’s Species Survival Commission.
"But in the longer term, if global warming continues, it will be such an influential forcing factor that they’re all going to be in decline," Bromaghin said.
He previously found that the overall polar bear population fell by a quarter in the first decade of the 21st century (ClimateWire, Nov. 19, 2014).
In late June, USGS scientists reported that polar bear populations face sharp declines by century’s end, even under a scenario where emissions peak around 2040.
Learning animal surgery in the field
The researchers battled ice, fog and wind. The complicated conditions called for numerous helicopters and a Coast Guard icebreaker, the Polar Sea.
The researchers studied around two dozen bears in the Beaufort Sea, north of Alaska and Canada, in 2008 and 2009. They would spot and capture a bear, place a collar to track GPS coordinates and temperatures, then release and eventually recapture it.
They also set up a field surgery station to implant temperature loggers in the bears’ abdomens, a technique that had never been used in a wild population before. Using that body heat as a proxy allowed the team to track the bears’ energy expenses. The study found no evidence of walking hibernation.
But it did find evidence of another biological coping mechanism. Bears can cool their outermost tissues to create an insulating shell and maintain a warm core. The phenomenon is called regional heterothermy.
That trick could be useful in helping the bears avoid extreme heat loss during frigid ocean swims, the authors said. A melting Arctic would make those swims longer and more frequent. In an earlier publication in Polar Biology, the team described how one of the bears it was tracking survived a nine-day, 400-mile swim from shore to ice, losing more than 20 percent of her body mass and her cub on the journey.
15-foot waves and crashing icebergs
Ben-David was surprised by the extent of sea ice melt when she was in the region on the icebreaker. The maps the team used to navigate did not match what she was seeing.
"What they called 100 percent ice was covered in ice, but it was all broken pieces of ice, not solid ice," she said. "That was an eye-opener."
In the past, wind would blow on top of the ice, without creating waves, but the shattered landscape offered a new setup.
"We had a bad storm from the Pacific that caused huge 15-foot waves to crash on the boat, carrying icebergs that were bigger than the bridge," she said. "It was extremely scary."
The vessel suffered no significant damage.
A helicopter flew out soon after to check on a bear and cub the team had sighted earlier. The mother bear was alone, having probably lost her cub in the storm.
"It was a realization that not only do they have to deal with less food and longer fasts, with longer swims in open water, but they also have to deal with types of storms and waves they didn’t have to before," said Ben-David. "It was something I probably would never have thought of unless I was there."
Reporter Christa Marshall contributed.