By Emily Stone
You've probably seen a photo or video of a lumbering polar bear, seemingly forlorn, stuck on a small piece of ice in what looks like an endless stretch of sea. The animals have become the symbol for what's at risk if the arctic sea ice continues to retreat each summer. But for all their emotional punch, very little is known about how the bears are coping with the dramatic changes in their habitat.
A group of researchers from the University of Wyoming is looking for some answers among the polar bears on the North Slope of Alaska. These bears are forced to decide between staying on land during the summer or following the ice edge north, past their normal feeding grounds to an area of deep ocean that may not hold much food. The researchers want to know what the polar bears are eating and how much they're exerting themselves if they stay on land versus head north.
The project's main question is: "Are they able to eat a lot during the summer, and if they're not eating a lot, how well are they able to fast," explains John Whiteman, a Ph.D. student on the project run by co-principal investigators Hank Harlow and Merav Ben-David.
The team is in the second year of the two-year project, which is based out of Barrow and Prudhoe Bay. They're using a combination of blood, muscle, fat and breath samples, which reveal what and how often the animals are eating, along with GPS tracking collars and internal body temperature and activity monitors that the bears wear during the summers. Together the information will show how the bears on land are faring versus those that head north.
The project grew out of earlier work Ben-David, associate professor of zoology and physiology, did with the U.S. Fish and Wildlife Service and the U.S. Geological Survey. The University of Wyoming team is continuing to partner with both agencies on this project, and flies out in tandem with the federal scientists to find bears.
They track the bears from helicopters and then sedate them so the team can take samples and fit the bear with a tracking collar and insert the tiny temperature logger, roughly the size of a stack of four dimes, into its abdomen.
Taking respiration samples from animals in the field is relatively unusual, Whiteman said. Back in the lab, the researchers analyze the carbon in the bear's breath, to see if it came from the animal's own fat stores or from something it ate. Animals that are fasting start using their own body reserves for energy.
"It's a little bizarre because you're in this helicopter with two big bags of polar bear breath," said Harlow, a professor of zoology and physiology.
The blood samples give additional information about what the bears are using for fuel and the muscle samples show how active the bears are and whether they're fasting.
Unlike brown and black bears, which fatten up on food during the summer to prepare for winter hibernation, polar bears generally lose weight during the summers. They normally hunt seals from the ice, and in the summer the ice is scarcest, so they are limited in how often they can eat. And, unlike brown and black bears, only pregnant female polar bears hibernate in the winter.
However, there is a theory that polar bears reduce their body temperature and metabolism in the summer in a state called "walking hibernation." There is little data on this, and the scientists hope that their temperature and activity loggers, along with the samples they take, will shed some light on the phenomenon. A drop of just two degrees Celsius could make a big difference in helping a bear conserve energy while it's not eating, Harlow said.
If summers are the most difficult feeding season for polar bears, then the recent retreat of summer sea ice is compounding the situation. The bears, on average, face about three extra weeks each summer without a sea ice edge near enough to shore to making feeding easy.
The team went to Prudhoe Bay in early and late summer both years to find bears under a National Science Foundation grant. Last year they did the full suite of sampling and attached collars and data loggers to 13 bears. This year they did 19. In October, the researchers are going on a U.S. Coast Guard icebreaker to track the bears that followed the ice edge north.
They haven't analyzed any of the data yet, though a few interesting pieces of information have emerged.
The bears don't necessarily stick with their decision to stay on land or follow the ice north in a given season. "We had animals from the ice swim to shore and animals on shore swim to the ice, and that's something that we didn't expect," Ben-David said.
There was also one remarkable swimmer in last year's group. An adult female went on a 500-600 kilometer trek, swimming for seven days straight, resting about half a day, probably on a small ice floe, then swimming another two, Whiteman said.
Anecdotally, the group noticed a shortage of juvenile bears in the 2- to 5-year-old group when they went tracking this summer. They're not sure what this might mean, or whether it was just a blip in the data, but are curious about it.
The group believes that most of the bears head north with the ice rather than stay on shore, though anecdotal evidence points to more bears staying on land in recent summers. Harlow said the bears they have seen there appear to be healthy and he assumes they're finding whale and seal carcasses to scavenge. But there isn't enough carrion to support a very big community of bears, he said.
Polar bears, with their awkward gait and huge fat reserves that cause them to quickly overheat, aren't going to easily switch to hunting caribou on the North Slope.
"Polar bears are not graceful" on land, Harlow said. "They're not going to adapt back to looking for big game."
Read John Whiteman's blog about the project at http://icestories.exploratorium.edu/dispatches/author/john-whiteman/