What Cameras on Polar Bears Show Us: It's Tough Out There

You wouldn’t necessarily know it by looking at a polar bear’s ample figure, but the world’s only marine bear is a fat-burning machine.

Polar bears can lose as much as four pounds of pure fat per day, according to Anthony Pagano, a wildlife research biologist with the United States Geological Survey. While dieters around the world would love to know the bears’ secret — to shave off a single pound in a day a human would have to run a marathon without eating any more than usual — their high metabolism is not necessarily a good thing, as Mr. Pagano details in a study published Thursday in the journal Science.

To maintain those stores of fat, the bears have to venture out onto Arctic sea ice to catch seals. But as global warming makes the sea ice more scarce, the bears have to search farther and wider for food, and the more they search, the more fat they burn — putting them at risk for starvation.

“Some of these bears moved up to 250 kilometers over the course of an eight- to 11-day period, so they could really cover quite a distance over a short period of time,” Mr. Pagano said, describing the study.

Despite their popular appeal, not much is known about how polar bears behave in the wild. And their wide-ranging expeditions out on the sea ice have historically made them hard to track.

“Most of their lives, if they have a choice, polar bears are out on the sea ice,” said Thea Bechshoft, a researcher at Aarhus University in Denmark who was not involved in the study.

Polar bears are mostly solitary creatures, outside of mating season, denning season — when females raise their cubs — and the town of Churchill, Manitoba, where bears congregate toward the end of summer as they await the return of winter sea ice.

“They’re spread out, so for us to go out and find them, be discreet enough for them not to notice us and for them to do their natural behaviors, is not feasible,” Dr. Bechshoft said.

A lot of what scientists do know comes from the Canadian biologist Ian Stirling, who in the 1970s and ’80s spent long stretches of time on the sea ice in the Western Hudson Bay observing bears. Dr. Stirling observed the bears from a distance to avoid being mauled.

In this study, Mr. Pagano and his colleagues affixed video cameras to GPS-equipped collars on nine female bears off Alaska on the Beaufort Sea. The researchers took helicopters to find the bears, tranquilized them to put on the collars, and took weight and blood samples. Eight to 11 days later they repeated the task to reclaim the video cameras and take more measurements.

While summer is a feasting period for other bear species, for polar bears it is a time of hunger. The sea ice has always retreated during summer, even before climate change exacerbated the trend. Spring, just before the sea ice begins its seasonal breakup, is a critical time for polar bears to gain the weight they need to carry them through until the sea ice re-forms, so the researchers tracked the bears in April of 2014, 2015 and 2016.

“Spring is the pupping season for the seals,” said Dr. Bechshoft. “They don’t move a whole lot so they’re basically laid out as one big buffet for the bear to go around.”

Of course this presumes that the bears can find the seals in the first place. Less sea ice means the bears have to work harder to find them.

The researchers hypothesize that the bears’ lazy hunting style — when it works — allows them to conserve energy, helping them survive through the summer months when food is scarce. But it’s not working, and the problem is exacerbated by their high metabolism — their base metabolic rate was 60 percent more than scientists expected.

Using the GPS and video collars and the metabolic information, the researchers determined that the more a bear had to search for food the more energy it used. But less sea ice means they have to walk or swim more in search of food. And every additional mile that a polar bear has to traverse under its own power is that much more food the bear has to eat. The only way for the bears to restore that lost energy was to catch more seals.

“We found that they were really dependent on their ability to catch seals,” Mr. Pagano said. “If they were successful they did quite well.”

Four of the bears, mostly the older ones, did gain as much as 10 percent of their body mass over the study period. Because of the video footage, the researchers know that these bears gained weight because they caught seals. But because of the small study size, it was not possible to tell if this was a trend in which age brought wisdom, or if it was just a lucky week for the older bears. One bear that managed to catch some seals lost a negligible amount of weight.

Four other bears lost up to 10 percent of their body mass over the same period, the result of not catching seals.

“The other bears either scavenged or they fasted, and they all lost mass,” as much as 20 percent of their body mass, Mr. Pagano said.

Although the study results seemingly reflect the roughly 40 percent decline in polar bear populations seen in this part of the Arctic, the study was not broad enough to draw large conclusions. But the new information on how dwindling sea ice affects the health of female polar bears could be important for conservation efforts.

“No sea ice, no polar bears, it’s really simple,” said Dr. Bechshoft.

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