A rare glimpse inside the everyday lives of polar bears may give scientists clues about their decline.
With bear-cams and activity trackers around the necks of nine polar bears, researchers monitored more than a week of their daily activity during peak hunting season in the Beaufort Sea above Alaska.
Five of the nine bears lost weight over that time, burning far more calories during their hunt than they were catching.
The study, published in the journal Science, raises questions about the top predators' future. As climate change melts their prime hunting grounds, scientists expect polar bears will have a harder time finding prey.
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Seal, a meal
The bears spend much of their time on sea ice, where it's easiest to catch their favorite meals: fatty, energy-rich seals.
But sea ice is shrinking by 14 percent per decade as the planet warms. In addition, the ice is breaking up earlier in the spring and freezing later in the fall.
Polar bear numbers are declining. They are not endangered, but they are vulnerable, as defined by the International Union for Conservation of Nature. That means their population is expected to drop by 30 percent or more within three polar bear generations.
But it's not clear if that's because the bears are not catching enough seals, or if they are having to work harder and travel farther to find them.
Biologist Anthony Pagano with the U.S. Geological Survey and colleagues went out on the ice-covered Beaufort Sea and captured nine polar bears with tranquilizer darts.
They weighed the bears and took blood samples to measure their metabolism. And they fitted each bear with a collar equipped with a camera, GPS and motion tracker.
They recaptured each bear about 10 days later, took measurements and retrieved the cameras.
Watching the video "was really quite fascinating," Pagano said. Polar bears spend most of their lives in very remote areas, so researchers don't often get a chance to see their daily lives.
Pagano's team did. They watched bears hunt. They watched them court and mate.
But mostly, they watched them sit.
"These bears are active about 35 percent of the time. Which means they're resting about 65 percent of the time," Pagano said. "Which means if you're watching video of bears and trying to document their behaviors, you're spending a good portion of your time watching them rest."
One of the most surprising findings was how much the bears' weight changed in the short time scientists watched them.
Five of the bears didn't catch anything while they were monitored. Four lost about 10 percent of their body weight.
For a 180-kilogram bear, Pagano said, "you're talking about 18 to 20 kilograms that they're losing over the course of 10 days. Which is a pretty remarkable amount of mass to lose over such a short amount of time."
They studied the bears in March and April, before the sea ice began to break up. "This is really the beginning of the time when they're supposed to be putting on a lot of body mass to sustain them for most of the rest of the year," he added.
The scientists also found that the bears' metabolism was more than one-and-a-half times as high as previous estimates.
"Polar bears need to be catching a lot of seals," Pagano explained. As sea ice continues to decline and break up earlier, he added, it's likely going to get harder for them.
Pagano noted that polar bears in other parts of the Arctic spend more time on land than do the bears he studied on the Beaufort Sea. If the sea ice melts completely in the summer, as it is expected to, many of them will likely move onto land.
But how well the land can sustain them is "a big unknown," Pagano added.