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Polar bears need lots of seal snacks — and a melting Arctic makes it hard to eat enough

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In the spring, one polar bear needs to snarf at least one adult ringed seal, or 19 newborn pups, every 10 to 12 days

Video: A. Pagano et al., Science (2018)

In the spring, during their prime hunting season, polar bears need an insanely large amount of food to stay healthy — over 12,000 kilocalories a day, or roughly six times what humans need, according to new research. That’s a lot more than what scientists previously thought these predators needed to stay healthy. And that means that, as Arctic ice keeps melting because of rising temperatures, polar bears may be in more trouble than we had anticipated.

In April of 2014, 2015, and 2016, researchers tracked nine female polar bears living on the sea ice of the Beaufort Sea, off the northern coast of Alaska, analyzing their blood and urine. The polar bears were also given GPS collars that recorded their movements and took videos of what the bears were doing. All this data showed that polar bears have a higher metabolism than we thought — requiring a meal of at least one adult ringed seal, or 19 newborn pups, every 10 to 12 days, according to a study published today in the journal Science. If the bears don’t catch this much food, they start to lose weight quickly: four of the bears in the study shed about 44 pounds in just 10 days, losing about 10 percent of their body mass.

Polar bear wearing a GPS video-camera collar on the sea ice of the Beaufort Sea
Photo by Maria Spriggs, Busch Gardens

Polar bears depend on sea ice to catch seals, but as temperatures increase, the sea ice in the Arctic is declining. In 2007, scientists estimated that if we keep pumping heat-trapping greenhouse gases into the atmosphere, allowing the ice to keep melting, we could lose two-thirds of polar bears by the 2050s, and all of them by the end of the century. By showing just how reliant polar bears are on their high-fat diet of seals to stay healthy, today’s study paints an even bleaker pictures for the Arctic’s top predator.

“Polar bears may start to suffer from declining sea ice more rapidly than they would have, had their metabolism been lower as was previously mentioned,” says Steven Amstrup, chief scientist at Polar Bears International, who was not involved in the research.

Polar bear on the sea ice.
Photo by Brian Battaile, USGS

Polar bears are particularly susceptible to changes in sea ice because of their hunting strategies: the bears eat as much as they can in the spring and early summer, when seals are having their pups, putting on lots of fat that will help them get through pretty much the rest of the year. But if sea ice is missing or is thinner, drifting longer distances, polar bears can’t catch as many seals, using more energy than they take in. That affects their health and their ability to make new pups: the number of polar bears in the Beaufort Sea has already declined by 40 percent from 2004 to 2007, likely because of changes in sea ice.

To get a better understanding of what exactly polar bears are doing on the ice, and how much they’re eating, researchers tracked nine female polar bears in the wild. The bears were darted from a helicopter, so that the scientists could take blood and urine samples, and attach GPS collars equipped with cameras around their necks. (The researchers picked females only because the males’ necks are wider than their heads, causing the collars to just slide off.) The bears were also injected with a harmless solution of heavy hydrogen and oxygen that allows scientists to calculate how much energy the bears are using during a certain amount of time. After eight to 11 days, the same bears were darted again, to get more samples and see how the values had changed.

The researchers found that polar bears spend about 60 percent more energy than we thought, needing to eat more high-fat seals than scientists had believed. In fact, the bears were found to need over 12,000 kilocalories a day to break even, and more than that to put on body fat required to stay healthy throughout the year. (How much energy the bears use in the winter is not really known, says John Patrick Whiteman, a biologist at the University of New Mexico.) Previous estimates had put that number at 7,700 kilocalories per day, says study co-author Anthony Pagano, a research wildlife biologist at the US Geological Survey’s Alaska Science Center. Polar bears live in freezing, remote regions, so they’re understudied in the wild — and that’s why the estimates were so off.

The videos also showed how polar bears hunt: 90 percent of the time, the bears just sat and waited for seals to come up breathing holes in the ice, while the remaining time they stalked the seals and attacked them by jumping on them. When the polar bears couldn’t catch enough seals, they lost a lot of their body mass. In about ten days, four of the bears in the study lost about 44 pounds. One bear didn’t only lose her fat preserves, but also part of her muscles. “We were certainly surprised to see such dramatic changes in body mass over a short period of time,” Pagano says. That shows just how much polar bears rely on seals to survive: if they don’t catch any, they’re going to suffer quickly. “Polar bears are seal-eating machines,” Whiteman says. And that doesn’t bode well for the animals.

A polar bear catches a seal.
Video: A. Pagano et al., Science (2018)

“Polar bears are walking this fine line between having enough energy and not, because they spend so little of their time feeding in a given year,” says Blaine Griffen, a marine biologist and ecologist at Brigham Young University, who was not involved in the research. “This paper shows that, yeah, actually they’re spending energy faster than we thought, that means that line that they’re walking is much thinner than we thought.”

The findings are based on only nine female polar bears, but that’s actually a pretty high number for such a ferocious predator living in remote areas, Whiteman says. Polar bears that are taking care of their young and lactating might also require even more energy than the female bears in the study. But overall, the results can be applied to polar bears living in other areas, Whiteman says.

Next, the researchers want to keep studying the bears to get a better understanding of how exactly declining sea ice is impacting them. If we don’t do anything to address climate change, the polar bears will disappear — and as such, they’re a great harbinger of just how much we’re changing the world, says Amstrup. “Polar bears are sending us a message about what we’re doing to the rest of the world,” he says. “And we need to take heed of that.”

Correction February 1st 5:55PM ET: This story incorrectly stated that polar bears need 6,000 times as many calories as humans. It’s six times as many. We regret the error.