Millions have seen the heart-wrenching video of a polar bear clinging to life, its white hair limply covering its thin, bony frame. Shot by Paul Nicklen and Cristina Mittermeier of the nonprofit group Sea Legacy, and published on National Geographic in early December, the video ignited a firestorm of debate about what scientists know, and don’t know, about the impacts of global warming on polar bears.
Without examining the bear in the video—thought to have died—it’s impossible to know for sure what ailed that individual, but now scientists have published new findings that shed more light on the risk to the species overall.
Because of melting sea ice, it is likely that more polar bears will soon starve, warns a new study that discovered the large carnivores need to eat 60 percent more than anyone had realized. Turns out they are high-energy beasts, burning through 12,325 calories a day—despite sitting around most of the time, according to a unique metabolic analysis of wild bears published Thursday in Science.
“Our study reveals polar bears’ utter dependence on seals,” said lead author Anthony Pagano, a wildlife biologist with the U.S. Geological Survey (USGS).
Polar bears rely almost exclusively on a calorie-loaded diet of seals. To minimize their energy consumption the bears still-hunt, waiting for hours by seals’ cone-shaped breathing holes in the sea ice. When a seal surfaces to breathe the bear stands on its hind legs and smacks it on the head with both of its front paws to stun it. Then the bear bites it on the neck and drags it onto the ice.
“They’re far more successful doing this than any other method of hunting,” Pagano said. That’s why the melting of the Arctic sea ice threatens polar bear survival.
DISAPPEARING ICE MAKES FOR HUNGRIER BEARSClimate change is heating up the Arctic faster than anywhere else, and sea ice is shrinking 14 percent per decade. Even today, in the middle of the bitter cold Arctic winter, satellites show there is about 770,000 square miles less sea ice than the 1981 to 2010 median (That's an area larger than Alaska and California combined).
In the late spring, the ice is breaking up sooner and forming later in the fall, forcing bears to burn huge amounts of energy walking or swimming long distances to get to any remaining ice. Or they stay on land longer, spending the summer and, increasingly, the fall fasting, living off their fat from the seals they caught in the spring.
Pagano’s study involved capturing nine female bears in the Beaufort Sea off Alaska last April, when there are normally lots of seals around. The bears were fitted with GPS collars that had cameras to record point-of-view videos of each. Blood and urine samples were also taken. Eight to 11 days later they were all re-captured. One bear had moved 155 miles away by that time. Blood and urine samples were taken again and the video and other data were downloaded.
The data showed the bears were active about 35 percent of the time and resting for the remainder, yet they burned through 12,325 calories a day, much of it from their body reserves. That’s about 60 percent more than previous studies had estimated. The videos revealed that four of the females weren’t able to catch a single seal. Measurements showed those animals lost 10 percent or more of their body mass.
One bear lost close to 44 pounds, including her lean muscle, in 10 days. This bear even leapt into the sea in a failed attempt to catch a seal swimming by. “She might have been desperate,” Pagano speculated.
“It’s a really strong study,” said Steven Amstrup, chief scientist for Polar Bears International, a conservation-focused organization, who was not involved with the work. “It shows that polar bears are more like the big cats—lions and tigers— predatory carnivores with high energy metabolisms,” Amstrup said.
As solitary hunters, the bears are more like tigers, except twice as big, some tipping scales at 1,100 pounds. And yet they are uniquely vulnerable in their almost total reliance on one prey species.
BIGGER IMPACT THAN PREVIOUSLY THOUGHT?If these results hold up, then it shows that the loss of sea ice may have a bigger impact on the bears than previously thought, said Amstrup, a former USGS polar bear expert. Amstrup’s own 2010 study projected that continued decline in sea ice would reduce the global population of bears by two thirds, to less than 10,000 by 2050.
Best estimates say there are 20,000 to 30,000 polar bears in 19 different groups or populations scattered across the top of the U.S., Canada, Greenland, Norway, and Russia. Four of these populations are considered to be declining. Bears in the Beaufort Sea region are among the best studied and their numbers have fallen 40 percent in the last ten years. Five populations are thought to be stable and there's not enough known about the others to judge. (See how scientists are trying to track polar bears across vast areas of Russia.)
Polar bears are considered endangered in the U.S. and are listed as “vulnerable” by the IUCN, because their sea ice habitat is under threat from climate change.
NOT BUILT FOR WALKINGAlthough it’s just a 10-day snapshot, the study confirms that polar bears aren’t made for walking, said Andrew Derocher, Canada’s leading polar bear expert and a professor at the University of Alberta. They’re not efficient walkers, but thanks to their high-energy diet of seals they can roam an area as large as 95,000 square miles, Derocher said in an interview.
Bears can lose weight fast but also gain it back quickly if they can catch seals. “I’ve seen a 500-kilogram [1,100-pound] male consume 100 kilograms [200 pounds] of seal in one meal,” he said.
The farther the bears have to travel to get on the ice to hunt the more weight they lose. Eventually they start losing muscle, hurting their chances of hunting success, which can lead to a downward spiral. Bears are also doing a lot more swimming as the sea ice declines, said Derocher.
Although capable of swimming long distances, polar bears burn far more energy doing so than walking, a recent study published in Polar Biology found.
“As the sea ice melts earlier and earlier, polar bears are forced to swim more and more, to reach seal populations,” said author Blaine Griffen, a biologist at BYU in a release. One female bear Griffen studied swam 426 miles over nine days. She lost 22 percent of her body weight and, worse, lost the nursing cub that had started the journey with her.
More swimming could lead to smaller bears, reduced reproduction rates, and even increased risk of death—something already being seen in western Hudson Bay and around the southern Beaufort Sea, Griffen said.
There’s no doubt that as the sea ice declines more and more bears are going to starve to death, said Amstrup. “I don’t know if that poor bear in that video was starving. I do know that the only solution for the long-term survival of the polar bear is to address climate change.”
By Stephen Leahy/ National Geographic