Polar Bears Are Fighting For Survival as Melting Arctic Ice Cuts Off Their Only Food Source

Polar bears use 1.6 times the energy previously thought, according to a study published in Science this week. Patrick Kelley, U.S. Coast Guard

Polar bears are shedding weight—fast. A new study for the first time calculated the amount of calories that polar bears require, and it doesn't look good for their survival. Their key source of food—various species of seals—are full of the blubber they need to satisfy their demands. But as Arctic sea ice melts, polar bears can't get to seals as readily. And there aren't any other food sources that could save them as a whole.

The study, published Thursday in Science, is the first in-the-field observation measuring the energy demand of the polar bears. The researchers focused on field metabolic rate, which indicates how much energy the animals are using in the wild. To discover this rate, the team of scientists studied nine female polar bears during the month of April from 2014 to 2016 in the Beaufort Sea, when hunting time was optimal for polar bears. They put GPS-tracking collars equipped with video cameras on the bears and used a method called "doubly labeled water" to track their metabolic rate and thereby how much energy their were using.

The researchers found that the field metabolic rate was 1.6 times higher than what was previously assumed based on what scientists knew about polar bear behavior, such as their "sit and wait" hunting strategy. The bears burn over 12,000 calories a day.

Field movements of nine female polar bears for a new study published this week in Science. Anthony M. Pagano/Science

Nine bears in the study even lost 10 percent of their body mass in under 11 days—that's just over four pounds per day. That's more than double the weight lost when polar bears are fasting on land, rather than sea ice, during the winter. One bear started to lose lean muscle too, not just fat reserves.

For the researchers, the finding drives home just how crucial it is for the polar bears to have ready access to food—more specifically, seals. "It really emphasized to us the reliance that these bears have on being able to catch seals," Anthony Pagano, lead author and research wildlife biologist at the U.S. Geological Survey, told Newsweek. "They are highly reliant on these energetically dense seal prey." Seals, which have a lot of fat and are high in calories, are the primary way polar bears get the calories they need to survive.

An adult female polar bear on the sea ice wearing a GPS satellite video camera collar. Anthony M. Pagano, U.S. Geological Survey

Seals have a lot of high-calorie blubber that satisfies the high energy demands of polar bears. That food source is not easily replaced. Pagano said that even scavenging for leftovers, which often consists mostly of seal muscle, does not contribute enough calories to fuel a polar bear. If polar bears are cut off from seals, they may not survive.

And that appears to be exactly what is happening. Melting sea ice in the wake of climate change is exacerbating the already naturally high energy demands of polar bears. As Arctic sea ice retreats more and more, seals become harder to hunt. The polar bears have to travel further to catch seals, which uses more energy.

"As sea ice has become thinner, it is more easily pushed around by ocean currents and by wind, so sea ice is actually drifting faster through time," John Whiteman, biology professor at the University of New Mexico, told Newsweek. Whiteman was not involved with the study but published a commentary paper in Science.

The drifting ice makes the base on which polar bears live and hunt moves like a "conveyor belt." As a result, the bears have to move quicker or stay on the sea ice for longer periods of time to hunt for seals. The latest paper led by the USGS confirmed that the quicker the polar bears have to move around, the more energy they are using each day.

An adult female polar bear and her two cubs traveling across the sea ice of the Arctic Ocean. In a recent study, nine female polar bears were tracked to study their energy use and behaviors. Mike Lockhart, U.S. Geological Survey

Other sources of food likely won't cut it for the polar bears survival. News often arises of polar bears finding other food sources, such as seabird colonies, washed up whale carcasses or polar bears hunting for beluga whales from above the sea ice. These alternatives have been documented in some places for some populations of polar bears. But none of these replace the high-calorie meal that seals provide. "I really want to emphasize that as a species, there isn't an alternative," Whiteman said. "They're a seal-eating species. It's what they do."

Without sea ice, says Whiteman, "they won't persist as a species."