Polar bears are on thin ice as a changing climate makes greater demands on their bodies.Credit: Ole Jorgen Liodden/NPL

Polar bears have become the poster child for climate change, but their situation might be worse than researchers thought, according to a recent study. These Arctic mammals need a lot more calories than scientists previously estimated — but with sea ice melting under their feet, the bears often struggle to get enough to eat.

The study's results, published on 1 February in Science1, have captured the best picture yet of how much energy it takes to be a polar bear (Ursus maritimus) — and ecologists are looking to incorporate the findings into their own work on the Arctic. The study also supports scientists’ concerns that receding sea ice harms the bears by hindering their hunts of fat-rich seals.

If polar bears can’t meet their energy demands, their already declining populations could shrink by more than 30% over the next four decades, says Andrew Derocher, an ecologist at the University of Alberta in Edmonton, Canada. “This study is going to become the new hallmark for understanding polar bears’ ecology,” Derocher says.

The latest study stands out because it measures the actual calories that these animals need and explains how sea ice loss may cause them to waste away, says ecologist Ian Stirling at the University of Alberta, who studies polar bear energy requirements. Previous work only estimated how much energy polar bears needed to survive2.

Enough to eat

To measure the energy needs of wild polar bears, a team of biologists tracked free-ranging bears on sea ice near the northern coast of Alaska during spring, the animals’ prime hunting season. In April 2014, 2015 and 2016, researchers outfitted nine female bears with global-positioning-system-equipped video collars and activity trackers. The team also measured the energy cost of activities such as walking and hunting, and monitored changes in the animals' body mass over 8 to 11 days, says Anthony Pagano, a biologist at the US Geological Survey’s Alaska Science Center in Anchorage who led the research.

On average, the bears needed nearly 12,325 kilocalories per day — 1.6 times more energy than previously thought. To meet such energy demands, a female bear on the spring sea ice should eat either one adult or 19 newborn ringed seals every 10 to 12 days, the scientists concluded.

But nearly half of the bears didn’t catch enough food — and were forced to fast or scavenge carcasses. These animals lost 10% of their body mass over about 10 days. “That’s dramatic,” says physiologist John Whiteman at the University of New Mexico in Albuquerque. It’s as if a person weighing 80 kilograms shed 8 kilograms in just over a week, he says.

Fast-walking for food

Catching enough to eat isn’t the only challenge polar bears face. As rising temperatures thin the sea ice, wind and currents make it drift faster on the ocean surface. “Think about a treadmill,” says Merav Ben-David, a wildlife ecologist at the University of Wyoming in Laramie. If the sea ice moves faster under their paws, polar bears have to walk faster — or for longer — to remain in the same spot3, which forces them to expend more energy, she says.

Data collected from the activity trackers confirmed that polar bears needed more calories the further they walked. “If I had to walk an extra thousand kilometres this year I would be a lot skinnier — or my energy intake would have to go up to compensate,” Derocher says.

This study is just a snapshot, Pagano cautions. He plans to track the polar bears’ activities over the entire year to work out whether and how their energy needs change.