Polar bear diets should be filled with a lot of fat. So much fat that some researchers study them just to understand how a species that consumes a diet with up to 80% blubber avoids heart disease and obesity. 

However, a new study shows that as the Arctic warms faster than any other part of the world, polar bears can't feast on blubber from multiple seals as they’re used to doing. Instead, they're finding fewer seals and eating more muscle, increasing their consumption of protein relative to fat. Blubber is highly digestible and provides twice the energy per gram compared to muscle.   

"Polar bears have evolved to specialize on a high fat diet composed primarily of seal blubber,” Karyn Rode, the lead author and a USGS research wildlife biologist, said. Her team’s new study shows that when polar bears ate less fat from fewer ringed seals, they were less likely to survive the year.  

“Understanding how polar bears respond to periods of reduced access to their primary prey, and consequently reduced dietary fat, helps us better understand the implications of Arctic warming on these predators while simultaneously shedding light on the status of their prey populations,” Rode said.  

Satellite images show that the Arctic is losing sea ice, which polar bears use for roaming and hunting. This new study reveals how the melting sea ice is also affecting the seals, whales, and other creatures that are part of the ecosystem beneath the ice.  

“Our study is the culmination of 15 years of work to develop methods to measure the protein and fat content of polar bear diets using hair,” Rode said. Her team tested those methods on polar bears in zoos, applied the tools to wild populations, and analyzed the results. The researchers measured stable carbon and nitrogen isotopes in polar bear hair to determine the ratio of fat to protein consumed by polar bear populations in the Northern and Southern Beaufort Seas, which sit above Alaska and the Northwest Territories and west of Canada’s Arctic islands.  

Scientists and land managers can use the study’s methods to differentiate between indirect and direct effects of sea ice loss on polar bears. Indirect effects impact the bears' potential prey that reside underneath the ice, while direct effects prevent polar bears from roaming large areas of sea ice in search of food.  

When direct and indirect effects occur simultaneously, as is the case with the populations the team studied, the polar bears are particularly vulnerable to environmental change. 

The research highlights how studying polar bears can reveal what’s happening to the ecosystem hidden beneath the remote Arctic sea ice. "Polar bears are a good indicator species on how the Arctic is affected by climate change and sea ice loss,” Rode said.  

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The study was published in the journal, Ecological Applications.