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September 29, 2022

Researchers studied chemical signatures in polar bear hair to determine that the apex predators are changing their diet in response to prey (mostly seal) populations beneath the sea ice. 

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.  

Graphical abstract showing role of diet and food intake affecting polar bear population dynamics in southern Beaufort Sea.
This is a graphical abstract for a publication by the USGS and collaborators that examines the role of diet and food intake affecting polar bear population dynamics. Polar bears consume diets consisting of high proportions of marine mammal blubber that they access from the sea ice. When prey availability is low, polar bears are less selective consuming both the muscle and blubber of their prey whereas when prey are abundant, polar bears can selectively consume primarily blubber. This study used carbon and nitrogen isotopes to estimate the prey species and ratio of fat to protein in polar bears diets in the Northern Beaufort and Southern Beaufort Sea populations to better understand if bears can maintain high fat, high energy diets by prey switching when the abundance of their primary prey, ringed seals, is low. They found that bears in the western Beaufort Sea (Alaska) consumed the lowest proportions of dietary fat and ringed seal blubber during a period when polar bear abundance declined. Polar bears in the Northern Beaufort Sea population which has been stable over recent decades consumed higher proportions of ringed seals and dietary fat. Bears consuming less fat and lower energy densities would have to increase food intake from 2.1 to 3.0 kg/day in order to maintain energy intake. Prey-switching and consumption of whale carcasses onshore appeared insufficient to augment diets when availability of their primary prey, ringed seals, is reduced. Estimating dietary blubber using predator hair applied in the paper provides a new metric to monitor predator-prey relationships that affect individual health and population demographics.  More information can be found in the following publication: 
Rode, K.D., B.D. Taras, C.A. Stricker, T.C. Atwood, N.P. Boucher, G.M. Durner, A.E. Derocher, E.S. Richardson, S.G. Cherry, L. Quakenbush, L. Horstmann, and J.F. Bromaghin. 2022. Diet energy density estimated from isotopes in predator hair associated with survival, habitat, and population dynamics.  Ecological Applications: Graphic by: Andres A. Aceves for USGS through the Virtual Student Federal Service program. 

The study was published in the journal, Ecological Applications

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