Scientists pilot a new method to measure the health of wild polar bears
Climate change threatens the health of polar bears across the Arctic. A study published in Conservation Physiology on March 5, introduces a new approach to measuring the health of polar bear populations, drawing inspiration from a well-known concept in human medicine: allostatic load.
Allostatic load refers to the “wear and tear” on the body that results from chronic stress. In humans, high allostatic load increases the risk for disease and death. A team of scientists used allostatic load principles to create a health scoring model for polar bears in Alaska and Canada’s Southern Beaufort Sea, where the population has declined by 25-50%. The model included measures of nutritional, immune, and chronic stress—factors that are all highly relevant, given the threats facing polar bears. This project was carried out by biologists from the University of Washington School of Aquatic and Fishery Sciences (UW SAFS), United States Geological Survey (USGS), and Fish and Wildlife Health Consulting.
Polar bears depend on sea ice as a hunting platform, and ice loss can lead to poorer nutritional condition. In summer and fall, when the ice retreats northward, bears in the Southern Beaufort Sea must choose between following the ice into less productive hunting areas or moving onto land until the ice refreezes. Increasingly, bears are coming ashore and scavenging human-provisioned foods—an option that may expose them to new pathogens and increase disease risk. Additionally, coming ashore heightens bears’ exposure to human disturbance in areas of expanding oil and gas development, potentially adding further stress to an already vulnerable population.
“Trying to survive with so many stressors is like carrying a backpack that keeps getting heavier. Eventually, it becomes too much to bear,” said Sarah Teman, lead author of the study and UW PhD student working with UW Professor, Kristin Laidre. “By studying allostatic load, we can understand how much ‘weight’ each bear is carrying, and how that affects populations.”
The scientists measured allostatic load through a suite of samples collected from polar bears during annual population health assessments. This involved analyzing blood samples to assess metabolism, fluid balance, organ function, and immune cells, along with hair samples to measure cortisol, a stress hormone.
One finding stood out to the team: adult females that summer on land have higher allostatic load than those that remain on the sea ice. This may be driven by onshore food sources that fail to meet their nutritional needs, coupled with immune stress. The number of adult females summering on land has tripled over the last few decades, as the length of the sea ice melt season has increased.
Other findings contradicted the scientists’ predictions. For instance, they found no overall trend of increasing allostatic load across the population. However, at the individual level, allostatic load fluctuated—rising in some bears over time, while decreasing in others. This suggests that allostatic load may be best understood through individual monitoring. The next step is to link measures of individual health to population vital rates, such as reproductive success.
“Now more than ever, it’s important to develop tools to measure polar bear health,” Teman said. “This gives us insight into the stressors they face, and how we can support their survival in a changing Arctic.”
For more information, contact Sarah Teman at steman@uw.edu