Hakai Magazine

Polar Bear (Ursus maritimus) walking along shoreline
A physiological response to stress may be helping polar bears cope with shrinking Arctic sea ice—but it likely won’t help them forever. Photo by Paul Souders/Biosphoto/Minden Pictures

Researchers See Signs of Chronic Stress in Polar Bears’ Blood

Abnormally long fasts linked to melting sea ice may be pushing polar bears to their limits.

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by Katarina Zimmer

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Since the 1980s, scientists have routinely helicoptered to the western edge of Manitoba’s Hudson Bay to find, immobilize, measure, and collect blood from polar bears to keep an eye on how they’re doing. What scientists are finding is worrying; climate change has driven sea ice to thaw sooner in the spring and freeze later in the fall, reducing the period when polar bears can hunt for seals—forcing them to fast far longer than normal.

Rudy Boonstra, an ecophysiologist at the University of Toronto Scarborough in Ontario, wanted to find out if that large collection of blood could tell a deeper story about how the bears have been coping with the physiological stress of fasting.

Using blood collected from 300 bears between 1983 and 2015, he and his colleagues began by measuring cortisol. In polar bears, as in humans, the hormone surges under stressful conditions, triggering the body to release its stores of carbohydrates and fat before moving on to protein in the muscles. This pumps the bear with enough energy to combat whatever is stressing it out. While this acute stress response is useful, chronically high levels of cortisol can be destructive, since they force the body to rapidly burn through precious energy reserves while suppressing other critical activities, such as growth, immune response, and reproduction.

The team found that between years prior to 1990, when sea ice conditions were suitable for hunting, and subsequent low-ice years, polar bears’ cortisol levels didn’t change much. Boonstra says that’s not surprising: cortisol levels are sensitive to immediate threats—getting chased by a helicopter would be enough to elevate them.

But when they examined a more robust measure of chronic stress—cortisosteroid binding globulin (CBG), a protein that regulates cortisol by binding to it—they found the bears’ CBG levels were significantly higher in years with little sea ice. Boonstra found this astonishing. In virtually all other mammal species studied so far, including humans and snowshoe hares, CBG declines under chronic stress.

Boonstra thinks the bears’ unusual response to chronic stress could have arisen as an evolutionary adaptation to historical climate swings. By mopping up excess cortisol, the high CBG levels could be protecting polar bears from its harmful effects, and perhaps staving off the point at which the bears would have to digest their own muscle to survive. “They are the most amazing bears,” says Boonstra.

But there may already be signs that this potential source of resilience is maxing out, says Kimberley Bennett, a physiologist at Scotland’s Abertay University who wasn’t involved in the research. To her, the team’s data suggests that some bears that have already endured abnormally long fasts are struggling to ramp up their CBG levels. She says this could further hinder the bears’ ability to handle short-term sources of stress, like tourist visits or mining activity.

As for the bears’ long-term fate, study coauthor Péter Molnár, a biologist at the University of Toronto Scarborough, recently projected the survival of 13 different polar bear subpopulations and found that even when using optimistic estimates for their metabolic rates, few high-Arctic populations would survive past 2100. Molnár says he doubts that the bears’ CBG adaptation is powerful enough to prevent that outcome.

Though polar bears may have evolved a secret weapon against climatic changes of the past, it probably won’t be enough to help them survive what’s hitting them now.