The polar bear has long been a poster child for global warming, but a laboratory at the University of Massachusetts is generating new evidence of the effects of rising temperatures on this threatened species.
Jerrold Meyer, a behavioral neuroendocrinologist at UMass Amherst, has found that levels of the stress hormone cortisol in polar bears of East Greenland are tightly linked to changes in temperature and ice cover. Meyer is careful to point out that his research can’t prove that global warming is causing increased stress in the bears. Still, he says the fact polar bears are showing physiological signs of stress in relation to climate change “raises real concerns.”
“I don’t think there’s any one study, research paper, or anything that’s going to galvanize public opinion.”
For their study, published in the journal Polar Biology, Meyer and a team of international colleagues set out to measure changes in cortisol concentrations over time. Thanks to an agreement with native hunters, who are allowed to hunt a certain number of polar bears each year, the lab analyzed hair samples collected between 1988 and 2009.
As you might expect, the lab found different levels of cortisol across the samples. But they later learned that a few samples in the mix dated a few years earlier than 1988. Meyer and his colleagues were shocked that those samples were more than a century old. They came from polar bears killed and preserved as museum specimens since the late 1800s, and they turned out to have the highest concentrations of cortisol.
Why would those nineteenth century polar bears be more stressed than today’s bears, which have been dealing with a rapidly changing environment? Meyer says that although mammals pump out a lot of cortisol in response to short-term elevations in stressors, hormonal activity actually decreases over time in the presence of chronic stress. So animals under constant stress might actually secrete less cortisol than animals under temporary stress. The relatively lower stress levels in today’s bears may represent an adaptive response to a changing environment.
For years, the stress hormone has been measured in the blood, saliva, urine, and feces of humans and animals. Cortisol levels in these body fluids provide a decent measure of an organism’s current stress level, but levels can vary wildly throughout the day and in response to immediate changes in the environment. Scientists needed a way to measure average cortisol levels over weeks and months, to get a better picture of chronic stress over time. The answer was in the hair.
Hair grows at a rate of about one centimeter per month. Throughout the month, blood delivers the nutrients needed to grow the hair, and along with the nutrients comes traces of that circulating cortisol. Imagine pulling out a piece of your own hair. As you move down the strand, away from your scalp, you are essentially moving back in time. Scientists can divide hair samples into segments, give each section a time mark, and measure the amount of cortisol present. Plot those levels across the preceding weeks and months, and you have a picture of an individual’s chronic stress over time.
Meyer says stress is something humans intuitively understand. It’s a word and a concept we use in our daily language, and usually carries a bad connotation. “Just saying that there’s new information suggesting polar bears may be under increased stress and it seems to be related to climate change could help raise public awareness.”
Yet when it comes to climate change, there may actually be a surplus of awareness. Andrew Revkin, a seasoned environmental reporter who writes the popular Dot Earth blog at the New York Times, recently gave a TED Talk about the “climate in our heads,” which makes the Earth’s climate change such a difficult problem to solve. In the talk he references the work of Dan Kahan, a professor of both psychology and law at Yale.
“He does empirical studies that show that more information doesn’t matter quite often. More information, more science, more education actually further divides people on issues like global warming,” Revkin said.
It’s not that Americans are unconcerned about the environment. Rather, it seems that, compared to other environmental issues, global warming falls to the bottom of the list. In the most recent Gallup poll on the issue, 34 percent of people surveyed said they worried “a great deal” about global warming, identical to how Americans felt about the issue in 1989. In contrast, 60 percent said they were worried about polluted drinking water.
Though a major finding, Meyer says the connection between polar bear stress and climate change alone won’t be enough to instigate change. “I don’t think there’s any one study, research paper, or anything that’s going to galvanize public opinion. It adds a piece to the story. I think it’s an important piece,” he said.
Indeed, it adds to the unfolding story of how polar bears are coping with a rapidly warming planet. In 2012, researchers at Penn State and the University at Buffalo found that the polar bear populations have drastically changed in size for millions of years, expanding during cooler periods and shrinking as temperatures rise.