Polar bears have been a face of climate change for years. We’ve long understood that the melting sea ice is drastically reducing their means of foraging for seals, their main food source. We even know that pollution puts their penile bone density at risk.
But now, scientists have identified another threat to polar bears: Summer ice melt.
Researchers from the University of Wyoming found (paywall) that polar bears aren’t able to adapt to low food supplies by reducing their metabolisms during the summer, as they do in winter.
“To save energy and heating in your home, you turn down the thermostat,” Merav Ben-David, a zoologist at the University of Wyoming and co-author of the study, told Quartz. “It’s exactly the same concept, except physiological mechanisms.”
Polar bears reduce their metabolic rate in winter as part of the hibernation process and, said Ben-David, scientists had assumed that they could dial it back in the summer, too, if needed. To test this theory, researchers tracked the internal temperatures and movements of 25 polar bears over two years near the Beaufort Sea, north of Alaska and Canada. Usually when bears lower their metabolic rates, their body temperatures drop from between 39 – 38 degrees Celsius (about 100 – 102 degrees Fahrenheit) to 35 degrees Celsius (about 95 degrees Fahrenheit) and remain there for months. With a lower internal temperature, bears need less energy, and therefore food, to stay alive.
Researchers had expected to see that as polar bears went for increasingly longer periods of time without food over the summer, they would lower their body temperatures. However, the bears never did. This means that if they can’t access seal meals via sea ice over the summer, polar bear populations will decrease.
“There is a limit to how long they can go without food,” Ben-David told Quartz. “The only way we can stop [polar bears’s decline] is to reverse sea ice loss.”