Scientists studying mercury levels in polar bears say that melting sea ice has forced the bears to change their diets. The researchers published their study in the journal Environmental Science & Technology.

Mercury is all around us. Some of it occurs naturally in plants, animals, and the soil. Some of it is our fault, the result of industrial pollution from coal and other fuels.

Regardless of its source, it piles up in living things the same way. Tiny animals eat plants containing mercury, and the mercury stays in their bodies even after all the plant matter is gone. Then those tiny animals are eaten by small animals, and on and on, onward up the food chain. Apex predators like swordfish and sharks are swimming globs of mercury, which is why we tell pregnant women not to eat them.

For the same reason, they shouldn’t eat polar bears, either (although we would kind of like to see them try). Previous tests on bears in the southern Beaufort Sea found alarmingly high mercury levels in the animals’ hair and body tissue—the result of the bears’ diet of mercury-saturated ringed seals. Or their former diet, we should say.

Researchers followed Beaufort Sea bears from 2004 to 2011, taking regular tissue and hair samples from sedated bears and by long-distance biopsy dart. Chemical analysis of the samples showed a clear and steady decline in mercury levels. Each year, the bears’ bodies contained 13 percent less mercury.

But it’s not as though these bears were getting 13 percent smaller. They weren’t wasting away. So what had happened?

It turns out their main prey, ringed seals, had grown scarce. Ringed seals spend most of their time on sea ice. But as the sea ice disappeared, so did they. In response, the bears shifted their predatory attention to bearded seals and bowhead whales, both of which carry less mercury. Even as their mercury levels dropped, the bears' BMI increased, perhaps because they were eating more blubber from the larger prey.

The authors note that the bears in their study might not represent all bears in that region. They could only take samples from the bears they could find along the coast. Less successful hunters might still have been struggling out at sea.

They also say that the polar bears’ prey-switching is, sadly, not a sustainable solution. We’re not exactly experiencing a surplus of whales, here.