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Maria Spriggs, Busch Gardens

By Bob Holmes

Being a polar bear is harder work than biologists thought. As the bears roam the Arctic sea ice hunting seals, they use more energy than expected. That has ominous implications for a future in which sea ice is ever scarcer.

Anthony Pagano of the US Geological Survey in Anchorage, Alaska, and his colleagues studied 9 adult female polar bears on the Beaufort Sea ice north of Alaska. They fitted the bears with collars carrying GPS trackers, accelerometers and video cameras. They also injected the bears with water labelled with stable isotopes of hydrogen and oxygen.

When the team recaptured the bears 8 to 11 days later, changes in the ratio of isotopes in their blood gave a measure of metabolic rate. The researchers could compare this with the bears’ activity and hunting success.

Researchers already knew that polar bears are inefficient walkers, but many assumed that they also saved energy by sitting quietly waiting for seals, and by reducing their metabolic rate when fasting. However, it seems these savings are small at best. The bears’ average metabolic rate during the study period was 1.6 times more than most previous estimates.

Not so thrifty

This means the bears must kill and eat more seals over the course of a year, to pay for this higher metabolic rate.

But as climate change melts ever more sea ice, the bears are likely to have to walk farther to find prey. “That has a cost, and the animal has to find the energy to do that – or take it out of growth, reproduction, or survival,” says polar bear biologist Andrew Derocher at the University of Alberta in Edmonton, Canada.

Though Pagano’s study focuses on just one population of polar bears, many of the other populations around the Arctic likely face the same challenges, says Derocher.

Journal reference: Science, DOI: 10.1126/science.aan8677