Polar bears rely on sea ice to get their primary meal: seals. But as climate change melts this ice, the bears have to work harder to find prey, and that’s taking a toll on their health, researchers report today in Science. To find out how much the animals need to eat, scientists captured nine female polar bears living in the Beaufort Sea each spring for 3 years and took blood samples and other measurements to gauge the bears’ metabolisms. Then they attached GPS collars affixed with video cameras and accelerometers to watch the bears forage for food. Polar bears living on sea ice need to eat more than 12,000 calories per day, the team found, which translates to at least one adult ringed seal—or the equivalent of nearly 220 Big Macs—every 10 to 12 days. That means the animals’ have higher metabolisms than previously thought. They also use a lot of energy when they walk, more than similarly sized animals. The researchers tracked the bears for about 10 days and found they walked for more than a quarter of that time. Bears that killed and ate ringed seals during the study gained or maintained weight, but more than half the bears lost weight and four lost at least 10% of their body weight. As sea ice continues to collapse, energy demands may outstrip the bears’ ability to find food and ultimately lead to their demise, the researchers say.