Feast or Fast

It’s easy to gain weight on a road trip—restaurant meals and junk food add up. But that’s not the case for some humpback whales. According to a recent study, a group of humpbacks lost an average of 24,000 pounds per adult during its annual migration—the equivalent of a city bus.

The whales feed around Antarctica. They filter the tiny organisms known as krill from the water. So by autumn, the whales are nice and fat. The 103 adults in the study averaged about 33 tons apiece.

When autumn arrives, the humpbacks head into warmer waters. They cruise along the Pacific coast of South America, and settle in their summer breeding grounds, near Colombia—a trip of several thousand miles. But they don’t feed along the way, so they lose weight.

Researchers used drones to shoot pictures and video of the whales at both ends of their journey. They measured the sizes of the whales, and found them to be a lot thinner from late winter through spring. From that, they calculated the change in body mass. The whales lost most of their blubber—more than a third of their total body weight.

The scientists also calculated what it would take to pack on that much weight: 125,000 pounds of krill for every whale. And that’s a problem. Our changing climate has reduced the supply of krill. It’s also changed where the krill are found. So, in the years ahead, it might be harder for the whales to fuel up before they hit the road.