The biggest animals to have ever lived on Earth gobble up much more food than scientists thought, according to a new study of filter-feeding whales that reveals just how important their eating habits could be for recycling nutrients in the ocean.
Baleen whales such as blue, fin, minke, and humpback whales consume, on average, around three times more each year than previous estimates suggested, researchers report in Nature. A blue whale in the eastern North Pacific, for example, might eat between 10 and 20 tons of food a day.
"That amount of food is somewhere in the range of 20 to 50 million calories," says Matthew Savoca, a researcher at Stanford University and the lead author of the new study. "That is about 70 to 80 thousand Big Macs. Probably decades of our eating is one day for them. So it's pretty remarkable."
Savoca first got interested in how much whales eat a few years ago, because he wanted to know how much pollution they might ingest along with their food. To his surprise, he says, the only numbers he could find on whales' prey consumption "didn't actually come from living, breathing whales in the wild."
Instead, researchers had made guesses based on extrapolations from the caloric needs of smaller animals. Or, they'd simply inspected the stomach contents of whales that had been hunted, relying on a snapshot in time that might not fully reflect how much a whale actually takes in over a day or a year.
A new way to count calories
Savoca realized that researchers could get more accurate estimates by using an underwater device that can measure the size and density of swarms of shrimp-like krill—the mainstay of these whales' diet. This kind of device sends out out pulses of sound that bounce off the swarms and return.
He and his colleagues gathered data on over 300 tagged whales as the huge animals fed in krill swarms by gulping in water to filter out the krill. The size of each whale determined how big of a mouthful of krill-filled water it could get at one go, and the researchers tracked the whales' movements to see how often they went for another gulp.