The calorie intake of a Baleen whale is mind-boggling

SARAH MCCAMMON, HOST:

If you've ever lived with teenage boys, you might be under the impression that you've experienced the creatures with the biggest appetites. But they cannot, in fact, compete with whales, the largest animals ever to have lived on Earth. In this encore presentation, NPR's Nell Greenfieldboyce reports on a study that suggests scientists have long underestimated just how much these giants can eat.

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NELL GREENFIELDBOYCE, BYLINE: Certain whales - like blue, fin and humpback whales - eat huge quantities of tiny shrimp-like krill. These whales gulp big mouthfuls of water and use bristly plates in their mouths to filter out the prey.

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Matthew Savoca is a biologist at Stanford University. He recently wondered, how much can these whales gobble down?

MATTHEW SAVOCA: I remember thinking, oh, well, we must know this, you know? This is something that seems like a basic question, and these are some of the most charismatic animals really in the history of life on Earth.

GREENFIELDBOYCE: He says if you just Google, how much does a blue whale eat? - you'll get an answer. But...

SAVOCA: What I was surprised to find out is when I dug into where that answer came from, it didn't actually come from living, breathing whales in the wild.

GREENFIELDBOYCE: All he could find were guesses based on extrapolations from the caloric needs of smaller animals or the stomach contents of dead whales that had been hunted.

SAVOCA: But, of course, that's just a snapshot from, you know, a morning of feeding or something like that, depending on when the whale was caught.

GREENFIELDBOYCE: Savoca realized that it was possible to get much better estimates by monitoring tagged whales as they fed on swarms of krill. He and his colleagues used underwater devices that can measure the size and density of a swarm by sending out pulses of sound.

SAVOCA: And then the instrument listens to the returning echoes reflected back.

GREENFIELDBOYCE: His group tracked hundreds of whales as they chowed down. The results, in the journal Nature, show that on average, these whales eat way more than expected - three times more. In just one day, a blue whale can eat between 10 and 20 tons of food.

SAVOCA: That amount of food is somewhere in the range of 20 to 50 million calories, which - you know, that's a lot of calories. And that is about 70- to 80,000 Big Macs.

GREENFIELDBOYCE: It's the equivalent of what a person would eat over decades. He says given how much whale populations can eat...

SAVOCA: How are these whales going to be able to recover when it appears that, at this time, they're food-limited?

GREENFIELDBOYCE: Krill populations have dramatically declined in recent decades. Some scientists blame the loss of whales that not only ate lots of food, but also made enormous amounts of poop. Whale poop contains iron and other nutrients. It fertilizes the ocean and helps krill.

Asha de Vos is the founder of Oceanswell, a marine conservation group in Sri Lanka. She says some researchers have talked about artificially fertilizing the ocean to boost krill and give whales more food.

ASHA DE VOS: I think it's really complicated, and I do think that unless it's done well, it can actually have unintended consequences that could be negative.

GREENFIELDBOYCE: And, she says, there's plenty of other ways to help whales, like protecting them from getting hit by ships or tangled in floating nets.

Nell Greenfieldboyce, NPR News. Transcript provided by NPR, Copyright NPR.