Blue whales are the largest and heaviest animals on the planet, spanning up to 25 metres and weighing around 140 tonnes.

The gigantic ocean-dwellers boast an appetite to match and can swallow half a million calories in a single mouthful.

Now, scientists off the coast of New Zealand have captured drone footage of a particularly voracious “bluey” tucking into a hearty lunch.

In one long motion, the whale turns on to its side, pumps its flukes, opens its mouth and swallows nearly an entire mass of krill.

The whales, which feed almost exclusively on the small crustaceans, are discerning when it comes to deciding which shoal to target.

“Every time a blue whale opens its mouth, it’s like putting on the brakes, so it slows way down,” said Leigh Torres, principal investigator at Oregon State University’s Marine Mammal Institute. “These animals have to make decisions about what it’s worth opening their mouths for.”

In the next shot in the video, a blue whale turns on its side as it approaches a small krill patch, signalling it’s ready to lunge.

But, at the last moment, it changes its mind, swimming straight through the shoal, rather than gobbling it up.

“Modeling studies of blue whales ‘lunge-feeding’ theorize that they will not put energy into feeding on low-reward prey patches,” Torres said. “Our footage shows this theory in action.”

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The researcher added that the study was made possible thanks to the use of small drones, which have several advantages over conventional techniques.

“It’s hard to get good footage from a ship,” Torres said, “and planes or helicopters can be invasive because of their noise. The drone allows us to get new angles on the whales without bothering them.”

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