It's strange to think that the biggest animal on earth feeds on one of the smallest, but that's why blue whales have to nosh a lot of food to survive. Just to fill their giant bellies, these unmatched behemoths must gulp an estimated 997 kilograms (2,200 lbs) of krill – and this new footage helps to explain how they choose the perfect "bite".

When you're the size of three school buses, just the act of eating is an immense energy suck. It would make sense, then, for the whales to first size up potential prey before gulping down only the meals with the greatest return on eating effort. Or so researchers have theorised.

This rare clip, filmed off the coast of New Zealand by researchers at Oregon State University's Hatfield Marine Science Center, confirms that hunch. 

"We can see the whale making choices, which is really extraordinary because aerial observations of blue whales feeding on krill are rare," says principal investigator Dr Leigh Torres. "The whale bypasses certain krill patches."

By analysing the footage, the team discovered that the act of opening its giant mouth slowed this particular blue whale from a 6.7 mph cruise to a crawl of just 1.1 mph. Once in feeding position, the whale is essentially pushing an underwater parachute against the current. In some whale species, the water taken on during this manoeuvre weighs more than the animal's own body. What's more, getting back up to speed after such oral acrobatics takes even more work.

Rorqual (filter-feeding) whales have an impressive toolkit of adaptations for such feeding feats – including inverting tongues and extra jaw joints – but lunge feeding is extremely demanding nonetheless. 

It's for this reason, explains Torres, that whales have to shop efficiently – and in bulk. They can't just stop at every corner store. "It would be like me driving a car and braking every 100 yards, then accelerating again. Whales need to be choosy about when to apply the brakes to feed on a patch of krill," she says.

Exactly how the whales size up their prey remains a mystery, but visual cues are thought to play an important role, especially here in the so-called "sunlight zone".

There's still much to learn about these true ocean giants, and the team hopes footage like this will continue to improve our understanding. 

"Blue whales in New Zealand are currently listed as a Migrant, and therefore are offered no specific conservation protection," says Torres. "These animals live in a busy, noisy environment, and we need to learn more about their ecology and biology so that we can protect their population and habitat."


Top header image: NOAA Office of National Marine Sanctuaries/Flickr