How whales open their huge mouths
Researchers have discovered how very large whales co-ordinate their jaw muscles and bones to take gigantic mouthfuls of prey.
For a blue whale, the largest animal that has ever existed, each mouthful can scoop up 100 tonnes of krill-filled water in less than 10 seconds.
Scientists have now found a sensory organ in whales' jaws which they say links bones and muscles to the brain, making the vast lunging gulps possible.
They report their findings in Nature.
A 500,000-Calorie lunge
When a rorqual whale, such as a blue whale, senses that there is sufficient prey suspended in the water, it dives. Then at some point in that dive it opens its mouth, rotates its body and accelerates in order to force krill-laden water into its mouth
A blue whale has separate right and left lower jawbones, allowing it to expand its gape to approximately 3m in width. Pleats of skin and blubber below the mouth, extending to the belly, form a stretchy cavern to accommodate the vast volume of water
Plates of a comb-like structure called baleen (pictured), which hang from a rorqual whale's upper jaw, trap up to 500kg of small marine creatures - that is approximately 500,000 Calories in a single mouthful
Writing in the journal, lead researcher Nicholas Pyenson, from the Smithsonian Institution in Washington DC, explained that the rorqual whales he studied had "one of the most extreme feeding methods in aquatic vertebrates".
This whale-specific mouth organ seems to facilitate that.
Dr Pyenson said the structure looked like a "gelatinous mess", which could be the reason it was previously overlooked and assumed simply to be a fluid-filled joint between the two lower jaw bones.
By dissecting whale carcasses in fine detail, the researchers found that the structure was actually far more complex.
Found at the front tip of the lower jaws, the structure is laden with nerve endings. The team says that these are sensors which pick up signals from the jaw as it starts to open. Nerves from the organ then send signals to the brain, triggering the whales' dramatic and complex feeding lunge.
The study, carried out with colleagues from the University of British Columbia, was possible because the team had access to carcasses landed at a whaling station in Iceland.
"We were able to work with tissue samples that were freshly dead," explained Dr Pyenson. "It was a unique opportunity to look at these animals' anatomy in detail.
"And that's what we'd been missing."
Dr Pyenson and his colleagues examined the jaws of fin and minke whales, dissecting them and using high resolution medical imaging to examine the carcasses.
Finding this structure, the researcher said, "answered a lot of outstanding questions".
A rorqual whale's feeding lunge was "one of the largest biomechanical events on Earth", said Dr Pyenson.
"This shows us how they do it so quickly, co-ordinating the inflation of the throat pouch with the opening of the jaws... and closing their mouth to prevent prey escaping - all in under 10 seconds."
Dr Bill Sellers, a zoologist from the University of Manchester, said that this was an "amazing discovery".
"They've found an organ we didn't know was there, which is remarkable considering people have been chopping up whales for hundreds of years."
Dr Gareth Fraser from the University of Sheffield added that the discovery revealed a unique adaptation that mammals had made to an "aquatic lifestyle".
It showed, he said, "how much we still have much to discover, even from the largest ocean residents".