Rorqual whales are the leviathans of the deep, yet they feed on some of the tiniest creatures of the sea: krill. In order to fill their enormous bellies, rorqual whales adopt a bite-more-than-you-can-chew approach to eating.
They open their mouths at a wide, jaw-dropping angle almost perpendicular to the rostrum. The undiscriminating behemoths then fill their mouths with prey-laden water. The volume of water can be greater than the volume of the whale. The water is then slowly 'sieved' through baleen plates so that the whale only eats the tasty krill and doesn't fill up on water.
Close up of Bryde's whale eating anchovy in Thailand (a rorqual whale), demonstrating how wide their jaws can extend / kajornyot via shutterstock
The method of eating is called 'lunge feeding,' but it isn't something that every species is suited to. Rorqual whales are well-equipped for lunge feeding because the floor of their oral cavity is stretchy and can expand to hold a whales-worth of water. It enlarges with the help of the tongue that inverts and stretches so that the whales can gulp more water.
And it's not just the tongue that stretches. Professor Wayne Vogl, an anatomist at the University of British Columbia and the study's lead author, told the BBC about the moment his co-author, Robert Shadwick, discovered stretchy nerves present in whales' mouths: "We were looking at the muscle in the floor of the mouth and there were these long white cords. Bob picked one up—about 3ft of it—grabbed each end and stretched it. He looked at me and said, 'Hey, look at this!'"
A whale nerve being stretched to demonstrate elasticity by Vogl et al. via Current Biology 2015
It is these flexible nerves that allow whales to open their mouths so wide. If the idea of stretchy nerves makes you feel squeamish, then that's probably because stretching nerves in humans often leads to pain, paralysis or even the detachment of nerve roots from the spinal cord.
Rorqual whales, however, have adapted to have nerves that bend and stretch to cope with extreme expansion. The nerves are composed of folded up fibers and are surrounded by a thick wall of collagen and elastin. These proteins are the same ones that keep our own skin elastic.
Unstretched and stretched nerve cell wall of a rorqual whale via Current Biology
When the whale opens its enormous mouth, these nerves unfold and elongate. They're even built with a natural stopping point—the collagen stiffens and keeps the ligaments in check to stop the whale opening its mouth too far. Then, just like a bungee cord, the nerves will 'snap back' to their original length.
Without these elongating nerves, satisfactory feeding would be impossible for the rorqual whales. It is truly a fascinating structural innovation.
The study was published in the journal Current Biology.
[Via Current Biology, Science Mag, BBC]
