For rorqual whales, some of the largest animals in the world, feeding is no simple task. In fact, some have described the act of a blue or fin whale opening its jaws 90 degrees wide and taking a giant gulp of water as the largest biomechanical event on earth.
Now, scientists at the University of British Columbia and the Smithsonian Institution in Washington, D.C., believe they've identified an organ that helps these huge mammals accomplish the gargantuan task of quickly opening and closing their giant mouths underwater while moving along relatively rapidly, a process known as lunge feeding.
"It really is pretty astonishing when you look at the fact that a blue whale, which has six-metre-long jaws, has to open and shut its mouth in under 10 seconds to actually capture any prey," said Nicholas D. Pyenson, curator of fossil marine mammals at the Smithsonian.
"In doing so, they engulf a volume of water that's about 100 cubic metres. Imagine a city bus — it's a volume of water that's larger than that."
Cavity filled with mechanoreceptors
Pyenson and his colleagues at UBC discovered an oval, gel-filled cavity in the chin of rorqual whales filled with blood vessels and nerves that act as mechanoreceptors relaying information to the whale's brain about the positioning of its enormous jaws.
"It just gives it information about where the jaw is as it goes through this very rapid and very dramatic expansion of the mouth," Pyenson said.
The researchers suspect the as-yet-unnamed organ is an important anatomical feature that enabled whales to evolve the very large body size they have today.
Their findings are published in the May 23 edition of the journal Nature.
Pyenson's collaborators on the study were Robert E. Shadwick, who holds the Canada research chair in integrative animal physiology at UBC, A. Wayne Vogl, a professor of cell and developmental biology at UBC, and Jeremy A. Goldbogen, now a postdoctoral researcher at the Cascadia Research Collective in Olympia, Wash., but previously a PhD student of Shadwick at UBC.
Rorqual whales fall under the group known as baleen whales, which instead of teeth have a sieve-like substance called baleen hanging from their upper jaw that filters fish, krill and plankton out of the water. Unlike toothed whales and dolphins, rorqual whales don't use echolocation to pursue a single prey but take in a jumble of marine life in one giant gulp of water with the help of a flexible accordion-like throat pouch that allows their mouth to expand.
They then let the water flow out through their baleen, which is made up partly of keratin, the protein found in human hair and fingernails, which filters out the prey.
"It's a very efficient form of feeding even though it's very costly in the sense of the physics involved, because you're no longer very hydrodynamic when you engulf a large volume of water, you're not very streamlined," Pyenson said.
Organ synched up with chin hairs
Another peculiarity of baleen whales is that their right and left jaws are not fused the way human jaws are. It is in the space between the two jaws that the researchers found the sensory organ. From the outside, the cavity appears as a bump at the edge of the chin that can be glimpsed when a whale opens its jaws.
The scientists discovered the organ while studying discarded tissue of dead whales caught at a commercial whaling station in Iceland. The species they studied were fin, minke and Bryde's whales, but, says Pyenson, they suspect the organ exists in all rorquals, which also include blue and humpback whales.
Exactly how rorqual whales detect prey without the aid of echolocation, the process of emitting a sound and using its echo to locate and glean information about objects, is a question that has stumped scientists, but Pyenson and his colleagues believe the sensory organ they've identified plays a role. Specifically, the organ links up with the hairs that rorqual whales have on the tip of their chins to help them locate prey.
"When we used MRI and CT data to look at the detailed anatomy, we discovered the same nerves that are going in this organ are those that also connect up to the chin hairs, so we think the chin hairs are pretty important for detecting clouds of prey," Pyenson said.
"Our hypothesis is that they likely detect a certain density of objects hitting it ... and that registers with the animal and says 'OK, time to open your mouth.'"
Pyenson and his fellow researchers are not the first to have spotted the organ, but previous research misidentified it as a cartilaginous joint, Pyenson said.
For most of the centuries that humans have been hunting and studying whales, the organ has gone unnoticed.
"I think it underscores how little we know about the anatomy of the largest animals that are alive on the planet today," he said.