Baleen whales have been hunted and ripped apart for hundreds of years, especially their mouths, where the baleen (extensions of the oral mucosa) was once used as stays in women’s corsets and in making buggy whips. So it’s really surprising that only now have scientists discovered that there is a unique organ in their mouths that helps them feed. This organ, described in a new paper in Nature by Nicholas Pyensen and five other authors (reference below), is found in rorqual whales, a subset of baleen whales that include blue whales, humpback whales, minke whales, and fin whales.
Baleen whales, which include the largest living animals on Earth, have a unique form of feeding: “lunge-feeding,” in which they suddenly ingest a huge mouthful of water and then expel it, leaving the edible contents sticking to the sieve that is formed by the baleen. They then lick off the prey. The process is described in a nicely written editorial in the same issue of Nature, “A whale of a story“:
Rorqual whales capture much of their food by an extraordinary procedure known as lunge feeding. When a rorqual comes across a dense patch of prey, it accelerates through the water and open its mouth. As it does so, its mouth fills with water, suspended within which are the tiny animals that the whale wants. The amount of water that flows into the whale can more than double the creature’s weight, and to accommodate it, blubbery pleats under the lower jaw expand, just as an accordion grows as it fills with air. The once sleek and streamlined whale now has the shape of a bloated tadpole. And it has a lot of water in its mouth.
To squeeze the water out again, the whale closes its jaws and pushes the water out through plates of keratin filters, which trap the food. In this way, rorqual whales can gulp and graze for hours, repeatedly slowing down then lunging through the water.
I was stunned that they take in an amount of water equal to their weight: these are the most massive creatures on Earth!
Now I’ve posted on baleen whales several times before, including discussions of transitional forms and vestigial tooth buds, and I also posted the following video of the lunge-feeding of a blue whale, which I reproduce again so you can see this amazing behavior (watch the first 10 seconds only):
So what did the authors find? Examining hunted specimens of fin and minke whales (both rorquals), they discovered a strange organ in the middle of the lower jaw (actually, the lower jaw consists of two bones that aren’t connected, allowing them to open their mouths even wider. Here’s a diagram of where that organ is (it’s the bit labeled “so” in the upper part of the diagram below; click to enlarge). There are also “vibrissae” (“vib”), or whiskerlike protrusions that probably aid in detecting prey.
Here are the keratin “bristles” on the baleen plates that filter out the prey (largely krill, or small crustaceans); the whales literally comb the prey out of the water:
The sensory organ is a unique feature in these animals. It’s a fibrous capsule, about 40 cm (16 inches) across, filled with a gel-like substance that contains connective tissue with nerves and nerve terminals, as well as “mechanoreceptors” (nerve receptors that sense changes in position or deformation) embedded in that tissue. Here’s a picture of the organ from the paper:
The authors suggest that, based on its position, its anatomy, and its possession of nerves that run to the brain, this organ provides sensory input to the brain when the whale is opening and twisting its jaws during bouts of lunge-feeding. In other words, it helps coordinate the critical feeding movement and makes sure it’s done correctly. The authors suggest this scenario for how lunge-feeding works. It’s in scientific jargon but not too hard to follow:
We propose a three-step lunge-feeding model to explain the organ’s role during a lunge: (1) using vibrissae present on the external surface of the chin, rorquals register prey fields of sufficient density; (2) the jaws disengage and rotate6, thereby compressing and shearing the organ; and (3) the oropharyngeal cavity reaches full expansion, with drag forces acting on the inside of the mouth transmitted to the organ through the YSF8 (Fig. 2). According to dynamic modelling studies, rorquals must actively control the rate of mouth openingand throat-pouch expansion to effectively maximize volume captured; we propose that the sensory organ has a key role in coordinating this movement.
It’s surprising that given the history of whaling, nobody either noticed or took an interest in the organ before. How it evolved, and what its precursor was, are mysteries that remained to be solved. It just goes to show that many of these mysteries have been right under our noses for a long time. What’s important is to notice them, get curious, and ask the right questions.
Peyensen, N.D., J. A. Goldbogen, A. W. Vogel, G. Szathmary, R. L. Drake and R. E. Shadwick. 2012. Discovery of a sensory organ that coordinates lunge feeding in rorqual whales. Nature 485: 498–501; doi:10.1038/nature11135