“To produce a mighty book, you must choose a mighty theme. No great and enduring volume can ever be written on the flea, though many there be that have tried it.” Herman Melville, the author of Moby Dick and of that quote, certainly knew a mighty theme when he saw one. His masterpiece is as much a study on the behaviours of the sperm whales that Melville saw while serving on a whaling ship as it is an exploration of the motives and characters of the men who pursue them.
Sperm whales (Physeter catodon) have teeth, as Melville and his fictional Captain Ahab knew all too well. So how they catch and eat their prey, although a mighty theme, is not much of a story. More interesting, and much more mysterious, is how rorqual whales — those toothless giants of the oceans — manage to synchronize the multiple complex processes that they use to survive on little more than krill and small fish.
These creatures, which include the fin (Balaenoptera physalus), humpback (Megaptera novaeangliae) and blue whale (Balaenoptera musculus), have long puzzled biologists. How can they survive and sustain their huge bodies on such a meagre diet? How can they get enough food? The answer involves one of the greatest feats in the animal kingdom.
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.
It is a unique process, and one that requires some special equipment. The shape of a rorqual's head is more reptilian than mammalian, with a giant skull and mandibles that are held together by unique jaw joints made from dense elastic matrices of fibre and cartilage and infused with oil. This system lets the whale open its jaws to nearly a right angle — ideal for gulping water. There is more: the lower mandible has a flexible joint at its centre, which allows the two sides to rotate. And the whale can turn its tongue inside out, pulling it backwards to create even more space for the water. As it does so, its open mouth experiences massive drag, which the creature mitigates by contracting its throat to force some of the water back out.
But how on Earth does a rorqual manage to coordinate this activity? On page 498, Nicholas Pyenson of the Smithsonian Institution in Washington DC and his colleagues describe a sensory organ that they discovered in the jaws of several species that might offer an answer. The organ, they say, seems to be involved in several lunge-feeding processes, including the opening of the jaws and the expansion of the throat. And that means that it may have helped to drive the evolution of the huge bodies of rorqual whales — their great and enduring volumes, as Melville might have put it.