The challenge–and the pleasure–that evolutionary biologists face in their work is deciphering the history of nature, no matter how weird it gets. And nature doesn’t get much weirder than a beluga whale singing through its nose to see the ocean.
Ordinary vision doesn’t work as well in the ocean as it does on land, thanks to the way light travels through water. Sound, on the other hand, travels over four times faster through water than air. Belugas and other toothed whales (such as orcas and sperm whales) take advantage of its underwater speed by using echolocation.
The process by which they generate echoes is a complicated one. As mammals, toothed whales have to breathe air into their lungs. Other mammals can breathe through their mouth, or their nose, which sits right on top of it. In toothed whales, the nasal passage runs up above its eyes, creating a blowhole on top of its head. When it surfaces, it opens the blowhole to breathe. But underwater, toothed whales can use the air in their nasal passage to vibrate sets surrounding muscles. They keep their blowhole shut as they push this air around inside their heads, using a series of chambers to store and recycle it.
The vibrations are then guided by the bizarre anatomy of a toothed whale’s head. Behind the vibrating muscles, the skull rises to a ridge, preventing the sound from moving backwards. In front of the muscles is a large blob of fatty tissue, called the melon. It sits on top of the large, shelf-shaped upper jaw of the whale. As the vibrations pass through the melon, they become focused. The whale also has massive muscles anchored to the sides of its upper jaw and surrounding bones that let the animals squeeze the melon into different shapes–and thus direct the beam of sound in different directions.
When the sound waves hit something in front of the whale–a coral reef, a fish, or some other object–some of them bounce back towards the whale. The whale boosts its hearing with its lower jaw, which contains a long cylindrical piece of fat running down each side. Vibrations that hit the jaw travel back to its ear, which can detect the sound. Studying dolphins in captivity, scientists have shown that they can recognize complicated shapes based on their echoes alone. They can even sense the texture with sound.
How did dolphins and other toothed whales get to this strange state? Fossils and evidence from DNA have helped scientists figure out how they evolved from land mammals. It’s a topic I first took up in my book At the Water’s Edge, and which I’ve been trying to keep up with ever since. Here’s a tree joining toothed whales to a selection of their living and extinct relatives. It comes from the recently-published second edition of my book The Tangled Bank. (You can see a bigger version here.)
Whales evolved from land mammals, sharing a close common ancestor with hippos. Starting about 50 million years ago, they gradually lost their limbs, evolving a body dedicated to swimming rather than walking. Many lineages of whales evolved and thrived and eventually became extinct. All living whales descend from two lineages that split from each other about 40 million years ago, known as baleen whales and toothed whales.
While the back end of whales provide the most dramatic evidence of their evolution–turning from legs and a thin tail to flippers and a flukes–it’s the front end where much of the most essential adaptation took place. Whales needed to use their heads to sense their underwater world and to grab food from it. Early whales evolved long toothy snouts to catch prey. But then baleen whales and toothed whales evolved two different updates on that anatomy.
Baleen whales lost their teeth (although they still have broken genes for making teeth today). In place of teeth, they evolved fronds of baleen they could use to filter food from giant gulps of water they engulfed. As I’ve writtenon the Loom, paleontologists are finding fossils of early baleen whales that bridge the transition from a hunter to a filter feeder.
Toothed whales continued to catch prey individual prey, the way ancient whales had done before. But at some point they added on their extraordinary echolocation equipment–the head reflector, the air recycling chambers, the buzzing lips, the melon, and the rest.
Charting the evolution of echolocation has been tough because so much of this anatomy rots. That is, once a dolphin dies, its melon decomposes, along with its lips, muscles, and other organs essential for making sounds. All that scientists have to go on when they look at a toothed whale fossil is the skull itself. While scientists have been finding toothed whale fossils for a century, none of the older ones display many of the traits that you’d expect from an echolocator. Scientists have thus been left to wonder how long after the split between baleen whales and toothed whales this marvelous acoustic equipment evolved.
Today in Nature, Jonathan Geisler of the New York Institute of Technology College of Osteopathic Medicine and his colleagues offer details of a new fossil of a toothed whale, dating back 28 million years ago. It will go a long way to answer our questions about echolocation.
The skull of the whale, dubbed Cotylocara macei, was found in a drainage ditch in South Carolina. It’s got lots of features that are found only in toothed whales, showing that it belongs to their lineage. And it also has a lot of traits that toothed whales use for echolocation. The hole where the nasal passage leaves the skull, for example, is surrounded by flanges that could control buzzing lips. The skull has cavities around the nasal opening that look like the chambers dolphins and other living toothed whales use to recycle air. The jaw bones are dense, perhaps allowing them to reflect sound waves into the melon. Its upper jaw forms a broad shelf, which would allow Cotylocara to anchor muscles for controlling its melon.
Taken together, Geisler and his colleagues write, these traits “make a compelling case that Cotylocara could echolocate.”
What makes Cotylocara even more interesting is how it’s related to dolphins and other living toothed whales. Short answer: not closely at all.
When the ancestors of toothed whales split from the ancestors of baleen whales, they then branched into new lineages. One of the earliest splits led to two branches. One branch eventually led to living toothed whales. The other branch led to a group of toothed whales, called xenorophids, which are all long extinct. Cotylocara belongs to that extinct xenorophid branch.
Scientists have dug up a number of other fossils of xenorophids in the past. If you look at any one species, it may have a few of the traits that we associate with echolocation, such as a broad upper jaw or cavities in its skull. But none of them come close to the new fossil, Cotylocara.
Yet the anatomy that Cotylocara might have used to echolocate is not identical to that of living toothed whales. Some of its cavities are located in places in the skull where you won’t find any in a dolphin, for example. It looks as if it evolved these traits on its own.
The explanation that Geisler and his colleagues favor is that echolocation had a complicated history. The ancestor of all toothed whales–living and extinct–had already evolved a crude sort of echolocation. Its descendants then branched off into new lineages. In at least two of those lineages, toothed whales evolved much more sophisticated muscles, bones, and various organs, giving them more control over the signals they sent out. While Cotylocara evolved some traits that were very much its own, it also evolved some of the same traits found in living toothed whales, such as dolphins. (This kind of evolution in parallel, which I’ve illustrated below, is called convergence.)
Geisler and his colleagues already know how they can test this hypothesis. If they’re right, then early toothed whales didn’t just have sophisticated anatomy for making sounds. They must have also had ears that were able to detect them. Cotylocara’s skull doesn’t preserve its ear region, so we can’t know yet how well it could hear. But Geisler and his colleagues predict that if paleontologists examine the ears of other early toothed whales, they should find signs that the animals were somewhat more adapted to hearing high-frequency echoes.
Not only does Cotylocara answer some questions we have about this weird bit of nature, but it tells us what new questions to ask in turn.
[Update: paper link fixed. Also, fixed spelling of xenorophids.]
