The Mystery of the Purring Elephant

The lowest registers of an elephant's voice are inaudible to humans—but they've been a longstanding puzzle all the same

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Lisa Jeffries / AP

An African elephant, Mara, 28, originally from San Jose's Happy Hollow Zoo, wanders in a field at PAWS ARK 2000 sanctuary in San Andreas, Calif.

Stand near an elephant herd, and you may feel a strange vibration in your chest. That’s not your heart beating in terror because you’re, well, standing next to an elephant herd. Or at least that’s not all it is. It’s also a sign that the elephants are talking to one another. Elephants are famous for their trumpeting, of course, but they also produce rumbles pitched so low that humans can’t hear them, only feel them as a sort of physical buzzing. Exactly how elephants do this has been a mystery — and while solving that mystery is not of first-order importance in understanding and preserving this largest of land animals, it would add new insight into how a whole range of species vocalize.

The best way to answer the question of elephant-talk would be to examine the animal’s larynx. Live elephants are notoriously touchy, and recently deceased elephants are hard to come by. So when a team of University of Vienna biologists who study animal sounds heard that an elderly zoo elephant in Berlin had died, they wasted no time requesting the larynx, which was soon on its way to Vienna on a bed of ice. What they did with it next is the focus of a paper in this week’s Science.

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There are two major theories behind elephant rumbling, explains Christian Herbst, a biophysics post-doc in the lab of cognitive biologist Tecumseh Fitch and lead author of the paper. One theory holds that the rumbles are made only by the elephants’ vocal cords, which, like ours, consist of two flaps of flesh in the larynx — though an elephant’s larynx is eight times larger than ours. Much like a slit made in a blade of grass, the flaps vibrate and produce sound when air rushes through them. We then use our throats and mouths to shape that sound — adding vowels, consonants, and other frills that turn it into language. The vibration itself, however, happens passively, without any muscular control, as soon we start to push air up through our throats.

The other theory is that elephants, like cats, are purring. Purring is not passive: each pulse of the purr is made by voluntary contraction of the muscles around the larynx. Muscles can contract only so fast, so purrs are low pitched. That’s why a tiny kitten, whose vocal cords are so minuscule it can only squeak when it meows, can make a deep rumble when it purrs. The question the team sought to answer, then, was would the elephant larynx still make a rumble if it were no longer connected to the elephant’s muscles? If so, it could not be a purr.

To test this, the researchers soon had the donor larynx wedged over the end of an aluminum tube. (“There was a certain amount of duct tape involved,” admits Fitch.) They fastened electrodes to the vocal cords, which would record whenever the two flaps vibrated against each other, and trained a high-speed camera and sensitive microphone on the contraption. Then, holding the whole thing in place with their hands, they sent a stream of air up through the tube and into the larynx.

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The sound made by an elephant larynx on a tube is difficult to describe. Herbst imitates it by gripping each of his cheeks in his fingers and shaking them until they slap.

Listen to the sound here.

That may be an unlovely sound but it’s an exciting one since, in the sonograms made from the recordings, the sounds, including the lowest registers, strongly resembled what you’d expect a rumble from a headless (and bodyless) elephant to look like—that is, without the extra resonance and shaping added by the throat and mouth. When William Langbauer, a biologist who was among the first to study elephant rumbles, look at the data, he was impressed: “The sonograms that they show in the paper — the pictures of what the sound looks like — look so incredibly similar to a vocalization of a live elephant.”

Listen to the audible part of a live elephant’s rumble here.

Muscle control, thus, isn’t required for the larynx to make a rumble. Elephant purring, alas, is mere fancy.

This doesn’t mean that the study has proven that live elephants refrain totally from using muscles in producing deep calls, notes Joseph Soltis, a research scientist at Disney’s Animal Kingdom who studies elephant communication. The muscles in the larynx and throat are likely very important in shaping the sound made by the vocal cords and expressing emotion in the voice. A frightened elephant would contract its throat and sound quite different from an elephant rumbling in pleasure. But the rhythmic contraction of purring, at least, seems to be unnecessary for getting the sound started.

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To biologists who study sound, the new findings have implications outside the elephant species. “This physical principle spans an extremely wide range of species — four orders of magnitude basically, both in frequency of the sound and body mass. That, I think, is fantastic,” says Herbst.

The next step up from an elephant, Fitch says, would be to study the larynx of a blue whale, whose deepest calls are also inaudible to humans. “A blue whale has a larynx the size of me, so to ever do that would mean building a truck-sized larynx apparatus and going on to a beach where a whale had washed up,” he says. “That’s a whole other level of army-style commando science. I don’t know if it will ever happen. But I dream of it sometimes.”

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