Do elephants purr?

Where does the elephant get its distinctive voice? These massive creatures are capable of communicating with one another at frequencies outside the limits of human perception over distances of several miles, but how they do it has long been a mystery. Now, a team of biologists has demonstrated that they likely employ the same mechanism as humans to communicate at remarkably low frequencies.

Elephants are capable of producing very low notes, at what are called "infrasonic" frequencies — that means the sounds they produce can actually extend below 20 hertz, the lowest frequency detectable by the human ear. Researchers have known about elephants' infrasonic abilities for some time — but how are these sounds actually generated? That's been less clear.


According to biologist Christian Herbst, first author of an enlightening new study on elephant vocalization, the vast majority of mammalian vocalizations rely on the vibration of vocal folds housed within the larynx, an organ known colloquially as the "voice box". These oscillating layers of tissue are capable of producing an extraordinary range of frequencies across species, ranging from 9 Hz in whales to over 110,000 Hz in bats. It stands to reason, notes Herbst in the latest issue of Science, that elephants rely on this method of sound production, as well.

Catspeak vs. Humanspeak

But an important question remains: what's controlling the vibration of the vocal folds? According to Herbst, there are two potential mechanisms. The first is called active muscular contraction, or "purring" mode; as it's name suggests, this is the same method your typical house cat uses to produce purring sounds. The second is the myoelastic-aerodynamic, or "flow-driven" mode, which is the way we humans generate sound.


When a cat purrs, it regulates the speed at which its vocal folds vibrate by actively contracting its laryngeal muscles. In humans, however, laryngeal muscle activity remains relatively steady; while human vocalization can be modulated by adjusting the tension of various laryngeal muscles, changes in the frequency at which your vocal folds vibrate are largely due to shifts in the air pressure being applied via your lungs. The figure shown here, borrowed from Herbst and his colleagues' publication, helps further illustrate the difference between the two mechanisms (click to enlarge).


When Dead Elephants Talk

The upshot of all this is that purring relies on finely tuned, neurally controlled muscular twitches, while flow-driven vocalization can function in the presence of air pressure alone. It's the difference between the reed on a woodwind instrument vibrating autonomously, and a reed vibrating because you're physically blowing air across it. If Herbst and his colleagues could produce infrasonic frequencies from an elephant larynx that had been disconnected from its nervous system, they could prove that a purring mechanism isn't actually necessary for elephant vocalization. To do this, the researchers designed an experiment that looks like it was ripped from the pages of MAKE magazine. Here's how it went down:


Step one: An African elephant at a Berlin zoo dies of natural causes. Herbst and his colleagues are permitted to use its body for research.

Step 2: The researchers excise the elephant's larynx, labelled here in red.

Step 3: Herbst and colleagues bring the vocal folds together so as to replicate their positioning during elephant vocalization.


Step 4: The excised larynx is attached to a tank of compressed air via a pressure regulator, humidifier, and artificial lung.

Step 5: Using the compressed air, the researchers attempt to simulate flow-driven, infrasonic vocalizations. The larynx is recorded with a microphone, a high-speed video camera, and carefully placed electrodes.


What's awesome about this experiment is that it looks like something you could set up in your garage. (If you've ever seen the episode of Inside Nature's Giants where a dead lion is made to roar through the use of a compressed air tank, the setup used by Herbst's team is very similar.) And here's the kicker: it worked.

The researchers were able to reproduce infrasound vocalizations at an average frequency of just 16.38 Hz, demonstrating that a live elephant doesn't have to purr to produce these low tones. Remember: an excised larynx is incapable of purring, as the necessary muscles are disconnected from the nervous system. As Herbst explains:

Although we can clearly rule out a role for active muscle twitching in our excised larynx preparation, we obviously cannot eliminate the possibility of such "purring" in a living elephant. However, our study demonstrates that there is no need for such twitching to produce loud low- frequency vocalizations such as elephant rumbles.


Be that as it may, the fact that infrasound noises can be produced by a simulated flow-driven system is pretty damn compelling, and underscores its utility as a vocalization system among mammalian species.

The researchers' findings are published in the latest issue of Science.

Top image via Shutterstock; kitteh opera via here and here; all other images via Herbst et al.

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