Elephants can talk silently, too

STEVE HAGERTY / MCCLATCHY NEWSPAPERS

Elephants talk to each other in tones too low for the human ear. A new study found how they produce the sounds.

With their trumpetlike calls, elephants may seem like some of the loudest animals on Earth. But we can't hear most of the sounds they make.

The creatures produce low-frequency noises between 1 and 20 hertz, known as infrasounds, that help them keep in touch over distances as large as 6 miles. A new study reveals for the first time how elephants produce these low notes.

Scientists first discovered that elephants made infrasounds in the 1980s. The head female in a herd may produce the noises to guide her group's movements, whereas a male who's in a mating state called musth might use the calls to thwart competition from other males. Mother elephants even rely on infrasounds to keep tabs on a separated calf, exchanging "I'm here" calls with the wayward offspring in a fashion similar to a game of Marco Polo.

These noises, which fall below the hearing range for humans, are often accompanied by strong rumbles with slightly higher frequencies that people can hear. By recording the rumbles and then speeding up the playback, the scientists can increase the frequency of the infrasounds, making them audible.

Researchers have speculated that the noises come from vibrations in the vocal folds of the elephant larynx. This could happen in two ways. In the first, called active muscular contraction, neural signals cause the muscles in the larynx to contract in a constant rhythm. Cats do this when they purr.

The second possibility is known as the myoelastic-aerodynamic method, and it occurs when air flows through the vocal folds causing them to vibrate - this also happens when humans talk.

The AMC is like a doorbell or buzzer that requires electricity, or in this case a brain signal, to work, while the MEAD is similar to a woodwind instrument such as the clarinet, which only needs airflow to produce sounds. Neural signals are only needed in this case to control breathing, but not produce the sound.

Until now, field researchers were unable to test which method was behind the infrasound. "You cannot just walk up to an African elephant and stick an endoscope in his mouth and ask him to say 'Ahh,' " says voice scientist and lead author Christian Herbst of the University of Vienna. So his team did the next best thing. They obtained an intact larynx from a female African elephant that died of natural causes at the Berlin Zoo.

With only the excised larynx, the researchers could not test for the AMC method, as it requires neural messages for the muscles to contract. To test the MEAD hypothesis, they mounted the larynx on a tube connected to an air tank that acted as a psuedolung, regulating airflow and pressure. The psuedolung was also connected to a heater and humidifier, which simulated the conditions of air in elephant lungs.

Using high-speed cameras, researchers found that the vocal folds vibrated as the air passed over them. They also recorded the sound from the vibrations and compared it with a database of more than 470 live elephant calls collected over four years. This analysis revealed that the excised larynx produced sounds similar to the low-frequency calls of live elephants.

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