With their trumpet-like calls, elephants may seem like some of the loudest animals on Earth. But we can’t hear most of the sounds they make. The elephants produce low-frequency noises are as low as the lowest notes of a pipe organ. These infrasounds between 1 to 20 Hertz help them keep in touch over distances as large as 10km.
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.
Although the sounds themselves have been studied for many years, it has remained unclear exactly how elephant infrasounds are made. One possibility, favoured by some scientists, is that the elephants tense and relax the muscles in their larynx for each pulse of sound. This mechanism, similar to cats purring, can produce sounds as low in pitch as desired, but the sounds produced are generally not very powerful.
The other possibility is that elephant infrasounds are produced like human speech or singing, but because the elephant larynx is so large, they are extremely low in frequency. Human humming is produced by vibrations of the vocal folds, which are set into vibration by a stream of air from the lungs, and don’t require periodic muscle activity. By this hypothesis, elephant infrasounds result simply from very long vocal folds slapping together at a low rate, and don’t require any periodic tensing of the laryngeal muscles.
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