Scientists Reveal How Elephants Produce Infrasound

A team of scientists from Germany, Austria and the United States, has found that elephants rely on the same mechanism that produces speech in humans – and the vocalizations of many other mammals – to hit extremely low notes.

African bush elephants in Tanzania (Ikiwaner / GNU-1.2)

African elephants are known to be great communicators that converse with extremely low-pitched vocalizations, known as infrasounds, over a distance of miles. These infrasounds occupy a very low frequency range – fewer than 20 Hz – that is generally below the threshold of human hearing.

“These vocalizations are called infrasounds because their fundamental frequency is below the range of human hearing,” explained Dr Christian Herbst of the University of Vienna, Austria, lead author of the paper published in the journal Science.

“We only hear the harmonics of such sounds, or multiples of that fundamental frequency. If an elephant’s vocal folds were to clap together at 10 Hz, for example, we would perceive some energy in that sound at 20, 30, 40 Hz and so on. But these higher overtones are usually weaker in amplitude.”

Until now, researchers have wondered whether these low, rumbling elephant infrasounds were created by intermittent muscle contractions, as a cat’s purr is, or by flow-induced vocal fold vibrations, fueled by air from the lungs, as is a human’s voice.

The team used the larynx of a recently deceased elephant to recreate some elephant infrasounds in a laboratory. The researchers removed the larynx and froze it within a few hours of the animal’s death. They then took it over to the laboratory in the Department of Cognitive Biology at the University of Vienna, where Dr Tecumseh Fitch, a senior author of the paper, studied it in depth.

The scientists imitated the elephant’s lungs by blowing controlled streams of warm, humid air through the excised larynx while adjusting the elephant vocal folds into a phonatory, or vocal-ready, position. In this way, they were able to coax the vocal folds into a periodic, low-frequency vibration that matched an elephant’s infrasound in every detail.

The fact that they were able to duplicate the elephant’s infrasounds in a laboratory demonstrates that the animals rely on a myoelastic-aerodynamic, or flow-driven, mode of speech to communicate in the wild. The elephant’s brain would have been required to recurrently tense and relax the vocal muscles if the other mechanism, which produces a cat’s purr, was involved.

This flow-induced mechanism demonstrated by the researchers is likely to be employed by a wide range of mammals. From echolocating bats with their incredibly high vocalizations to African elephants and their extremely low-pitched infrasounds, this mode of voice production seems to span four to five orders of magnitude across a wide range of body sizes and sonic frequencies.

_______

Bibliographic information: Herbst et al. 2012. How Low Can You Go? Physical Production Mechanism of Elephant Infrasonic Vocalizations. Science, vol. 337 no. 6094 pp. 595-599; doi: 10.1126/science.1219712