August 3, 2012
Elephants And Humans Have Similar Vocal Mechanisms
April Flowers for redOrbit.com - Your Universe Online
African elephants are known to communicate with infrasound - extremely low-pitched vocalizations - over a distance of miles. These infrasounds are at a very low frequency rate, fewer than 20 Hertz per second, and are generally below the threshold of human hearing.A new study, published in the August 3 issue of Science, shows that elephants rely on the same mechanism that produces human speech, and the vocalizations of other mammals, to hit those really low notes. The research team, led by Christian Herbst from the University of Vienna, used the larynx of a recently deceased elephant from the Berlin Zoo to recreate some elephant infrasounds in a laboratory.
"These vocalizations are called infrasounds because their fundamental frequency is below the range of human hearing," explained Herbst during a phone interview with the American Association for the Advancement of 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 Hertz, for example, we would perceive some energy in that sound at 20, 30, 40 Hertz and so on. But these higher overtones are usually weaker in amplitude."
Until now, researchers have wondered whether these low rumbling vocalizations 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 like a human voice.
The elephant's natural death gave the research team a rather serendipitous chance to find out for sure. The team removed the elephant's larynx and froze it within a few hours of the animal's death. Tecumseh Fitch, at the Department of Cognitive Biology at the University of Vienna, studied the larynx in detail.
The researchers were able to imitate the elephant's low-frequency vibrations perfectly by blowing a steady stream of warm, humid air through the larynx while holding it in a phonatory, or vocal ready, position. This demonstrates that the animals rely on a myoelastci-aerodynamic, or "flow driven", mode of speech to communicate in the wild. The elephant's brain, not present in this test obviously, would be necessary to recurrently tense and relax the vocal muscles if the other mechanism, which produces a cat's purr, was involved.
This flow-induced mechanism for vocalizing is likely to be used by a wide range of mammals from the high pitched echolocating bats to the low pitched infrasounds of African elephants. This mode of voice production seems to span four to five orders of magnitude across a wide range of body sizes and sonic frequencies.
Some rather interesting "non-linear phenomena" were seen in the way the elephant's vocal folds vibrated. These mostly irregular patterns occur when babies cry or heavy metal singers scream. The physical mechanisms used is again identical to that seen in humans.
"If I scream, it's no longer a periodic vibration," said Herbst. "It becomes chaotic and you can hear a certain degree of roughness. This can also be observed in young elephants, in situations of high excitement."
Herbst asserts that these findings could not have been obtained without collaboration between biologist and voice scientists, and that voice science is an essential aspect of our social and economic lives.