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Vocalizations:
Elephants and humans use same mechanism
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 Hertz, or cycles, per second -- that is generally below
the threshold of human hearing.
Now, a new study shows that elephants rely on the same mechanism that
produces speech in humans (and the vocalizations of many other mammals)
to hit those extremely low notes. Christian Herbst from the University
of Vienna, along with colleagues from Germany, Austria and the United
States, used the larynx of a recently deceased elephant to recreate some
elephant infrasounds in a laboratory. Their findings are published in
the August 3 issue of the journal Science, which is published by AAAS,
the nonprofit science society.
“These vocalizations are called infrasounds because their fundamental
frequency is below the range of human hearing,” explained Herbst during
a phone interview. “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
elephant infrasounds were created by intermittent muscle contractions,
as a cat's purr is, or by flow-induced vocal fold vibrations, fuelled by
air from the lungs, as is a human's voice. But, the natural death of an
elephant at a zoo in Berlin gave Herbst and his colleagues a somewhat
serendipitous chance to study the mechanism firsthand.
The researchers removed the elephant's larynx and froze it within a
few hours of the animal's death. They then took it over to the larynx
laboratory in the Department of Cognitive Biology at the University of
Vienna, where Tecumseh Fitch, a senior author of the Science paper,
studied it in - depth.
Herbst and the other researchers 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, the scientists 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, they say.
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.The researchers also saw some interesting
“nonlinear phenomena” in the way the elephant vocal folds vibrated.
These mostly irregular patterns of vibration occur when babies cry or
heavy metal singers scream and the physical mechanism that elephants use
is again identical to that seen in humans, they say.
“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 says that the findings were only made possible by a
collaborative effort between voice scientists and biologists, and that
voice science is an essential aspect of our social and economic lives.
-ScienceDaily |
