Sep. 7, 2004 BLOOMINGTON, Ind. -- When it comes to making noise, both parrots and humans rely on extremely specialized vibrating organs in their throats. Now scientists at Indiana University and Leiden University in The Netherlands have shown for the first time that parrots, like humans, also can use their tongues to craft and shape sound.
"This is the first direct evidence that parrots are able to use their large tongues to change the acoustic properties of their vocalizations," said IU Bloomington neurologist Roderick Suthers, who participated in the research. "The basic idea here, we believe, is that motor control of tongue movements is an important part of communication, just as it is in humans."
It's known that to produce sound, a parrot uses its syrinx, a voice box organ nestled between the trachea and lungs. The lingering question has been: What happens to that sound as it moves up and out of the throat? Ornithologists and bird enthusiasts have long noticed that parrots bob their tongues back and forth while they vocalize, but it wasn't known whether the tongue motions contributed significantly to sound-making.
The report by Suthers and biologists Gabriel Beckers and Brian Nelson in the Sept. 7 issue of Current Biology shows that even tiny changes in the position of a parrot's tongue can lead to big differences in sound.
"Birdsong is an excellent model for human speech and also for the development of communicative behavior," Beckers said. "Song is something that has to be learned, and it can only be learned by listening. Very specific areas of the bird's brain aid song and imitation. Humans have language centers. Before, we used to think all the complexity of parrot communication was because of the syrinx. Now we think it's likely the tongue is involved, just like with human speech."
Beckers, the study's lead author and a former IUB postdoctoral fellow, is now a research fellow at Leiden University.
The syrinx operates similarly in all parrots. For this study the scientists used five monk parakeets, small parrots native to South America. While a speaker swept through a series of tones, from 500 to 11,000 Hz (from the B above a piano's middle C to a high-pitched F beyond the keyboard's range), the researchers measured how much the birds' tongue position influenced the outgoing sound.
They found that a change of just a fraction of a millimeter in tongue position could significantly affect the qualities of the emerging sound. "By analogy, it's larger than the difference between an A and an O in human speech," Beckers said.
The scientists also believe they are the first to identify four acoustic "formants" in parakeet sounds. Formants are small ranges of frequencies that remain strongly audible as sound travels past the throat, tongue, mouth and nasal cavities. The geometry of these passages deadens some frequencies but leaves others relatively unaffected. What's left distinguishes the character of a sound -- in this case, the voice of a parakeet.
Human voices also have formants. Because of differences in genetics, development and behavior, these fingerprint-like formants vary greatly from individual to individual. By studying parrots, Beckers, Suthers and Nelson hope to learn more about what structures are responsible for that variation in both birds and humans.
The study was funded by a grant from the National Institute of Neurological Disorders and Stroke (N.I.H.-N.I.N.D.S. grant R01 NS029467). Monk parakeets were provided by Michael Avery of the U.S. Department of Agriculture's National Wildlife Research Center.
To hear a simulation of the sound sweep Beckers and Suthers used for the experiment, go to http://miles.ucs.indiana.edu/~iuinfot/pub/libs/images/usr/392_h.wav (WAV format).
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