Labial vibrations as the principal sound source in the songbird syrinx
by Franz Goller
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Songbirds sing elaborate, species-specific songs, which are acquired during ontogeny. Although birdsong is a model system for learned vocal behavior and, for that reason, has been studied extensively, the mechanisms of sound generation are poorly understood.
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Singing behavior in songbirds is characterized by high acoustic complexity and diversity.
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According to the classical model of sound production in the songbird vocal organ, the syrinx, vibrations of the thin medial tympaniform membranes generate sound. This model was derived from morphological studies and physiological experiments on the excised syrinx. Endoscopic studies of the intact syrinx, however, reveal that the medial and lateral labia are prominently involved in phonation. Prior to phonation both labia are moved into the lumen of each bronchus, upon which Bernoulli and elastic recoil forces establish vibrations of the labia. The medial tympaniform membranes are continuous with the medial labia and therefore are also set into vibratory motion.
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New model of sound generation illustrated in a schematic frontal section of the syrinx: The two labia (ML and LL) are moved into the airstream and vibrate (Bernoulli forces). The medial tympaniform membranes (MTM) are not the principal sound generators. a, quiet position; b, phonatory position.
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To determine if vibrations of the medial tympaniform membranes are critical for sound production, they were surgically removed in cardinals (Cardinalis cardinalis) and zebra finches (Taeniopygia guttata). In contrast to the predictions of the classical model, this procedure did not prevent phonation in either species. Furthermore, zebra finches were able to sing normal songs after recovery from the surgical procedures despite complete bilateral destruction of the medial tympaniform membranes.
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The medial tympaniform membranes are not required for sound generation. After surgical removal of the membranes zebra finch song remains almost normal (bottom panel: control song; top panel: song 26 days after MTM removal).
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With this new model postulating the labia as the principal sound generators, the sound generating mechanisms in the human larynx and the songbird syrinx show striking similarities despite the difference in morphological structures.
Goller, F. and O.N. Larsen (1997): A new mechanism of sound generation in songbirds. Proceedings of the National Academy of Sciences, 94:14787-14791.
Larsen, O.N. and F. Goller (1999): Role of syringeal vibrations in bird vocalisations. Proc. Roy. Soc. Lond.B., 266:1609-1615.
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