The Role of the Anterior and Posterior Cricothyroid Muscles in the Production of Echolocative Pulses by Mormoopidae
Pteronotus parnellii in the family Mormoopidae emits pulses having a long initial CF portion followed by a short downward FM sweep. Most of the acoustic energy is in the second harmonic at 60 kHz, but the fundamental and higher harmonics can also be introduced when greater bandwidth is needed (Fig. 1). Griffiths (1978) reported that the cricothyroid muscle (CTL) of Mormoopidae has two distinct bodies—an anterior longitudinal portion (aCTM) and a posterior transverse portion (pCTM). The latter attaches to the flexible lateral walls of the laryngeal cavity. On the basis of its anatomy, Griffiths suggested that it might enable the bat to tune its larynx, in the manner of a variable Helmholtz resonating amplifier, by altering the cross-sectional area of the laryngeal cavity anterior to a larger post-laryngeal chamber formed by the enlarged trachea. We have examined the effect of a light gas mixture (20% oxygen, 80% helium) on the harmonic content and frequency structure of orientation pulses emitted before and after selective denervation of particular parts of the CTM in an effort to determine if the pCTM is responsible for varying the harmonic content of the emitted pulse by changing the vocal tract resonance as suggested by Griffiths.
KeywordsHigh Harmonic Vocal Tract Acoustic Energy Harmonic Content Distinct Body
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