Laryngeal mechanisms for the emission of CF-FM sounds in the Doppler-shift compensating bat,Rhinolophus ferrumequinum

Abstract

1. The laryngeal mechanisms for the production of CF-FM orientation sounds in the horseshoe bat,Rhinolophus ferrumequinum, were studied with 3 different methods: a) denervation of laryngeal muscles, b) recordings of electrical activity of the cricothyroid muscle during vocalization and Doppler-shift compensation, and c) electrical stimulation of the cricothyroid muscles. 2. Unilateral section of the inferior laryngeal nerve (i.e. denervation of the intrinsic laryngeal muscles) had no effect on the sound pattern, the frequency of the orientation sounds and the Doppler-shift compensation. Bilateral denervation led always to suffocation. 3. Unilateral section of the superior laryngeal nerve (i.e. the denervation of the cricothyroid muscles) did not change the sound pattern, but caused a decrease in the frequency of the emitted CF by 4–6 kHz. Doppler-shift compensation was possible, but it was unstable and was not accurate. 4. Bilateral denervation of the cricothyroid muscles introduced several strong harmonics in the orientation sounds. The fundamental frequencies changed considerably between 12 and 42 kHz after surgery. The frequency pattern in each harmonic is the same as in normal orientation sounds (Fig. 1). 5. The spike number per vocalization of cricothyroid muscle fibres was proportional to the frequency of the CF component of the orientation sound in a range between the resting frequency and 5 kHz below it (Fig. 2). 6. Electrical stimulation of the cricothyroid muscles increased the frequency of the emitted CF. The maximum frequency change due to electrical stimulation was about 45 Hz per ms when measured during the CF component of the orientation sound (Fig. 3). 7. Tetanus fusion frequency of the cricothyroid muscle was about 200 Hz for continuous stimulation and 400 Hz for short stimulation applied during vocalization.