Japanese macaque phonatory physiology

Abstract

While the call repertoire and its communicative function is relatively well explored in Japanese macaques (Macaca fuscata), little empirical data is available on the physics and the physiology of this species' vocal production mechanism. Here, a 6 year old female Japanese macaque was trained to phonate under an operant conditioning paradigm. The resulting “coo” calls, and spontaneously uttered “growl” and “chirp” calls, were recorded with sound pressure level (SPL) calibrated microphones and electroglottography (EGG), a non-invasive method for assessing the dynamics of phonation. A total of 448 calls were recorded, complemented by ex vivo recordings on an excised Japanese macaque larynx. In this novel multidimensional investigative paradigm, in vivo and ex vivo data were matched via comparable EGG waveforms. Subsequent analysis suggests that the vocal range (range of fundamental frequency and SPL) was comparable to that of a 7-10 year old human, with the exception of low-intensity chirps, whose production may be facilitated by the species' vocal membranes. In coo calls, redundant control of fundamental frequency in relation to SPL was also comparable to humans. EGG data revealed that growls, coos, and chirps were produced by distinct laryngeal vibratory mechanisms. EGG further suggested changes in the degree of vocal fold adduction in vivo, resulting in spectral variation within the emitted coo calls, ranging from “breathy” (including aerodynamic noise components) to “non-breathy”. This is again analogous to humans, corroborating the notion that phonation in humans and non-human primates is based on universal physical and physiological principles.