On the Mechanism of Modulating the Volume of the Voice in Howling Monkeys

Abstract

It is proposed that the howling monkeys intensify the low pitched sounds, which are emitted in their howls, by considerably narrowing the laryngeal vestibule in order to build up a pressure capable of distending the collapsed thyrohyoid canal. In this manner, the vibrating air column can enter the hyoid bulla and the latter act as a resonator. The narrowing of the laryngeal inlet is achieved by a pneumatic sphincter—the pharyngolaryngeal sacs—activated by extralaryngeal muscles.