Muscular control of vocal complexity in the túngara frog (Engystomops pustulosus)
Many animals use communication to increase their chance of successful reproduction. Advertisement signals vary in complexity and their diversity can be best understood through the study of the simplest case, in which a species with a single-note mating call evolves a second note. Túngara frogs ( Engystomops pustulosus ) provide the best-known example of complex calling among frogs. The initial part of the mating call, called "whine", is always present and mediates species recognition. The frog can facultatively add one to seven additional notes called "chucks" to the mating call. Frogs initiate phonation by compressing pulmonary air to pass it through the larynx, inducing vibration of vocal cords. In male túngara frogs, a pair of fibrous masses is attached to the vocal cords and vibrates to produce the chuck. The mechanism that controls the optional production of the chucks was previously unknown. This study examined the muscles of the larynx thought most likely to be involved in controlling the production of the chuck. Four pairs of laryngeal muscles were previously known in amphibians. The laryngeal dilator muscle is the largest of them and has the role of opening the larynx. In the túngara frog, the laryngeal dilator muscle is divided into two bundles of fibers with distinct attachments. The superficial bundle of the dilator muscle originates at the posteromedial process of the hyoid cartilage and inserts medially into the arytenoid cartilages. The deep bundle attaches to the medial aspect of the arytenoid cartilages and to the base of the fibrous mass, extending through a fissure between the cricoid and arytenoid cartilages. This attachment to the fibrous mass may allow the deep bundle of the laryngeal dilator muscle to control the vibration of the fibrous mass and the production of the chuck. This possibility was evaluated by examination of the muscle innervations and by electrical stimulation. The long laryngeal nerve innervates all laryngeal muscles except the deep portion of the laryngeal dilator muscle, which is innervated by the short laryngeal nerve. Electrical stimulation confirmed that the long laryngeal nerve stimulates all laryngeal muscles, except the deep portion of the laryngeal dilator muscle. Stimulation of the short laryngeal nerve only resulted in contraction of the deep portion of the laryngeal dilator muscle, which leads to the fibrous mass. These results indicate that the short laryngeal nerve controls the deep portion of the laryngeal dilator muscle, which may position the fibrous masses in a way that allows them to vibrate with airflow adding chucks to the mating call. The deep portion of the laryngeal dilator should be recognized as a separate muscle in the amphibian larynx, for having distinct attachments, innervation and role. The recognition of this muscle resolves the issue of multiple innervation of the laryngeal dilator muscle in amphibians. Based on innervation, the new muscle is likely homologous to the mammalian cricothyroid muscle.
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