Anatomical and Acoustical Determination of Sound Production in the West Indian (Trichechus manatus) and Amazonian (T. inunguis) Manatees

West Indian (Trichechus manatus) and Amazonian (T. inunguis) manatees are an endangered species. They are vocal mammals, with most sounds related to communication between mother and calf.  Although their hearing levels and vocalizations have been characterized, the sound production mechanism is unknown. Anatomical examination and sound localization were used in this study to determine the source of sound generation. Anatomical examinations were carried out in five Florida and two Antillean manatee carcasses. During necropsies, the larynx, trachea and nasal areas were dissected, with particular focus on identifying musculature and soft tissues capable of vibrating or constricting the airway. Acoustical recordings were performed on live semi-captive and captive manatees to determine focal point of sound production. Access was provided during veterinary examinations of two Antillean manatees in Puerto Rico and eight Amazonian manatees in Iquitos, Peru. Recordings were carried out using four SS03-10 Sea Phone directional hydrophones with 30.5 cm suction cups and an Edirol-R44 digital recorder. Anatomical results indicate that the manatee has two raised areas of tissue in the lateral walls of the laryngeal lumen that appear to be vocal folds. They oppose each other and can regulate airflow between them when they are adducted or abducted by muscular control of arytenoid cartilage. Results from sound localization indicate that the focal point corresponds to the ventral throat. Vocal fold vibrations are thought to generate these sounds. The anatomical and acoustical study indicates a strong point of transmission in the larynx region, thus the vocal folds.