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Terrestrial, Semiaquatic, and Fully Aquatic Mammal Sound Production Mechanisms
Aquatic mammals generate sound underwater but use air-driven systems derived from terrestrial ancestors. How do they do it without drowning?
Terrestrial mammals produce sound in air mainly for communication, while many aquatic mammals can communicate by vocalizing in air or underwater. A sub- set of aquatic mammals called odontocetes (toothed whales, including dolphins and porpoises) can also use echolocation sounds for navigation and prey track- ing. In all cases, mammals use pneumatic (air-driven) mechanisms to generate these sounds, but the sources and transmission pathways differ depending upon whether sounds are emitted into air or water.
Terrestrial Mammals
Email:
joy.reidenberg@mssm.edu
Joy S. Reidenberg
Postal:
Center for Anatomy and Functional Morphology Icahn School of Medicine at Mount Sinai 1 Gustave L. Levy Place Mail Box 1007 New York, New York 10029-6574 USA
The voice box, or larynx, is the organ of vocalization used by most terrestrial mam- mals. It initially evolved from the protective anatomy used to keep water out of a buoyancy organ in fish (the swim bladder). The main function of the larynx remains protection, only now it prevents incursions of foreign material into the “windpipe” (trachea) and lungs of mammals.
The entrance of the larynx is sealed by a pair of vocal “cords” (vocal folds). In ad- dition, there are tall cartilages (epiglottic and corniculate) that act as splashguards to deflect food and water away from the opening. These cartilages overlap in front with the soft palate and behind with the posterior wall of the airspace (pharyn- geal wall) to interlock the larynx with the rear of the nasal cavity (Figure 1). This interlock channels airflow through the larynx and isolates it from the swallowing pathway that is now diverted around the interlock. This division of the airspace into two separate pathways allows simultaneous breathing and swallowing, a trait that allows prey to detect the scent of a predator even while constantly feeding (Laitman and Reidenberg, 2016).
Over time, additional roles beyond coordinating swallowing were added to the larynx, including stabilizing the ribcage (for weight lifting or for “bearing down”; Laitman and Reidenberg, 2016). One of the last functions to be added was phona- tion, producing the initial sound (fundamental frequency) of vocalizations.
The vocal folds attach between the thyroid cartilage in front and the arytenoid car- tilage in the rear, and lie across the airway oriented perpendicular to the direction of tracheal airflow (Figure 1). Closed vocal folds protect the opening and elevate internal pressure; opened vocal folds release pressure and allow respiration. The intermediate positions, however, can be used to generate sounds. Air rushing be- tween vocal folds pushes on the free edges, causing them to vibrate (see Video 1, https://youtu.be/7RV4mBk53kQ).
The vocal folds are not actually free string-like “cords,” but rather are shelves of tissue (thus the term “folds”). Therefore, as they oscillate, they repeatedly com-
©2017 Acoustical Society of America. All rights reserved. volume 13, issue 2 | Summer 2017 | Acoustics Today | 35