Page 40 - Summer2017

Sound Production in Aquatic Mammals
Figure 4. Schematic drawing of the larynx (red) and the vocal tract (blue) in a mysticete whale. A: close-up of the larynx, indicating the U-shaped pair of vocal folds guarding the opening into the laryngeal sac and oriented parallel to the trachea. The front of the larynx is closed by opposition of the epiglottic and corniculate cartilages, caus- ing diversion of airflow (blue arrow) from the trachea into the laryn- geal sac. This airflow causes fold vibrations, generating low-frequency sounds (larger green arcs). Air in the laryngeal sac may be recycled back to the lungs to be used in the next vocalization (curved blue arrow). B: closed vocal folds and airflow passing above the epiglottis and between the paired flaps of the corniculate cartilage. These flaps may clap together to produce pulsed sounds (smaller green arcs). Printed with permission from © 2017 Mount Sinai Health System. Illustration by Christopher M. Smith.
Sirenians vocalize only underwater (Schevill and Watkins, 1965) and, as in most other semiaquatic and terrestrial mammals, the sound source appears to be the larynx (Reyn- olds and Odell, 1991). Although sirenian vocal folds are thick and don’t have a sharp edge, they lie perpendicular to tracheal airflow (Figure 2D) and can still oppose each other and regulate airflow to cause sound-generating vibrations (Landrau-Giovannetti et al., 2014). Sirenian sounds are not complex (listen to audio recording at http://acousticstoday. org/manatee) and the simple squeaks are probably used only for communication. The larynx is not well protected from water incursions (Figure 2D), thus risking drowning through parted vocal folds if the animal opens the mouth to vocalize. Therefore, sound production probably occurs only when the mouth is closed. The sound transmission path- way is not known but may involve transferring vibrations through the fat of the neck or the thin membrane covering the bony nasal aperture (an opening of the skull just behind the nostrils; Landrau-Giovannetti et al., 2014).
The evolutionary transition from a terrestrial life to a fully aquatic one appears to have had little effect on the mecha- nism of sound production in sirenians. Vocalizing still in- volves airflow, just as it did in their terrestrial ancestors. The
Figure 5. Schematic drawing of the larynx (red), nasal region (red and yellow), and vocal tract (blue) in an odontocete whale. A: close- up of the larynx, indicating a single vocal fold in the midline of the larynx. It is oriented parallel to the direction of airflow (blue ar- row). The airstream (double blue lines) is divided to flow along either side of the fold. Sounds are hypothesized to arise from fold vibrations (larger green arcs). B: the nasal region where echoloca- tion clicks and whistles are produced. Elevating the larynx pushes air toward the closed blowhole (crescent mark at the top of the head). Airflow (blue arrow) passes between the two pairs of phonic lips and causes high-frequency vibrations (smaller green arcs). These pulses pass into the adjacent melon (yellow oblong body) where they are focused into a beam to be transmitted out of the forehead. Air used in this process (curved blue arrows) is collected in the nasal air sacs (red) and may be recycled back to be used again in the next pho- nation. Printed with permission from © 2017 Mount Sinai Health System. Illustration by Christopher M. Smith.
vocal folds have returned to their original role of providing protection from incursions (particularly during swallowing) while keeping the new role of producing sound (although not simultaneously with swallowing; Reidenberg and Lait- man, 2010).
Cetaceans are also fully aquatic mammals that vocalize underwater. Unlike sirenians, cetaceans make complex sounds including clicks and whistles (see the review in Tyack and Clark, 2000; listen to audio recordings at http:// acousticstoday.org/mmammals). As in other terrestrial and aquatic mammals, cetaceans also use a pneumatic mechanism to generate sound. However, their respiratory tract is exquisitely protected from incursions of water (Figure 3). Therefore, simultaneous vocalizing and underwater open-mouthed behaviors (e.g., sucking in water during prey capture) do not risk the animal drowning.
Cetaceans use airflow to cause respiratory tissues to vibrate, generating pressure waves in both the surrounding tissues and air in the supralaryngeal vocal tract. Unlike terrestrial mammals, it is the vibrations in the tissues rather than in
38 | Acoustics Today | Summer 2017