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Figure 3. Schematic drawing of laryngeal position in a baleen (mys- ticete) whale (A) and an toothed (odontocete) whale (B). Note that the epiglottic and corniculate cartilages provide protection to the la- ryngeal entrance by interlocking into the nasal region through over- lap with the soft palate and posterior pharyngeal wall. This interlock enables function of the vocal folds during open-mouthed behaviors that would otherwise risk drowning. Vocal fold position is indicated by a line spanning from point 1 (blue) to point 2 (red). In mystice- tes (A), the folds are rotated approximately 90° (counterclockwise or anticlockwise, if viewed from the left side of the animal) from the terrestrial position (Figure 1) to be approximately parallel to the long axis of the trachea. The front attachment point has left the thyroid cartilage and been extended rearward, all the way to the trachea. This may be related to expansion of the laryngeal sac (the pouch lo- cated under the larynx). In odontocetes (B), the fold is rotated in the exact opposite direction (clockwise). The front attachment has left the thyroid cartilage and is extended further forward along the epiglot- tis. In both mysticetes and odontocetes, the rear attachment is placed more inferiorly due to elongation of the arytenoid cartilage. The ceta- cean patterns involve rotations in opposite directions, but both place the vocal folds parallel to the long axis of the trachea. Printed with permission from © 2017 Mount Sinai Health System. Illustration by Christopher M. Smith.
Hippopotamuses have adapted to produce sounds in air, underwater, or simultaneously both above and below water (Barklow, 2004). Interestingly, the vocal folds are oriented parallel to the tracheal airflow (Figure 2A), a condition seen again in their close relatives the cetaceans (whales, including dolphins and porpoises; Figure 3). As in other semiaquatic mammals, hippos use a terrestrial mechanism to transmit sound to air: they vibrate the vocal folds to generate a funda- mental frequency and modify it in the supralaryngeal vocal
tract. The larynx is protected by tall cartilages that interlock the larynx with the nasal cavity (Figure 2A). This ensures that air is directed into the larynx while food is swallowed laterally around the interlock. This protection also enables hippos to vocalize during underwater open-mouthed behav- iors. Underwater sound emission likely involves the transfer of vibrations through the overlying throat tissues of the ven- tral neck (see Video 2, https://youtu.be/VqxHMDIfxkk).
Pinnipeds (seals, fur seals, sea lions, walruses) also produce sounds with the larynx (Poulter, 1965) both in air and un- derwater. These sounds can be in the form of songs, pulses, sliding chords, and bell-like sounds (see review in Ballard, 1993). (Readers interested in hearing recordings can go to http://acousticstoday.org/pinni where many of these sounds can be found. For more information on how marine mam- mals use sound for communication, see the article by Peter Tyack in this issue of Acoustics Today.)
Pinniped vocal folds may be oriented perpendicular relative to tracheal airflow (e.g., seal; Figure 2B) in a terrestrial pat- tern (e.g., deer; Figure 1) or parallel (e.g., sea lion; Figure 2C) in a cetacean pattern (e.g., dolphin; Figure 3B). The pin- niped larynx is not well protected from flooding with water because the protective cartilages surrounding the opening are very short, particularly in the rear of the larynx (Figure 2B,C). Therefore, open-mouthed behaviors with open vo- cal folds could result in the animal drowning. It follows that underwater laryngeal sound production is risky and occurs primarily when the mouth is closed. Some pinnipeds evolved a clever “work-around” solution involving pharyngeal or tracheal membranes, valves, or diverticulae (see review in Reidenberg and Laitman, 2010). These secondary vibrators enable sound production during open-mouthed behaviors without risking drowning because the vocal folds of the lar- ynx can remain closed. The transmission route into water, however, is still unknown because pinniped vocal anatomy is not well understood. It is likely that pharyngeal/tracheal vibrations are transmitted through adjoining neck tissues. This ability may differ between the sexes because males often have thicker necks than females.
Fully Aquatic Mammals
Fully aquatic mammals are restricted to living in water only and include sirenians (manatees and dugongs) and cetaceans (mysticetes or baleen whales and odontocetes). Fully aquatic mammals emit sounds nearly exclusively underwater; how- ever, they still generate them with a pneumatic mechanism.
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