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the supralaryngeal vocal tract air column that are propa- gated through the cetacean head or neck and emitted into the water. These tissues have a density close to seawater, thus enabling the efficient transfer of sound (i.e., acting as a transducer with minimal transmission loss at the density interface). The mechanism of sound generation, while still pneumatic, differs between mysticetes and odontocetes. These two taxa have evolved along different trajectories (Fig- ure 3) using different sources within the respiratory tract (Figures 4 and 5) that produce sounds on opposite ends of the frequency spectrum (mysticetes generate infrasound, odontocetes generate ultrasound).
Mysticetes (Baleen Whales)
Mysticetes are well-known for their ability to sing underwa- ter (listen to audio recordings at http://acousticstoday.org/ baleen). Because low-frequency sounds travel farther than high frequencies before attenuating, mysticetes generally use low frequencies (infrasound) to communicate over long distances. Their sounds can thus be heard many miles away. Mysticetes, similar to other mammals, produce these sounds laryngeally.
Mysticetes have evolved enhanced protection of the larynx from water incursions (Figure 3A). Although all mammals protect the larynx from the front using the epiglottic carti- lage, mysticetes have evolved tall corniculate cartilages that also protect the larynx from behind (Reidenberg and Lait- man, 2007). This creates circumferential protection around the larynx, keeping its opening completely interlocked with the nasal region. The cartilages are sandwiched between the soft palate and the posterior pharyngeal wall. Thus, food and water are swallowed past the larynx but cannot slip through the interlock and end up accidentally inhaled.
The mysticete larynx is both absolutely and relatively large, being larger than either one of the whale’s lungs (Reidenberg and Laitman, 2010). The increased size is correlated with loud and low-frequency sounds; bigger vocal folds and larg- er resonant volumes correspond with longer wavelengths and increased amplification (Aroyan et al., 2000; Cazau et al., 2013).
Mysticetes have vocal folds that make a U-shape (Reidenberg and Laitman, 2007). Interestingly, the vocal folds do not lie in the typical terrestrial position perpendicular to tracheal airflow (Figure 1). Instead, they are rotated approximately 90° (counterclockwise or anticlockwise, if viewed from the left side of the animal) to lie parallel to the long axis of the
trachea (Figure 3A). The ventral (inferior) attachment has moved caudally (rearward) from the thyroid cartilage to the base of the trachea, and the dorsal (superior) attachment has moved ventrally (inferiorly) and caudally with elongation of the arytenoid cartilage. Air can still flow through the gap between the vocal folds and into the laryngeal sac (a pouch located just below the larynx; Figure 4A).
Parted vocal folds allow air to flow between the lungs and the laryngeal sac. Vocal folds are controlled by cartilages that are moved by muscle contractions, similar to those of other mammals. Regulating vocal fold opening, orientation, thickness, and tension can control airflow volume and rate, setting the whale’s folds in motion to produce the fundamen- tal frequency (Adam et al., 2013; Cazau et al., 2013, 2016).
The laryngeal sac is very stretchy, but its volume can be com- pressed by a thick surrounding muscle layer (Reidenberg and Laitman, 2008). The sac provides an important reservoir for increasing total air volume available during diving for use in both respiratory and buoyancy control (Gandilhon et al., 2014). The laryngeal sac expands as air is used during vocal- ization, and on contraction it recirculates the air back to the lungs to be used again in the next vocalization (Figure 4A). The inflated air sac and contiguous nasal passageways may also serve as resonant spaces because larger volumes mean that the whale can project longer, louder, and lower sounds. This ability can advertise the whale’s size and strength par- ticularly in diving whales, where the air volume is severely constrained by deep depths. Laryngeal sac walls may act as a pulsing drumhead to transfer vocal fold vibrations through the overlying throat muscles, blubber, and skin and into the water (Reidenberg and Laitman, 2007).
The mysticete larynx also has a second, perhaps unique, site that may also generate sound: a pair of broad tissue flaps supported by the corniculate cartilages (Figure 4B). These flaps lie in opposition across the laryngeal opening and may clap together to generate pulsed sounds (Reidenberg and Laitman, 2007). Mysticetes have been documented to pro- duce pulses (e.g., Thompson et al., 1992). This is intriguing because it supports the idea that mysticetes may echolocate, an idea originally proposed by Norris (1969). Infrasonic fre- quencies have long wavelengths and therefore may only be useful in conveying the presence of large or distant objects, e.g., seamounts or schooling prey. Bowhead whales, for ex- ample, may navigate around ice obstacles using cues from the echoes of their vocalizations (George et al., 1989; see Video 3, https://youtu.be/rysR5SNwrn8).
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