Page 43 - Summer2017

ly, a path for conducting incoming sounds has been modeled for the throat region of deep-diving odontocetes (Cranford et al., 2008). Perhaps the same tissue properties that channel sounds into the head may allow sound to travel out as well.
Although laryngeal sound generation (and transmission) is anatomically possible, this has not yet been demonstrated in living odontocetes. The overwhelming evidence from stud- ies of captive dolphins and other odontocetes indicates that the laryngeal region remains quiescent during sound gen- eration while the nasal region is active (reviewed in Cran- ford and Amundin, 2003). Therefore, the main functions of the larynx appear to be limited to protecting the respiratory tract while feeding and channeling air to, or pressurizing the air in, the nasal region during sound production.
Odontocete Nose
The odontocete nose is highly modified from the typical ter- restrial nose. As in all whales, it is located on the top of the head, enabling efficient breathing at the surface without hav- ing to lift the head out of the water. The nostrils (blowholes) are paired in mysticetes, but they have become fused into only one opening (blowhole) in odontocetes.
Immediately below this nostril are two nasal passageways and a complex arrangement of air sacs, nasal plugs, and fatty structures (Figure 5). These epicranial structures are homologous to the facial tissues (e.g., lips, nasal cartilages, facial muscles) of terrestrial mammals, as evidenced by their pigmented surfaces (indicating that they derive from tissues whose skin coverings were located on the outside of the face in an ancestor). Some of this tissue comprises the muscular nasal plugs that close the nasal passageways to protect them from water while submerged or that can retract to expose them for breathing. The remainder of these tissues are in- volved in sound generation, including bilaterally paired sets of air sacs, a large fatty structure called the melon, and two sets of paired fat bodies called “phonic lips” that are the so- nar sources (Cranford, 1999; Figure 5B).
Each phonic lip opposes its pair and, as air passes between them, they can either clap against each other to create clicks or channel air along ridges of their surface to generate whis- tles (Figure 5B). There is only one pair of these phonic lips in sperm whales (see Video 4, https://youtu.be/sW7o5IC2io0). High-speed video endoscopy on live dolphins shows that phonic lip vibrations correlate with produced sounds, and the two sets of paired phonic lips can operate independently (Cranford et al., 2011; see Video 5, which is on the linked
page http://acousticstoday.org/bdolphin). This confirms that dolphins can use both pairs of phonic lips to produce two different frequency click trains simultaneously or emit whistles while clicking).
Air is next directed into (usually) three pairs of air sacs locat- ed underneath the blowhole (Mead, 1975; Rodionov, 2001; Figure 5B). The blowhole is closed during sound produc- tion, thus preventing water flooding in or air escaping out. As in mysticetes, inflating air sacs delays system pressuriza- tion (thus allowing continued airflow for sound production) while capturing air for reuse. Muscles covering the nasal sacs contract, sending air back to the lungs. The air is recycled, producing more sounds as it flows again through the phonic lips and into the nasal sacs.
The phonic lips vibrate very quickly, generating ultrasonic click trains that are then transmitted as vibrations to at- tached fat bodies called dorsal bursae. Vibrations are trans- ferred through the anterior dorsal bursa to a contiguous, but larger, fat body in the forehead area called the “melon” (Harper et al., 2008; McKenna at al., 2012; Figure 5B).
The melon is a biconvex shaped organ, comprised largely of isovaleric acid (Koopman et al., 2003) that is of similar density to seawater. It can therefore transfer sound waves to water with minimal transmission loss (from reflection, re- fraction, impedance mismatch, or attenuation). The chemi- cal structure of the melon may modify the sounds, perhaps filtering out certain frequencies (Koopman et al., 2003). The shape of the melon may be altered through facial muscle contractions to act as a lens that focuses the width and di- rection of the sound beam.
Odontocetes use these sounds to explore their environment. The emitted sound travels forward and reflects from objects in the path. The returning echoes are used to understand surface characteristics, composition, and location/move- ment of nearby objects, thus allowing navigation or prey capture (Tyack and Clark, 2000).
Conclusion/Summary
The transition from a terrestrial to a semiaquatic lifestyle did not result in substantial changes in vocalizations mecha- nisms because semiaquatic mammals continue to vocalize largely in air. Underwater sound production in semiaquat- ic mammals involved redirecting the sound transmission pathway through overlying tissues without radical changes to the larynx. Even the transition to a fully aquatic life was
Summer 2017 | Acoustics Today | 41