Page 38 - Summer2017

Sound Production in Aquatic Mammals
Figure 1. Schematic drawing of the larynx in a deer. Enlargement shows the upper laryngeal cartilages (epiglottic and corniculate) in- terlocking with the soft tissues surrounding the nasopharynx (soft palate and posterior pharyngeal wall). This interlock divides the air- space into two separate pathways for breathing/vocalizing (shown in gray) and swallowing (not shown because they are lateral to the lar- ynx but then merge in the midline to become the esophagus shown in yellow behind the cricoid cartilage). The right vocal fold is indicated in the inset, spanning between the thyroid and arytenoid cartilages. Vocal fold position is schematized in the whole head as a line span- ning from point 1 (blue; at front of larynx) to point 2 (red; at back of larynx). The paired vocal folds lie perpendicular to the long axis of the trachea, a pattern found in most terrestrial mammals. Air flows upward and between the two vocal folds to the supralaryngeal vocal tract, causing vibrations that generate the fundamental frequencies of vocalizations. Printed with permission from © 2017 Mount Sinai Health System. Illustration by Christopher M. Smith.
press air in the space surrounding the folds (vocal tract). These compressions generate the fundamental frequency, i.e., pressure waves in air. The waves propagate along the airspace above the larynx (supralaryngeal vocal tract) to- ward the nose and/or mouth for release as a vocalization.
Characteristics of the fundamental frequency are deter- mined by variable parameters such as the size of the gap between the folds and the tension, elongation, and stiffness of vocal fold tissue (Dickson and Maue-Dickson, 1982). The fundamental frequency is then modified by the geometry of the supralaryngeal vocal tract. This results in a complex sound (e.g., fundamental frequency plus harmonics) emit- ted from the mouth and/or nose. The vocal folds are the “source” of the vibrations that generate the fundamental fre- quency, and the supralaryngeal vocal tract is the “filter” that modifies the emitted sounds.
Semiaquatic Mammals
Mammals that spend a considerable amount of time both on land and in water are termed “semiaquatic” (e.g., hippopota- mus, seal, fur seal, sea lion, walrus, polar bear, various otters, various aquatic rodents). Performing underwater open-
36 | Acoustics Today | Summer 2017
Figure 2. Schematic drawing of the larynx in a hippopotamus (A), seal (B), sea lion (C), and manatee (D). Note that the epiglottis over- laps with the soft palate. This provides protection from swallowed water or food, diverting it away from the laryngeal opening. In the rear of the larynx, the corniculate cartilage is opposed to the posterior wall of the pharynx. It provides good protection in the hippo (A) but does not have enough overlap to provide protection for pinnipeds (B and C) or the manatee (D). Vocal fold orientation is indicated by a line connecting points 1 (blue; attached to thyroid cartilage) and 2 (red; attached to arytenoid cartilage). In hippos (A), the folds are reoriented (counterclockwise or anticlockwise, as viewed from the left side of the animal) to be approximately parallel to tracheal airflow. The thyroid cartilage attachment is extended inferiorly. This pattern is very similar to that in mysticete whales (see Figure 3A). Note that the vocal folds are oriented perpendicular to tracheal airflow in the seal (B) and manatee (D), showing a pattern similar to that found in terrestrial mammals (see Figure 1). The vocal folds are also reorient- ed nearly parallel to airflow in the sea lion (C) due to elongation of the arytenoid cartilage. This pattern is very similar to that in odon- tocete whales (see Figure 3B). Printed with permission from © 2017 Mount Sinai Health System. Illustration by Christopher M. Smith.
mouthed/open-nosed behaviors (e.g., sucking in water) can risk drowning. However, as long as the larynx is closed, the respiratory tract is protected. However, this will also prevent sound production underwater because the vocal folds are sealed against each other. Accordingly, these semiaquatic mammals vocalize in air as terrestrial mammals do. Vo- calizations in the air can be emitted nasally (e.g., whining, whistling) or orally (e.g., barks, growls, and roars). Regard- less of the transmission path, it involves the same pneumatic mechanism of generating vibrations of the vocal folds in the larynx, filtering sounds in the supralaryngeal vocal tract, and emitting them in air from the nose or mouth (Reiden- berg and Laitman, 2010).