Comparative Anatomy of the Larynx in Pinnipeds (Seal, Sea lion, Walrus)

Abstract

Pinnipeds (e.g., seal, sea lion, walrus) are marine mammals that spend time both in and out of the water. All are able to vocalize in air using the pneumatically driven vocal fold vibrations, a mechanism common to terrestrial mammals. This is no surprise, as their ancestors were once terrestrial mammals. Pinnipeds, however, are also able to vocalize underwater. It is not known how they generate or transmit these sounds. This study explores pinniped laryngeal anatomy in: harbor seal (Phoca vitulina), gray seal (Halichoerus grypus), elephant seal (Mirounga angustirostris), California sea lion (Zalophus californianus), and walrus (Odobenus rosmarus). Comparisons were made with the laryngeal anatomy of several closely related terrestrial species including: dog (Canis familiaris) and black bear (Ursus americanus), and several semiaquatic species including: polar bear (Ursus maritimus), sea otter (Enhydra lutris), river otter (Lontra canadensis), beaver (Castor canadensis), muskrat (Ondatra zibethicus), and hippo (Hippopotamus amphibius). Comparisons were also made to fully aquatic mammals that can vocalize underwater including: manatee (Trichechus manatus), baleen whales (Caperea marginata, Balaena glacialis, Megaptera novaeangliae, Balaenoptera musculus, B. physalus, B. borealis, B. acutorostrata), and over 20 species of toothed whales (including sperm whales, beaked whales, dolphins, and porpoises). Results indicate that the harbor and gray seals have an unremarkable larynx that appears grossly similar to that of most other mammals. Elephant seal, sea lion, and walrus larynges, however, were unusual in having arytenoid cartilages that are extremely large and rounded on their rostral surface. The arytenoids form a valve across approximately half of the laryngeal lumen. Interestingly, this feature also occurs in sea otters. The corniculate and cuneiform cartilages are difficult to detect in the elephant seal, sea lion, and walrus. The epiglottic cartilage is very short in the sea lion and walrus, similar to the manatee, but robust in the elephant seal. The vestibular fold of the elephant seal is very prominent, and appears connected to the arytenoid cartilage. Harbor seal, gray seal, and walrus vocal folds are oriented perpendicular to airflow, as in other terrestrial mammals. Elephant seal and sea lion vocal folds are rotated, compared with other terrestrial mammals, with the ventral attachment located more rostrally. Their vocal folds are thus positioned near the ventral aspect of the larynx. This orientation is similar to those of the hippo, baleen whales, and toothed whales. None of the pinnipeds have vocal folds resembling the smooth sloped vocal folds of the manatee, or the thickened vocal folds of the polar bear. The unusual arytenoid size and vocal fold orientation in some pinnipeds may indicate evolutionary convergence with other underwater laryngeal vocalizers (e.g., hippo, baleen whales). It is unclear whether laryngeal morphologies of elephant seal, sea lion, and walrus are functional adaptations representing autapomorphies (uniquely derived features) of these species. Further work should be directed towards understanding whether laryngeal specializations may relate to variations in underwater sounds or in‐air vocalizations. More pinniped species need to be studied, particularly deep diving seals that are very vocal (e.g., Weddell seal) or fur seals (having external morphology and behaviors very similar to sea lions).This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.