Anatomical Mechanism for Protecting the Airway in Whales Involves Interlocking the Larynx

Abstract

The mammalian respiratory and digestive tracts are configured to enable simultaneous function of both systems, while preventing accidental choking on food. This protection is critical in mysticetes (baleen whales), especially as feeding involves large pressures and potential inundation of water.Dissections were performed on 43 specimens representing 8 mysticete species: fin, blue, sei, minke, humpback, gray, right, and pygmy right whales. 36 specimens were necropsied as whole carcasses, 5 were obtained as whole heads, 2 were whole fetuses. All were recovered post mortem from US beach strandings. Dissections focused on hyolaryngeal anatomy, soft palate, and nasopharynx.Results indicate that the mysticete larynx retains an intranarial position during feeding, swallowing, breathing, and sound production. The soft palate forms a muscular ring that keeps the larynx interlocked into the nasopharynx. The epiglottis overlaps the soft palate extensively, nesting rostrally over a shelf that forms a pocket lined by nasopharyngeal pits. The soft palate articulates tightly with the epiglottic base, and lateral portions arc around larynx to meet in the caudal midline. Posteriorly, the soft palate forms another smaller shelf that interlocks with the caudally curling paired corniculate cartilages. These two points of interlocking keep the larynx intranarially positioned during deglutition. The paired lateral food channels (piriform sinuses) pass on either side of the epiglottis and corniculate cartilages and unite caudally to form the esophagus. Food can easily pass around the interlocked larynx using these lateral food channels.Suprahyoid muscles elevate the oral floor, squeezing out water and pushing the tongue towards the oral roof to compress the food bolus and force it through the oropharyngeal inlet. The oropharyngeal inlet appears regulated by muscles that likely relax to allow dilation. Suprahyoid muscles also ensure the larynx‐soft palate interlock, as the epiglottis is pushed rostrally to maximally overlap the soft palate. Food encountering the interlocked soft palate‐epiglottis is diverted laterally into paired lateral food channels. Infrahyoid muscles retract the larynx caudally, returning it to its rest position with moderate soft palate overlap. Continued contraction would allow the long epiglottis to slide along its overlap with the soft palate, giving mobility to enlarge the depth of the lateral food channels if necessary, without unlocking the larynx. This interlock preserves the protection of the airway.Unlocking the epiglottis necessitates elevating the soft palate while withdrawing the epiglottis. This may occur to channel air into the oral cavity for bubble cloud production. However, the larynx is at risk of inundation from water in this position, as it forms a very poor seal with the corniculate cartilages and their flexible flaps. The rigid epiglottis could not be folded caudally manually to meet the corniculates. No muscles were noted that could retract the epiglottis towards the corniculates. Similarly, no muscles were found that could rigidify the corniculate flaps or cause them to seal in opposition. Therefore, the sealing mechanism of the larynx is limited to opposition of the arytenoid cartilages supporting the U‐shaped vocal fold.