Fusion of the Pharyngeal Wall to the Soft Palate Around the Laryngeal Inlet Isolates the Airway in Odontocete Cetaceans.

Abstract

In odontocete cetaceans (toothed whales, dolphins, and porpoises), a palatopharyngeal sphincter permanently anchors the laryngeal inlet in the nasopharynx and isolates the airway from the gut tube. Using an MRI data set of a pacific white-sided dolphin (Aethalodelphis obliquidens), together with dissections of the dolphin and two neonatal harbour porpoises (Phocoena phocoena), we show that the sphincter is formed by fusion of part of the superior constrictor of the pharyngeal wall to the soft palate. The part of the superior constrictor that participates in forming the sphincter is brought close to the midline by the presence of palatine plates that: (1) project medially from the ventral aspects of the pterygoid bone on each side, (2) lie caudal to the palatine bones of the hard palate where the soft palate normally attaches in other mammals, and (3) divide rostral parts of the soft palate into a part dorsal to the plates and a part ventral to the plates. Based on the presence and the anatomy of major muscles of the soft palate, we present a biomechanical model for how the palatopharyngeal sphincter functions and is positioned during swallowing and breathing. We conclude that the sphincter is most contracted during swallowing to hold the laryngeal inlet in position while large prey items pass through the laryngopharynx, and is most relaxed during breathing when the laryngeal inlet maximumly opens to allow unrestricted airflow.