Functional morphology of the hyolaryngeal complex of the harbor porpoise (Phocoena phocoena): implications for its role in sound production and respiration.

In several publications, it was shown that echolocation sound generation in the nasal (epicranial) complex of toothed whales (Odontoceti) is pneumatically driven. Modern hypotheses consider the larynx and its surrounding musculature to produce the initial air pressure: (a) contraction of the strong pipelike palatopharyngeal sphincter muscle complex, which connects the choanae with the epiglottic spout of the larynx, should provide much of the power for this process and (b) muscles suspending the larynx/hyoid complex from the skull base and the mandibles may support these pistonlike laryngeal movements. Here, we describe the morphology and topography of the larynx, the hyoid apparatus, and the relevant musculature in the harbor porpoise (Phocoena phocoena) with respect to odontocete vocalization and respiration. We demonstrate that the hyoid apparatus, reminiscent of a "swinging cage," may not only be a stable framework in which the larynx can move but should support laryngeal actions by its own movements. Rostrocaudal relocations of the hyoid apparatus may thus support pistonlike actions of the larynx creating air flow into the nasal complex for sound production. The lift of the hyoid apparatus with the thick larynx in the direction of the skull base may squeeze the pharynx in the region of the piriform recesses and thus help to secure the (waterproof) tracheochoanal connection during respiration when the palatopharyngeal sphincter cannot be contracted maximally, because the air passage must remain open at the epiglottic spout.