Sisters of the sinuses: cetacean air sacs.

This overview assesses some distinguishing features of the cetacean (whale, dolphin, porpoise) air sac system that may relate to the anatomy and function of the paranasal sinuses in terrestrial mammals. The cetacean respiratory tract has been modified through evolution to accommodate living in water. Lack of paranasal sinuses in modern cetaceans may be a diving adaptation. Bone-enclosed air chambers are detrimental, as their rigid walls may fracture during descent/ascent due to contracting/re-expanding air volumes. Flexible-walled "sinuses" (extracranial diverticula) are a logical adaptation to diving. Odontocetes (toothed whales) exhibit several pairs of paranasal air sacs. Although fossil evidence indicates that paranasal sinuses occur in archaeocetes (ancestors/relatives of living cetaceans), it is not known whether the paranasal sacs derive from these sinuses. Sac pigmentation indicates that they derived from invaginations of the integument. Unlike sinuses, paranasal sacs are not circumferentially enclosed in bone, and therefore can accommodate air volume changes that accompany diving pressure changes. Paired pterygoid sacs, located ventrally along the cetacean skull, connect the pharynx and middle ear cavities. Mysticetes (baleen whales) have a large midline laryngeal sac. Although cetacean air sacs do not appear to be homologous to paranasal sinuses, they may serve some analogous respiratory, vocal, or structural functions. For example, these sacs may participate in gas exchange, thermoregulation, resonance, and skeletal pneumatization. In addition, they may subserve unique aquatic functions, such as increasing inspiratory volume, mitigating pressure-induced volume change, air shunting to reduce respiratory dead space, and facilitating underwater sound production and transmission.