The comparative anatomy of the nasal tract and the function of the spermaceti organ in the Physeteridae (Mammalia, Odontoceti)

The nasal complex of Kogia has been compared with that of Physeter and shown to be homologous with that of less specialized ondotocetes. The nasal complex of the Physeterids is structurally related to that of the Platanistids and Ziphioids. The spermaceti organ unique to the Physeteridae forms a part of the functional morphology of the right naris and is not a bilaterally symmetrical structure like the melon of other odontocetes. Separation of the two nares in the Physeteridae is caused by the asymmetrical development of the spermaceti organ. In Kogia part of the musculature of the left naris actuates the opening of the right naris, a feature unique amongst the Mammalia. The right naris is not used in external respiration but forms an air reservoir subservient to phonation at extreme depths. The spermaceti organ assists in evacuation of the lungs prior to a deep dive. The fat depots of the spermaceti organ in Physeter and the spermaceti organ and adipose cushion in Kogia may serve to absorb nitrogen at extreme pressure. The air reservoirs could perform a useful hydrostatic function during the ascent from great depths.