Determination of the delphinid sonar signal generation site has eluded cetologists for decades. Recent interest in developing a bionic sonar system has reinvigorated the effort to characterize the apparatus and its operation. We studied activities within the pharyngeal and nasal cavities of two bottlenose dolphins Tursiops truncatus during echolocation. A high-speed dual-camera video system provided synchronized windows for recording two concomitant events: movements visible through an endoscope and oscilloscope traces of acoustic pressure at a hydrophone placed near the animal's head. Dolphins have two tissue complexes (Cranford et al. 1996), one located on either side, and just above, the membranous nasal septum. They apparently generate acoustic pulses by pushing air across sets of internal flips.' The acoustic pulse occurs coincident with one oscillatory cycle of the lips. Changes in acoustic pulse repetition rate and the lip's vibration cycles are simultaneous, indicating that their rates and periods are synchronous. We did not find other structures in the airways vibrating synchronously with each acoustic pulse generation event. The palatopharyngeal muscle complex compresses air for the system. These observations settle a long-standing controversy over the site of biosonar signal generation in odontocetes and open a vista of potential avenues for future investigations. (Work supported by the Office of Naval Research)
Abstract: