Visualizing dolphin sonar signal generation using high-speed video endoscopy

Abstract

Determining the site of the delphinid sonar signal generation has eluded cetologists for decades. Activities within the pharyngeal and nasal cavities of two bottlenose dolphins (Tursiops truncatus) were studied during sound production and echolocation. A high-speed dual-camera video system provided synchronized windows for recording two concomitant events: (1) movements visible through an endoscope and (2) oscilloscope traces of acoustic pressure at a hydrophone placed near the animal’s head. Dolphins have two tissue complexes, one located on either side, and just above, the membranous nasal septum [Cranford et al., J. Morph. 228, 223–285 (1996)]. They apparently generate acoustic pulses by pushing air across sets of internal ‘‘lips.’’ The acoustic pulse occurs coincident with one oscillatory cycle of the lips. Changes in the acoustic pulse repetition rate and the vibration cycles of the lips are simultaneous, indicating that their rates and periods are synchronous. No other structures were found to vibrate in synchrony 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.]