Abstract:

Some indirect evidence exists for multiple sonar signal generators in odontocetes. Direct evidence was collected from two bottlenose dolphins by simultaneously measuring and digitizing internal nasal cavity pressure, nasal tissue motion, and acoustic pressure. Small catheters measured pneumatic pressure changes at the same depth within each bony nasal passage. A high-speed video endoscopy system revealed tissue motion at both pairs of phonic lips while two hydrophones measured acoustic pressure during biosonar target discrimination. The records clearly demonstrate that acoustic pulses can be generated at the phonic lips on the left and right sides, independently or simultaneously. We have only seen the left phonic lips generate whistles. Air pressure in the bony nasal passages rises and falls together, even if the activity patterns at the two pairs of phonic lips are different. Increasing pulse repetition rate or sound pressure level, and whistle production, are all normally accompanied by increasing nasal air pressure. Whistle production apparently requires greater nasal air pressure than does pulse generation. Discovering multiple sonar sources in a dolphin's head should cause us to reinterpret previous findings and re-evaluate physiologic limits on pulse repetition rate, signal bandwidth, frequency composition, and projection beam geometry. [Work sponsored by Office of Naval Research.]