Sound production mechanisms have been studied in two delphinid species - the harbour porpoise, Phocoena phocoena (L.), and the bottlenosed dolphin, Tursiops truncatus (Montagu). It was found that, in both species, the click sound production was coupled to a considerable pressure increase in the bony nares. The maximum pressure recorded in Phocoena was approximately 54 kPa and in Tursiops close to 81 kPa; it was equal in time and amplitude in both nares. The nasal plug muscle was found to be active up to 450 ms prior to and during sound production. Sound production without such activity was not seen. The results suggest that an identical mechanism underlies click production in both species, with pressurized air being the driving force and the nasal plug muscle having some active regulating function.

Probes were inserted into the bony nares of three harbour porpoises, Phocoena phocoena, and one bottlenosed dolphin, Tursiops truncatus, in order to record air pressure variations together with sound production. Sounds were picked up by a hydrophone manually held to the forehead of the animals. In several of the Phocoena recordings, electromyographic activity in the nasal plug muscle was also recorded.

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