Abstract

A previous study indicated no automatic gain control (AGC) in the auditory system of a harbor porpoise (Phocoena phocoena) as revealed by recording auditory evoked potentials to simulated echoes (Beedholm et al., 2006). The same harbor porpoise did change the rate and amplitude of its echolocation clicks during stationary echolocation when presented with an artificial target at a fixed delay. The animal spontaneously changed its click rate in such a way that the emitted level (in dB, arbitrary reference) of a click decreased as the inter-click interval (ICI) decreased (click emission rate increases), according to a 14.5 log (ICI) function. This same relationship was found when the animal swam toward a target (a fish). It reduced the amplitude of its clicks as it approached the target with a -14 to -17 log r (best-fit), which is close to the expected -20 log r found in other studies. The combined results indicate an incomplete AGC working on the transmitter side and might be explained by constraints in the sound production apparatus that couple the sound amplitude to the click rate.