Hearing loss and echolocation signal change in dolphins

Abstract

Recent studies and ongoing research have shown that echolocating dolphins can change the structure of their emitted echolocation signals during active echo-investigation of targets. The presumption has been that the animal adjusts various parameters (source level, peak frequency, etc.) of the emitted signal to maximize the information return in the target echo as a function of task or environmental constraints and requirements. Other work has suggested that the frequency range over which this dynamic control is exerted may change due changes in the animals hearing ability. Specifically, dolphins that develop high frequency hearing loss, for example from age, noise exposure or ototoxic drugs, shift the center frequency of the emitted echolocation click to lower frequency ranges. Observations of several Navy Marine Mammal Program animals with known high frequency hearing loss have demonstrated these frequency shifts. In this paper we will elaborate and extend ongoing analysis of emitted echolocation signals of several dolphins that show hearing loss associated changes in emitted signal structure, discuss the implications of these measures and suggest approaches that may prove useful for evaluating basic hearing capabilities from collected echolocation signals.