Bottlenose dolphin (Tursiops truncatus) discrimination of harmonic stimuli with range-dependent signal degradation

The importance of perceived sound source distance has been noted in controlled exposure studies with free-ranging marine mammals. Different behavioral reactions have been observed for sonar exposures with a similar received level but differing source distances. This psychophysical study examined bottlenose dolphins' use of range-dependent acoustic features in classifying frequency-modulated tonal stimuli (∼10-kHz fundamental). Repetitive tones with simulated range-dependent high-frequency attenuation (HFA) and reverberation (REV) were presented with roving levels (levels varied ±10 dB). The dolphins were trained to produce a phonic response upon hearing tones simulating relatively distant 30-km sources and to withhold response for closer-range tones. Once this behavior was reliably performed, probe trials with intermediate ranges were used to examine stimulus classification based on HFA and REV. Dolphins responded to nearly all probe trials with ranges of 10 and 20 km, while responses were less frequent at 1 and 2 km. Probes with HFA and REV decoupled from simulated source distance indicated that the dolphins used HFA to a greater degree than REV in response decisions. These results suggest that dolphins can classify harmonic signals based on range-dependent HFA and REV independent of received level, making these cues potentially useful in deciding behavioral reactions to acoustic sources.