UC San Diego

Dolphins use an advanced biosonar system to accurately navigate and forage in their environment. They have the unique capability to rapidly detect and classify underwater targets in a complex acoustic environment and do so more precisely than man-made underwater sonars. Dolphins use the fine-scale temporal and spectral features of the echo-waveforms to determine different attributes of the target and rely on echo-delay, the time between their emitted sound pulse “click” and the return of the reflected echo to determine the range to a target. In this dissertation, phantom echoes are used to determine the effect that changes in mean echo-delay, i.e. range, have on echo-delay discrimination abilities and which fine-scale acoustic features dolphins use to create a coherent mental representation of a target. In Chapter 1, the dolphin’s ability to detect changes in echo-delay as a function of range was determined. Chapter 2 then tested the dolphin’s ability to discriminate between the ranges of two targets at different mean ranges. In this experiment targets were presented successively, meaning, the dolphin had to hold the range of one target in memory while they compared it to the second target. Chapters 1 and 2 conclude that as mean range increases greater than 10 m the dolphin’s ability to discriminate between echo-delays degrades rapidly. Chapter 3 explores the possibility of the dolphin using spectral cues to determine the difference in range between two targets that are presented simultaneously. First, the dolphin’s peripheral auditory system was modeled to determine available spectral cues. The dolphin’s ability to discriminate the difference in range between two targets was then tested and compared under successive and simultaneous conditions. Although spectral cues were available to the dolphin, the results from the simultaneous condition suggest limited improvement when compared to the successive condition. Lastly, Chapter 4 focuses on the auditory perceived cues the dolphin may use to convert fine-scale spectral and temporal information into a coherent mental representation. By manipulating echo-phase information the results suggest that for fine-scale echo-delays the dolphins may use a pitch cue to discriminate between complex targets.