Range resolution in bottlenose dolphins: Effects of absolute target range and high-frequency hearing sensitivity

Sonar range resolution is defined by the smallest distance for which multiple targets along the same bearing can be discriminated from a single target. This study used a phantom echolocation task to measure range resolution in bottlenose dolphin biosonar. Dolphins were trained to produce an acoustic response when presented with a two-highlight echo (S + ) and to withhold the response for a single-highlight echo (S−). The internal delay of the S + was progressively reduced to determine the limits of range resolution. Measurements were conducted at simulated target ranges between 2.5 and 80 m. Two dolphins with normal high-frequency hearing resolved internal delays between 2 and 3 μs. A third dolphin with high-frequency hearing loss required internal delays between 3 and 5 μs to reliably identify the S + . These results are consistent with performance based on the perception of a low-pass filter effect at the highest audible echo frequencies, a result of spectral interference patterns in the S + . Unlike previous measures of the accuracy of range determination that were based on small changes in echo envelope timing, range resolution capabilities did not degrade as simulated target range increased. [Work funded by NIWC Pacific In-House Innovation Program.]