Differences between a successive versus simultaneous range discrimination task in an echolocating bottlenose dolphin (Tursiops truncatus).

A primary feature of bat and dolphin biosonar is the ability to measure echo-delay, both to determine absolute target range and to resolve range differences between targets. Measurements of range (i.e., echo-delay) discrimination thresholds in echolocating animals, however, vary across studies. This variance may reflect different test methods, such as whether the animal could simultaneously receive echoes from two compared targets (simultaneous condition), or only one target at a time (successive condition). The present study compares these test conditions directly in a dolphin. Dolphin echo-delay discrimination thresholds were measured under both simultaneous and successive conditions at simulated mean ranges of 3, 10, and 14 m to assess potential interactions across target range. The potential for spectral cues at various range differences under a simultaneous condition was also evaluated using a model of the dolphin peripheral auditory system. Results indicate that the availability of spectral cues during the simultaneous condition imparted limited improvement relative to the successive condition where spectral cues were unavailable. Echo-delay discrimination thresholds showed an overall increase with an increase in range in both the simultaneous and successive conditions.