HORIZONTAL ANGULAR DISCRIMINATION BY AN ECHOLOCATING BOTTLENOSE DOLPHINTURSIOPS TRUNCATUS

Abstract

A bottlenose dolphin was tested on its ability to echoically discriminate horizontal angular differences between two arrays of vertical, air-filled, PVC rods. The blindfolded dolphin was required to station in a submerged, vertically-oriented hoop, 2 radial metres from the stimuli, and indicate whether an array with four rods (S+) was to the left or the right of an array with two rods (S-), by pressing a corresponding paddle. The angular separation between the rods within each array (6w) was maintained at 2 degrees but the angular separation between the two arrays (6h) was varied to produce angular differences (b.6 = eb-ew) ranging from 0.25 degrees to 4 degrees. A modified method of constant stimuli was used to test for angular discrimination ability, and yielded a psychometric function having a 75% correct threshold of 1.6 degrees. This threshold fell between the passive listening minimum audible angle thresholds of 0.9 degrees for click signals and 2.1 degrees for a pure tone signal (Renaud & Popper 1975). Analyses of response times, number of clicks and inter-click intervals failed to detect any significant adaptive behaviour occurring as the task became more difficult. These results help to define angular resolution capabilities of dolphin sonar that may play an important role in representing spatial information in the dolphin's environment.

Dolphin sonar—modelling a new receiver concept

“…Thus, the system is in principle capable of unambiguously localizing a target to an accuracy of a tenth of a degree. In practice, noise and asymmetry between the two rows of teeth limit performance, and bottlenose dolphins are found to have an accuracy and resolution of about ±1 • when actively echolocating (Branstetter et al 2003).…”