Beamwidth control and angular target detection in an echolocating bottlenose dolphin (Tursiops truncatus)

Bottlenose dolphin (Tursiops truncatus) echolocation beams are typically characterized as symmetrical -3 dB beamwidths; however, the functional width of the beam during target detection has not been explored. Angular target detection thresholds of an echolocating dolphin were examined to more fully describe the functional characteristics of the echolocation beam. The dolphin performed an echolocation detection task with its head held in a fixed orientation. Targets were placed 9 m in front of the dolphin [0 degrees position (P(0))] and systematically moved right or left until target detection reached chance probability. A 24-element hydrophone array placed 1 m in front of the dolphin was used to measure vertical and horizontal echolocation beamwidths. Detection thresholds were 26 degrees left and 21 degrees right of P(0) for a sphere target and 19 degrees left and 13 degrees right of P(0) for a cylinder target. Estimates of maximum horizontal and vertical beamwidths ranged up to 40 degrees and 29 degrees , respectively, and exhibited large variability. The dolphin nominally steered the maximum response axis of the echolocation beam up to 18 degrees in the horizontal, 12 degrees in the upward vertical, and 4 degrees in the downward vertical. These results suggest that the dolphin can steer and modify the width of the echolocation beam.