Beamwidth control and angular target detection in an echolocating bottlenose dolphin (Tursiops truncatus)
Biteplate apparatus used to ensure immobility of the dolphin during the experiment. Located directly below the biteplate is a set of receive hydrophones used to collect target echoes (data to be presented in a subsequent paper).
Experimental setup: the shaded rectangles correspond to spanners placed across the pen and from which the equipment was staged. Targets placed at were 9 m away from the dolphin. The hydrophone array was placed 1 m in front of the location of the dolphin’s phonic lips. The response paddle was placed to the right of the biteplate stationing apparatus. Targets were progressively moved either to the left or to the right. The angle of displacement at the point of a 50% detection probability was used to define the angular detection threshold.
The 24-element hydrophone array used to measure the beam pattern of the dolphin during target detection trials. The left graphic shows a planar display of the array arc (shown on the right). The star within the middle of the graphic denotes the hydrophone, the hydrophone that was aligned with the main axis of the dolphin to the target when the target was placed directly in front of the dolphin at .
Right/left angular detection thresholds of the bottlenose dolphin for two target shapes. Detection thresholds for the cylinder (CYL) are associated with the dotted lines and those for the sphere (SPH) are associated with the dashed line. Angular detection thresholds were (left) and (right) greater for the SPH than the CYL.
Change in the average number of clicks emitted per trial as a function of target displacement from . Lowercase letters over a displacement category indicate that a significant difference exists between it and another category [(a) “ left,” (b) “ left,” (c) “ right,” and (d) “ right”].
Change in the average peak level of clicks emitted per trial as a function of target displacement from . Lowercase letters over a displacement category indicate that a significant difference exists between it and another category [(a) “ left,” (b) “ left,” (c) “ right,” and (d) “ right”].
(a) Composite horizontal beam pattern from the bottlenose dolphin for targets presented directly in front of him (at ). Open circles correspond to the composite beam recorded in this study; filled circles correspond to the composite beam recorded by Au et al. (1986) from the same animal. (b) Representative beams for when the target was placed far to the left (open squares) or far to the right (filled squares) of .
Distribution of the vertical and horizontal beamwidths measured throughout all threshold session trials. Note the bimodal nature of the horizontal beamwidth distribution (between and ).
Distribution of the hydrophones that registered echolocation clicks as being on-MRA during threshold sessions (bars denote number of clicks). Gray squares correspond to the location of hydrophones within the array (see Fig. 3 for a reference of hydrophone location); black squares correspond to empty space. The hydrophones are separated by in the horizontal and in the vertical. The distributions correspond to the following test scenarios: (a) CYL target displaced to the left, (b) CYL displaced right, (c) SPH displaced left, and (d) SPH displaced right.
Characteristics of echolocation clicks produced during the threshold sessions. The degree of target displacement from corresponds to the following: , . Values are means .
Horizontal and vertical beamwidths of the dolphin echolocation beam measured during target (CYL and SPH) presentations to the right and left of . The degree of target displacement from corresponds to the following: , . Beamwidth values are means .
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