Comparison of directional selectivity of hearing in a beluga whale and a bottlenose dolphin.

Hearing thresholds as a function of sound-source azimuth were measured in a beluga whale Delphinapterus leucas and a bottlenose dolphin Tursiops truncatus in identical conditions using the auditory evoked-potential method. In both the beluga whale and bottlenose dolphin, the receiving beam width narrowed with frequency increase. At all frequencies, the receiving beam was markedly wider in the beluga whale than in the bottlenose dolphin. In particular, the 3-dB beam width in the beluga whale narrowed from +/-33.5 degrees at 8 kHz frequency to +/-14.3 degrees at 128 kHz; the 6-dB beam width narrowed from +/-56.9 degrees to +/-18.9 degrees , respectively. In the bottlenose dolphin, the 3-dB beam width decreased from +/-19.9 degrees at 8 kHz to +/-6.3 degrees at 128 kHz; the 6-dB beam width decreased from +/-33.1 degrees to +/-8.4 degrees, respectively. In the bottlenose dolphin, the axis of the low-frequency receiving beam deviated from the midline up to 15 degrees; in the beluga whale, this effect was not detected. The audiograms of both the beluga whale and bottlenose dolphin were azimuth-dependent: from an audiogram featuring the best sensitivity at intermediate frequencies at 0 degrees to that featuring monotonous threshold increase with frequency increase at 90 degrees. In the beluga whale, this dependence was less prominent than in the bottlenose dolphin.