Directional hearing sensitivity for 2–30 kHz sounds in the bottlenose dolphin (Tursiops truncatus)

Bottlenose dolphins (Tursiops truncatus) depend on sounds with frequencies <30 kHz for social communication and foraging, but little information on the directional dependence of hearing thresholds for these sounds exists. This study measured behavioral hearing thresholds for 2, 10, 20, and 30-kHz tones projected from eight different angular positions around the dolphin in both the horizontal and vertical planes to determine whether the receiving beam at these frequencies was directional. Omni-directional hearing was hypothesized for sounds below 30 kHz, but this hypothesis was rejected. Results from two bottlenose dolphins demonstrated a positive relationship between directivity and the frequency of the test tone, with asymmetric beam patterns. Directional hearing sensitivity declined most dramatically between 10 kHz and 2 kHz. The results suggest that dolphins’ directional hearing is more pronounced for lower frequencies than previously predicted.