Hearing sensation change with loud sound warning in the false killer whale

Abstract

Work on hearing during echolocation has demonstrated that a whale was capable of changing its hearing sensitivity while it echolocated, perhaps to protect its hearing from its own intense emitted pulses. Would a whale similarly change hearing sensitivity when warned prior to receiving a loud sound? Hearing was measured using auditory evoked potentials (AEP). The whale had been trained to station within a hoop while wearing surface electrodes. Baseline AEP dependence on test-sound level and an auditory threshold were first established for a 20 kHz tone. In a second phase, the test sound was followed by a sudden increase in amplitude up to 170 dB re 1 μPa. Thus, the faint test sounds took on the role of a warning signal for the ensuing loud (unconditioned) sound. After a few trials, the test stimuli revealed a substantial reduction of hearing sensitivity before the loud sound. When the delay between the warning tone onset and loud tone was short (varied randomly from 1 to 9 s), the whale increased its hearing thresholds (reduced sensitivity) by around 13 dB. The data indicate that: (1) the whale learned to change hearing sensitivity when warned that the loud sound was about to arrive, and (2) the learning acted only when warnings were immediate.