SUMMARY

We investigated the possibility of conditioned dampening of whale hearing thresholds when a loud sound is preceded by a warning sound. The loud sound was a tone of 20 kHz, 170 dB re. 1 μPa, 5 s. Hearing sensitivity was measured using pip-train test stimuli and auditory evoked potential recording. The same test-sound stimuli served as warning sounds. The durations of the warning sounds were varied randomly to avoid locking an anticipated conditioning effect to the timing immediately before the loud sound. When the warning sound lasted from 1 to 9 s or from 5 to 35 s prior to the loud sound, hearing thresholds before the loud sound increased, relative to the baseline, by 12.7 and 7.3 dB, respectively. When the warning sound duration varied within a range of 20 to 140 s, the threshold increase was as low as 3.0 dB. The observed hearing threshold increase was not a result of the unconditioned effect of the loud sound, like a temporary threshold shift, so it was considered to be a manifestation of a conditioned dampening of hearing when the subject anticipated the quick appearance of a loud sound, most likely to protect its hearing.