Variability in Click-Evoked Potentials in Killer Whales (Orcinus orca) and Determination of a Hearing Impairment in a Rehabilitated Killer Whale

Abstract

An immature female killer whale (Orcinus orca) stranded in the Wadden Sea in 2010 and was later transferred to Loro Parque, Tenerife, Spain, for rehabilitation. The killer whale, named “Morgan,” was suspected to have a hearing impairment. To test whether Morgan has a hearing deficit, auditory brainstem responses to short-duration, broadband click stimuli were recorded. The same procedure was conducted with five other killer whales at Loro Parque for comparative purposes. Stereotypical click-evoked responses were recorded in all of the killer whales except Morgan, even at the highest click level that could be projected. Reductions in the amplitude of the click-evoked response paralleled reductions in the stimulus amplitude of the clicks presented to all of the other whales. The lack of a click-evoked response in Morgan indicates that she suffers from a hearing deficit. The magnitude and frequency range over which the hearing deficit occurs cannot be specified with the techniques used here. Nevertheless, it can be concluded that Morgan’s hearing sensitivity to broadband signals is at least 20 to 30 dB worse than the hearing sensitivity of the other killer whales tested. Morgan potentially suffers from a profound hearing deficit or even a complete loss of hearing, but this cannot be determined through the electrophysiological tests used in this experiment.