Abstract

Four female adult Pacific bottlenose dolphins (Tursiops truncatus) were kept in outdoor concrete pool for 13 years since they were subadults. Auditory evoked potentials (AEP) were recorded in response to sinusoidally amplitude modulated (SAM) tone bursts modulated at 1000 Hz. AEP thresholds were determined at 10, 30, 50, 60, 70, 100, 115, 130 and 150 kHz. Audiograms were similar to previous findings in bottlenose dolphin hearing tests. Hearing thresholds from 10 to 115 kHz were all less than 85 dB (re 1 µPa r.m.s), indicating good hearing ability within this range, while differences in higher frequency hearing were exhibited among these four dolphins. Dolphin No. 5 had higher hearing thresholds at 115 kHz (79.6 dB; others: 58.6–68.4 dB) and 130 kHz (120.1 dB; others: 69.4–79 dB) compared to the other three animals, which might be a mild high-frequency hearing impairment. Dolphin No. 5 is the only one individual having prior aminoglycoside (amikacin) treatment via intramuscular injection (3 periods, 40 days in total in 2013). The aminoglycoside amikacin has not only been implicated in the loss of hearing in humans but in odontocetes as well. Although there is no controlled study of the effects of aminoglycoside on sensorineural hearing loss in cetaceans, the possibility that exposure to aminoglycoside antibiotics in captive dolphins and stranded dolphins under rehabilitation might contribute to hearing loss in dolphins deserves consideration.

* Presenting author