Hearing abilities and acoustic signalization of the bottlenose dolphin Tursiops Truncatus with early hearing loss

Common bottlenose dolphins ( Tursiops truncatus ) are among non-human mammals that possess the ability for vocal production learning, which strongly depends on learning from conspecifics. The hearing sensitivity and acoustic signals of a 4-year-old captive-born bottlenose dolphin suspected of severe hearing loss were investigated. True deafness was confirmed using a non-invasive auditory evoked potential (AEP) recording method in response to rhythmic click stimuli. The deaf dolphin showed no response compared to normal-hearing animals. The vocal repertoire of the deaf dolphin was analyzed and compared to that of two normal-hearing conspecifics. Acoustic activity recordings of the dolphins were conducted individually in a partially isolated pool. The deaf dolphin produced whistles, clicks and burst pulses typical of bottlenose dolphins. However, his whistles were primarily combined with clicks, and we could not identify his individually distinctive signature whistle. Besides, the three dolphins shared a unique combined signal. The fundamental frequency of its flat whistle component matched the frequency of the trainer’s bridge signal. Our results indicate that the dolphin’s deafness likely occurred after birth, prior to signature whistle development. This research provides valuable insights into the impact of hearing loss on acoustic signalization in bottlenose dolphins.