Possible age-related hearing loss (presbycusis) and corresponding change in echolocation parameters in a stranded Indo-Pacific humpback dolphin

The hearing and echolocation clicks of a stranded Indo-Pacific humpback dolphin (Sousa chinensis) in Zhuhai, China, were studied. This animal had been repeatedly observed in the wild before it was stranded and its age was estimated to be similar to 40. years. The animal's hearing was measured using a non-invasive auditory evoked potential (AEP) method. Echolocation clicks produced by the dolphin were recorded when the animal was freely swimming in a 7.5m (width) x22m (length)x4.8m (structural depth) pool with a water depth of similar to 2.5m. The hearing and echolocation clicks of the studied dolphin were compared with those of a conspecific younger individual, similar to 13 years of age. The results suggested that the cut-off frequency of the high-frequency hearing of the studied dolphin was similar to 30-40 kHz lower than that of the younger individual. The peak and centre frequencies of the clicks produced by the older dolphin were similar to 16 kHz lower than those of the clicks produced by the younger animal. Considering that the older dolphin was similar to 40years old, its lower high-frequency hearing range with lower click peak and centre frequencies could probably be explained by age-related hearing loss (presbycusis).