Evoked-potential audiogram of an Indo-Pacific humpback dolphin (Sousa chinensis)

Summary An evoked-potential audiogram was measured for an Indo-Pacific humpback dolphin (Sousa chinensis) living in the Dolphinarium of Nanning Zoo, China. Rhythmic 20 ms pip trains composed of cosine-enveloped 0.25 ms tone pips at a pip rate of 1 kHz were presented as sound stimuli. The dolphin was trained to remain still at the water surface and to wear soft latex suction-cup electroencephalography (EEG) electrodes used to measure the animal's envelope-following evoked potentials to the sound stimuli. Responses to 1000 rhythmic 20 ms pip trains for each amplitude/frequency combination were averaged and analysed using a fast Fourier transform to obtain an evoked auditory response. The hearing threshold was defined as the zero crossing point of the response input-output function using linear regression. Fourteen frequencies ranging from 5.6 to 152 kHz were studied. The results showed that most of the thresholds were lower than 90 dB re. 1μPa (root mean square, r.m.s.), covering frequency range from 11.2 to 128 kHz, and the lowest threshold of 47 dB was measured at 45 kHz. The audiogram, which is a function of hearing threshold-versus-stimulus carrier frequency, presented a 'U'-shape with a region of high hearing sensitivity (within 20 dB of the lowest threshold) between approximately 20 and 120 kHz. At frequencies lower than this high-sensitivity region, thresholds increased at a rate of approximately 11 dB/octave, up to 93 dB at 5.6 kHz. The thresholds at high frequencies above 108 kHz increased steeply with a rate of 130 dB/octave, up to 127 dB at 152 kHz.