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

Abstract

SUMMARYAn 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 EEG electrodes used to measure the animalʼs 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.1Pa (r.m.s.), covering a 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 -1 up to 93dB at 5.6kHz. The thresholds at high frequencies above 108kHz increased steeply at a rate of 130dBoctave -1 up to 127dB at 152kHz.

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

“…Three suction-cup electroencephalography (EEG) electrodes were non-invasively attached to the back of the dolphin for the AEP recording. Examination of electrical noise before the hearing experiment confirmed that the electrical noise level for the AEP recording was comparable to that in our previous study (Li et al, 2012). The background acoustic noise in the experimental pool was recorded before the hearing measurement commenced and when the water depth in the pool was ~2.5m.…”

“…To better evaluate the dolphin's hearing, its echolocation clicks were also recorded on 15 April 2012. Both the hearing and echolocation clicks of the present subject were compared with those of a conspecific younger male, ~13years old, which was rescued from stranding on the coast of Beihai Bay, China (Fig.1B), in August 2007 (see Li et al, 2012). The methodology and equipment used for the two dolphins was the same or comparable.…”

“…During the hearing measurement, the method for sound stimulus presentation and calibration, AEP recording and hearing threshold determination was the same as previously described (Li et al, 2012) and is briefly described here. The sound stimuli were rhythmic pip trains composed of cosine-enveloped 0.25ms tone pips with a 1kHz pip rate and a variable carrier frequency.…”

“…1μPa) of the projecting sound stimuli were measured and calibrated in root mean square (r.m.s.) of the whole pip train, including both the pips and inter-pip pauses (Li et al, 2012), by positioning a calibrated receiving hydrophone at the same location as the animal's lower jaw area. Carrier frequencies varied from 5.6 to 128kHz, and were chosen to be 5.…”

“…The hearing and echolocation clicks of the subject were compared with those of a conspecific younger individual that was recently investigated using the same methods (Li et al, 2012). The comparisons of hearing and echolocation click parameters between the two individuals provide a unique opportunity to understand and interpret the hearing capabilities of the recently stranded dolphin, and to understand how changes in hearing might affect echolocation click production in the Indo-Pacific humpback dolphin.…”