Evoked-potential audiogram of an Indo-Pacific humpback dolphin (Sousa chinensis).
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 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. 1 μPa (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.
Marine Mammal Research Laboratory, Tropical Marine Science Institute, National University of Singapore, 18 Kent Ridge Road, Singapore 119227. tmsshl@nus.edu.sg
Songhai
S
Li
Ding
D
Wang
Kexiong
K
Wang
Elizabeth A
EA
Taylor
Emilie
E
Cros
Wenjing
W
Shi
Zhitao
Z
Wang
Liang
L
Fang
Yuefei
Y
Chen
Fanming
F
Kong
D000161:Acoustic Stimulation; D000818:Animals; D001299:Audiometry / Q000379:methods*; D002681:China; D004289:Dolphins / Q000502:physiology*; D005072:Evoked Potentials, Auditory / Q000502:physiology*; D005843:Geography; D006309:Hearing / Q000502:physiology; D008297:Male; D010140:Pacific Ocean; D013018:Sound Spectrography; D014828:Vocalization, Animal / Q000502:physiology; D058067:Wavelet Analysis