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First published online October 21, 2005
Journal of Experimental Biology 208, 4181-4188 (2005)
Published by The Company of Biologists 2005
doi: 10.1242/jeb.01876

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Hearing measurements from a stranded infant Risso's dolphin, Grampus griseus

Paul E. Nachtigall, Michelle M. L. Yuen^*, T. Aran Mooney and Kristen A. Taylor

Marine Mammal Research Program, Hawaii Institute of Marine Biology, University of Hawaii, PO Box 1106, Kailua, HI 96734, USA

^* Author for correspondence (e-mail: myuen{at}hawaii.edu)

Accepted 8 September 2005

An infant Risso's dolphin (Grampus griseus) was rescued from the beach in Southern Portugal, and an audiogram was measured using auditory evoked potentials (AEP) and envelope following response (EFR) techniques for frequencies from 4 to 150 kHz. The stimuli used were custom sinusoidally amplitude-modulated (SAM) tone-bursts, and the AEP responses were collected, averaged and analyzed to quantify the animal's physiological response and, thereby, hearing thresholds. The infant animal showed a wide range of best sensitivity, with the lowest threshold of 49.5 dB re. 1 µPa at 90 kHz. The audiogram showed a typical mammalian -shape with a gradual, low-frequency slope of 16.4 dB octave^-1 and a sharp high-frequency increase of 95 dB octave^-1. When compared with an audiogram of an older Risso's dolphin obtained using behavioral methods, the threshold values at upper frequencies were much lower for this infant animal, and this infant heard higher frequencies. These results redefine the hearing capabilities of Risso's dolphins by demonstrating very high-frequency sensitivity.

Key words: Risso's dolphin, Grampus griseus, hearing, audiogram, auditory threshold, auditory evoked potential, AEP, envelope following response, EFR, sinusoidally amplitude-modulated tone-burst, SAM

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