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

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 microPa at 90 kHz. The audiogram showed a typical mammalian union or logical sum-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.