The underwater audiogram of the West Indian manatee (Trichechus manatus)

The hearing thresholds of two adult manatees were measured using a forced-c hoice two alternative paradigm and an up/down staircase psychometric method . This is the first behavioral audiogram measured for any Sirenian, as well as the first underwater infrasonic psychometric test with a marine mammal. Auditory thresholds were obtained from 0.4 to 46 kHz, and detection thresh olds of possible vibrotactile origin were measured at 0.015-0.2 kHz. The U- shaped audiogram demonstrates an upper limit of functional hearing at 46 kH z with peak frequency sensitivity at 16 and 18 kHz (50 dB re: 1 mu Pa). The range of best hearing is 6-20 kHz (approximately 9 dB down from maximum se nsitivity). Sensitivity falls 20 dB per octave below 0.8 kHz and approximat ely 40 dB per octave above 26 kHz. The audiogram demonstrates a wider range of hearing and greater sensitivity than was suggested from evoked potentia l and anatomical studies. High frequency sensitivity may be an adaptation t o shallow water, where the propagation of low frequency sound is limited by physical boundary effects. Hearing abilities of manatees and other marine mammals may have also been shaped by ambient and thermal noise curves in th e sea. Inadequate hearing sensitivity at low frequencies may be a contribut ing factor to the manatees' inability to effectively detect boat noise and avoid collisions with boats. (C) 1999 Acoustical Society of America. [S0001 -4966(99)02005-6].