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

The hearing abilities of two adult manatees were tested using a forced two-choice paradigm and an up/down staircase psychometric technique. Prior to this research, no hearing tests or audiograms had been measured for any Sirenian species. This test was also the first controlled underwater infrasonic hearing test conducted on any marine mammal. The audiogram demonstrated a wider range of hearing (0.5 to 38 kHz) and greater sensitivity (50 to 102 dB re:1muPa) throughout this range than had previously been suggested by averaged evoked potential and anatomical studies. The audiogram was a typical U-shaped mammalian hearing curve and was compared with other marine mammal underwater audiograms. Both manatees were most sensitive to frequencies between 12-18 kHz at 50-54 dB. The manatees' hearing sensitivity dropped more than 20 dB per octave below 1600 Hz. This higher frequency sensitivity may have evolved as an adaptation to a shallow water existence where low frequency sound propagation is limited. Limited hearing sensitivity at lower frequencies may be a contributing factor to the manatees' vulnerability to boat collisions.