Manatees get run down by motorboats because they can't hear them coming, a new study suggests.
"It's a common belief that they must be too slow or too stupid to get out of the way," says Edmund Gerstein of the Florida Atlantic University in Boca Raton. Yet the animals can swim at several metres per second and are intelligent enough to remember hazards and avoid them.
That led Gerstein to investigate the animals' hearing. His team trained two captive-born males, Stormy and Dundee, to take a hearing test. The manatees were taught to wait underwater for a strobe light to flash. If a tone preceded the light, the animals were trained to press a striped paddle. If there was no tone, they pressed a white paddle.
The researchers found the threshold intensities below which the animals could no longer hear different frequencies. Their most acute hearing was at about 17 kilohertz, with a threshold intensity of 50 decibels. In contrast, at 0á5 kilohertz the threshold was about 50 decibels higher-meaning that a sound had to be 100 000 times as intense before the animals could hear it (The Journal of the Acoustical Society of America, vol 105, p 3575).
The frequency of a boat's motor is roughly 2 kilohertz, which is at the poor end of manatees' hearing range. Boats that slow down in an effort to avoid manatees produce even lower-frequency sounds, making it more difficult for the animals. Near the surface of the water, low tones also propagate poorly because of a phenomenon called Lloyd's mirror effect, where soundwaves bouncing off the water's surface interfere with one another.
"This work hopefully will help solve this problem," says Whitlow Au, a bioacoustics expert at the University of Hawaii in Manoa. Gerstein is already working on a high-frequency sonic beacon that could be attached to boat motors in order to warn manatees away.
Author: Philip Cohen
New Scientist issue 31st July 1999
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