19 June 2003



 Why Dolphins Aren’t Deaf


  Dolphins, bats, and submarines all send out high-pitched sounds to deftly navigate and hunt in their dark habitats. But loud sounds can hurt sensitive ears, just as a bright light can be painful to humans. Now scientists have uncovered a handy technique dolphins use to avoid deafening themselves with the echoes from their sonar.

The discovery was made by Whitlow Au and Kelly Benoit-Bird of the University of Hawaii's Institute of Marine Biology in Kaneohe. While cataloguing the echolocation sounds made by several dolphin species around the world, they noticed that all the animals seemed to use a similar pattern of signals. As the dolphins approached the hydrophones used to record their sonar clicks, the sounds became progressively softer. The volume dropped by 6 decibels every time they covered half the distance to the hydrophones, the researchers report in the 19 June issue of Nature.

This habit solves a potential problem. In order to sense a distant object, dolphins need to emit a strong signal. But the same strong signal bouncing off an object nearby would create a deafening echo, says Au, so dolphins use quieter signals as they get closer to their prey. It's possible that dolphins regulate the signal because they can't adjust the sensitivity of their ears, as bats do by flexing their middle ear muscles, says Au.

"It is surprising to see results with such consistency from field work from three different species," says Bertel Møhlof the University of Aarhus in Denmark. "The research implicates new mechanisms in the control of biosonar signals. It opens a new window of opportunity for biosonar research."


Related sites
Hawaii Institute of Marine Biology
Dolphin Research Center


 © 2002 by the American Association for the Advancement of Science.