Those friendly clicks dolphins make could actually be weapons in an impressive hunting arsenal, say American researchers.
The theory that dolphins stun their prey with sound has been around for almost twenty years, but no one had ever seen them do it, or even shown that the type of sounds dolphins make can stun or kill fish.
Now Ken Marten of Earthtrust in Hawaii and Denise Herzing from Florida Atlantic University say they have hard evidence on videotape. 鈥淚t鈥檚 the first time anyone鈥檚 got shots like this,鈥� Marten says.
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Most data in the field comes from researchers on boats recording dolphin sounds beneath and guessing what was going on. But Marten says he has taped a dolphin emitting a sequence of low- frequency 鈥渂angs鈥� while chasing a fish.
He had noticed before that dolphins close to herring would emit low bangs at the frequency the fish hear best at, and had suggested the bangs were designed to damage the fish鈥檚 hearing apparatus. He thinks this is happening on his tape. 鈥淪he was aiming right at him. Pow, pow, pow,鈥� he says.
Swimming in circles
In a further experiment, Marten showed that low sounds with similar acoustic properties to dolphins鈥� clicks disorientated anchovies to the point where they swam in circles, remained still or died. 鈥淚t could also mess up their schooling,鈥� he says.
Meanwhile, Herzing has found evidence of a different strategy. She recorded wild Atlantic spotted dolphins emitting a medium-frequency buzz while searching for prey in sand on the seabed. She says buried eels jumped out of the sand, and either stopped completely or moved sluggishly as if they were stunned. This gave the dolphin time to catch them. 鈥淢aybe it was just too loud in there,鈥� she says.
Other researchers remain unconvinced. 鈥淚f they can prove it, I鈥檇 hate to be negative,鈥� says Pete Tyack from the Woods Hole Oceanographic Institution near Boston.
But Tyack thinks the dolphins may be using sound merely to locate their prey, not to stun it. 鈥淭he underwater world is unfamiliar to us and you have to be careful with interpretations,鈥� he says.
Marten and Herzing have submitted their work to The Journal of Aquatic Mammals.