Dolphins killer sonar confirmed
US researchers may have finally proved what many have long suspected - that dolphins are capable of stunning their prey using a blast of sound.
This week's New Scientist reports that Ken Marten of Earthtrust in Hawaii and Denise Herzing from Florida Atlantic University now have hard proof of this theory that has been around for almost twenty years.
In work submitted to The Journal of Aquatic Mammals, Marten and Herzing claim to have hard evidence of the behaviour captured acoustically and visually on videotape.
"It's the first time anyone's got shots like this," Marten says.
Marten 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's hearing apparatus. He has now taped a dolphin emitting a sequence of low- frequency "bangs" while chasing a fish.
In a further experiment, Marten showed that low sounds with similar acoustic properties to dolphins' clicks disorientated anchovies to the point where they swam in circles, remained still or died. "It could also mess up their schooling," 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, giving the dolphin time to catch them.
"Dolphin sonar is more complex than any man made technology," Wendy Dunn of the Dolphin Research Unit told ABC Science Online. "What this is all about is understanding the properties of dolphin sonar and what is capable of doing."
"If the researchers have actually been able to combine an acoustic recording with a visual recording, and been able to use artificially produced dolphin clicks to stun anchovies, then that will be a first."
"This sort of thing has been suspected for so long. To actually get some confirmation of this would be really exciting," she says.
Other researchers remain unconvinced, says New Scientist. "If they can prove it, I'd hate to be negative," 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. "The underwater world is unfamiliar to us and you have to be careful with interpretations," he says.