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How Dolphins Steer Their Sonar
18 March 2009 (All day)
Dolphins and their close relatives that use sound to navigate can "steer" their sonar beams by merging two sound pulses together, a new study suggests. "It's the acoustic equivalent of moving your eyes without moving your head," says marine biologist Marc Lammers of the Hawaii Institute of Marine Biology at the University of Hawaii, Kaneohe. This ability may be unique in the animal kingdom, scientists say.
Biologists have long known that odontocetes, or "toothed whales," a group that includes sperm whales, beluga whales, and dolphins, navigate and hunt using sonar. Like bats, they emit high-frequency clicks that bounce off objects and then interpret the echoes. Until recently, biologists believed that sonar was a bit like a pair of headlights--it could be aimed only in the direction in which the creature's head was pointed. But in 2008, Patrick Moore of Space and Naval Warfare Systems Command, a U.S. Navy research facility in San Diego, California, and colleagues found that bottlenose dolphins can sweep their echolocation beams to the left and right by about 20° without moving their heads. What Moore's team couldn't determine was just how the animals were doing it.
One idea was that the dolphin manipulates a large fatty organ in its head called the melon to aim the beam. Alternatively, scientists posited that the dolphin was producing two sonar clicks separated by a slight time delay. This would cancel frequencies in certain regions of space but enhance them in others, effectively channeling the sound in a direction other than straight ahead.
The latter theory appears to be correct, according to a study that will be published this summer in Biology Letters. In the work, Lammers and Manuel Castellote of L'Oceanogràfic aquarium in Valencia, Spain, trained a 9-year-old beluga whale to echolocate on command and then measured the sounds using two underwater hydrophones--one placed directly in front of the whale and another one that could be moved in an arc around the whale's head. The hydrophones picked up two distinct sonar pulses produced less than a second apart.
After ruling out the possibility that the second pulse was from reflections inside the whale's head or off the walls of the pool, the scientists concluded that both pulses were produced by the whale itself. Past anatomical studies revealed that toothed whales have two sound generators--called "phonic lips"--inside their heads. But it was unknown whether the creatures were using only one or both of the lips, either independently or together. Now it seems they work in concert. "This work presents the first empirical demonstration of the double-pulse component being a real phenomenon," Lammers says.
Although the new study does not show that belugas, like dolphins, can actually steer their sonar beams, "the work demonstrates for the first time a plausible mechanism for how moderate beam steering ... can come about," says whale researcher Magnus Wahlberg of the University of Southern Denmark in Odense.
Lammers speculates that whales developed the ability to steer their beams as a way of expanding their acoustic field of view. "Many dolphins can't move their necks," he says. "Beam steering could allow them to 'look' around" without reorienting their entire bodies. An echolocation beam produced using both phonic lips might also travel farther and scan more frequencies than a beam produced using only one lip, he says.
Lee Miller, a biosonar expert also at the University of Southern Denmark who studies echolocation in whales as well as bats, says beam steering may be unique to toothed whales. Bats shouldn't be able to do it, he says, because they have only one sound generator: their vocal cords.