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Researchers find that beached dolphins are often deaf
Instead, researchers looked for reactions to the sound inside the animals' brains. The researchers affixed sensors to the creatures' heads with suction cups, which could detect electrical activity in the brain. They then played a series of tones: If the animals could hear them, the sensors would detect millions of neurons firing to process the sound.
But the researchers found severe to near-total hearing loss in two species. Among bottlenose dolphins, four out of seven stranded animals had hearing problems. Among rough-toothed dolphins, the total was five out of 14.
That, they said, could be a serious problem for animals that live in often-murky waters. Bottlenose dolphins often use sound to find each other: Each has a "signature whistle" all its own.
In addition to hearing sounds made by other creatures, dolphins use their own sonar to hunt and locate the seafloor and other objects. Scientists say these rapid-fire sounds - a series of clicks to human ears - return to give the dolphins information about the size and shape of prey.
"These animals are living in an environment where vision can't play the same role it does on land," said Randall Wells, a senior conservation scientist at the Chicago Zoological Society who was another of the study's authors. "Sound is probably the most important sense that they have."
Without the ability to hear these sounds, scientists said dolphins can be helpless. In some cases, the animals had lost more than 99 percent of their echo-locating capacity: If a normal animal could detect prey at 100 yards, these dolphins could do it only at a yard or less.
The research did not indicate what might have caused the animals to lose their hearing. Mann said he thinks the problem is most likely a combination of old age, birth defects and disease.
But other researchers have also identified a contentious and growing issue: too much noise in the ocean. Dolphins evolved when the only source of loud sounds underwater would have been thunderstorms or unusual events such as volcanic eruptions.
Now, however, there are the sounds of powerboats and huge oceangoing ships. Oil and gas exploration efforts use loud noises to conduct seismic tests of the seabed. Navy exercises fill the water with the sounds of explosions and sonar.
The association between marine mammal strandings and sonar has spawned several major lawsuits from the Natural Resources Defense Council and others. One resulted in an injunction against the Navy's use of sonar in some areas with high marine mammal populations. In 2008, the Supreme Court overturned that decision, saying the Navy needed an unfettered right to test sonar even if whales and dolphins might be harmed.
Other research has shown that North Atlantic right whales are making louder noises than in generations past, seemingly "shouting" to be heard over the ocean's background noise. Other work has predicted that as carbon emissions make the oceans more acidic, they may only conduct sound better - worsening the din.
In Sarasota Bay, Fla., home to about 160 dolphins, researchers have calculated that a powerboat passes within 100 yards of every dolphin every six minutes.
"These animals that are very finely tuned acoustic machines are now having . . . to deal with noises, with sounds that their ancestors never knew," Wells said. He said it's possible, but not certain, that chronic noise played a role in damaging some dolphins' hearing.
In the short term, Mann said he hopes the research will encourage organizations that rescue stranded dolphins to give the animals hearing tests. If the animals are found to have serious hearing damage, he said, they might be kept at aquariums or other protected locations instead of being released.
A spokeswoman for NOAA, which oversees a network of nonprofit and government marine mammal rescue operations, said these tests are done when resources permit. She said she was not sure what percentage of stranded animals got the test.
In Virginia Beach, the Virginia Aquarium and Marine Science Center oversees rescues for all dolphins and whales found along that state's coast. Aquarium official Mark Swingle said his group does not have the money, or the specialized training, to test the hearing of dolphins found alive.
But, he said, the number of dolphins affected was small, given the grim math of rescues. Out of 70 to 75 animals found stranded every year in his area, he said, only 1 or 2 percent were found alive.