New research into the cause of dolphin "strandings" - incidents in which weakened or dead dolphins are found near shore - has shown that in some species, many stranded creatures share the same problem.
They are nearly deaf, in a world where hearing can be as valuable as sight.
That understanding - gained from a study of dolphins' brain activity - could help explain why such intelligent animals do something so seemingly dumb. Unable to use sound to find food or family members, dolphins can wind up weak and disoriented.
Researchers are unsure what is causing the hearing loss: It might be old age, birth defects or a cacophony of man-made noise in the ocean, including Navy sonar, which has been associated with some marine mammal strandings in recent years.
The news, researchers say, is a warning for those who rescue and release injured dolphins: In some cases, the animals might be going back to a world they can't hear.
"Rehab is pretty time-consuming and pretty expensive," said David Mann, a professor at the University of South Florida and the study's lead author. If the dolphin can't hear, he said, "there's almost no point in rehabbing it and releasing it."
The study, published Nov. 3 in the journal PLoS One, examined several species of marine mammals - including dolphins and small whales - in the Gulf of Mexico, Atlantic Ocean and Caribbean Sea. The animals had been found stranded in the wild and taken in for medical treatment and feeding.
Each year, 1,200 to 1,600 whales and dolphins are found stranded off the U.S. coast, according to the National Oceanic and Atmospheric Administration. Most are dead: In 2007, the most recent year with data, 195 out of 1,263 animals were found alive.
But many are euthanized on the scene or die later. Others survive but are too young or too debilitated to be returned to the wild.
Of the 195 animals found alive that year, five were released.
Trying to study what put these animals in distress, the researchers faced a puzzle. How do test a dolphin's hearing?
"They can't raise their flipper" if they hear a tone, Mann said.
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.
In some of the species they studied, the tests showed that stranded animals could still hear normally. Three Risso's dolphins, two pygmy killer whales and a spinner dolphin showed no problems.
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.