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.