Some dolphins are also born with impaired hearing; certain drugs used to treat the animals' other health problems can also cause hearing loss. Other causes are chronic exposure to noise (such as from shipping), or exposure to short-lived intense noise (such as explosions).
Many studies have investigated the effects of military sonar on dolphin hearing. "There's mounting evidence that midfrequency sonar may be impacting dolphins and whales," St. Leger said.
The animals may lose hearing for a short time and then recover -- the so-called rock-concert effect, Houser said. But they'd have to be pretty close to the source of the sonar and be exposed to it repeatedly, he added. Studies have shown temporary hearing loss from sonar, but less is known about its long-term effects. The bigger concern is how sonar could disrupt the dolphins' behavior. For example, the high-frequency pings can mask the calls of dolphins and whales and scare them away from their habitats.
As for deafness in dolphins, researchers are still trying to get a handle on the problem's prevalence, which may not be as pronounced as the PLOS ONE study suggested, Houser said. "But I think, in time, we're going to answer the question," he said.