Volume control for whales

“It’s equivalent to plugging your ears when a jet flies over,” said Paul E. Nachtigall,a marine biologist at the University of Hawaii who led the discovery team. “It’s like a volume control.”

The finding,while preliminary,is already raising hopes for the development of warning signals that would alert whales,dolphins and other sea mammals to auditory danger.

Peter Madsen,a professor of marine biology at Aarhus University in Denmark,cautioned against letting the discovery slow global efforts to reduce the oceanic roar,which would aid the beleaguered sea mammals more directly.

The noise threat arises because of the basic properties of seawater. Typically,light can travel for hundreds of feet through ocean water before diminishing to nothingness. But sound can travel for hundreds of miles.

The world’s oceans have been getting noisier as companies and governments expand their undersea activities. Researchers have linked the growing racket to deafness,tissue damage,mass strandings and disorientation in creatures that rely on hearing to navigate,find food and care for their young.

Scientists at a research facility off Oahu are studying how dolphins and toothed whales hear. In nature,the mammals emit sounds and listen for returning echoes in a sensory behaviour known as echolocation. In captivity,scientists taught the creatures to wear suction-cup electrodes,which revealed the patterns of brainwaves involved in hearing.

The discovery came in steps. First,Nachtigall and his team found that the animals could adjust their hearing in response to their own loud sounds of echolocation. The scientists then wondered if the animals could also protect their ears from incoming blasts.

The team focused on a false killer whale named Kina and sought to teach her a conditioned behaviour similar to how Pavlov taught dogs to salivate upon hearing a bell.

First,the scientists played a gentle tone repeatedly. Then they followed the gentle pulse with a loud sound. After a few trials,the warning signal alone caused Kina to decrease the sensitivity of her hearing.

“It shows promise as a way to mitigate the effects of loud sounds,” said Nachtigall. In May,he and his colleagues presented the findings to acoustic scientists and groups meeting in Hong Kong,including the Acoustical Society of America. The team cited the protective deafening as a potential way to help sea mammals cope with noisy blasts from naval sonars,civilian air guns and other equipment.

In the future,the team plans to expand the research to other species in captivity and ultimately to animals in the wild.

Scientists say the extraordinary hearing of sea mammals evolved to compensate for poor visibility beneath the waves and to take advantage of the unique qualities of seawater. Sound travels five times faster than in air and undergoes far less diminishment.

The heads of whales and dolphins are mazes of resonant chambers and acoustic lenses that give the animals not only extraordinary hearing but complex voices. The distinctive songs of humpback whales appear to be sung exclusively by males seeking mates.

In recent decades,scientists have linked the human cacophony to reductions in mammalian vocalisation,which suggests declines in foraging and breeding. And the problem is poised to get worse: In May,the U.S.Navy disclosed draft environmental impact statements (Atlantic and Pacific operations) that said planned expansions could raise the annual hearing losses among sea mammals to more than one million.

Nachtigall said the research was costly because sea mammals need high levels of care. “I’m pulling in money where I can,” he remarked.

But he called it revealing and rewarding. “When it comes to whales and sound,” Nachtigall said,“we’re just starting to understand.”

In September 2002,more than a dozen beaked whales beached themselves in the Canary Islands. Rescuers tried to water down the stranded animals and keep them cool. But all of them eventually died.

Nearby,NATO naval forces were testing sonar devices meant to detect enemy submarines,and public knowledge of the deaths eventually came to strengthen suspicions of a link between whale distress and loud ocean noises.

For decades,environmentalists have worked to reduce the undersea din—usually with little success,given the growing industrialisation and militarisation of the oceans. They have filed lawsuits and waged letter-writing campaigns,including a recent petition that asks the Navy to drop its testing of underwater sound equipment.

The discovery by biologists in Hawaii that whales can decrease the sensitivity of their hearing to protect their ears from loud noise adds another dimension to the debate.

“A lot more work needs to be done,” said Michael Jasny,a senior policy analyst with the Natural Resources Defense Council,in an interview. “Could it be replicated in the wild? It’s a huge question.”

“It’s important to understand that it’s limited,” he said of the proposed method. “It won’t be a silver bullet.”