Scientists have long known that man-made, underwater noises — from engines, sonar, weapons testing, and such industrial tools as air guns used in oil and gas exploration — are deafening whales and other sea mammals.
The United States Navy estimates that loud booms from its underwater listening devices, mainly sonar, result in temporary or permanent hearing loss for more than a quarter-million sea creatures every year, a number that is rising.
Now, scientists have discovered that whales can decrease the sensitivity of their hearing to protect their ears from loud noise. Humans tend to do this with index fingers; scientists haven’t pinpointed how whales do it, but they have seen the first evidence.
“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, said he applauded the Hawaiian team for its “elegant study” and the promise of innovative ways of “getting at some of the noise problems.” But he 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 stranding and disorientation in creatures that rely on hearing to navigate, find food and care for their young.
In 2008, the US Supreme Court heard a lawsuit by the National Resources Defence Council against the US Navy over ocean noise; the court ruled that naval vessels had the right to test sonar systems for hunting submarines. But environmentalists saw a victory in getting the court even to consider the health of sea mammals in a debate over national security.
The latest development took place at a research facility off Oahu. Scientists there 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.
Nachtigall and his team found the animals could adjust their hearing in response to their sounds of echolocation, mainly sharp clicks. 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.
First, they 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.