Toothed whales navigate through sometimes dark and murky waters by emitting clicks and then interpreting the pattern of sound that bounces back. The animals' hearing can pick up faint echoes, but that sensitivity can be a liability around loud noises. Now researchers have discovered that whales may protect their ears by lowering their hearing sensitivity when warned of an imminent loud sound.
The scientists will present the finding at the Acoustics 2012 meeting in Hong Kong, May 13-18, a joint meeting of the Acoustical Society of America (ASA), Acoustical Society of China, Western Pacific Acoustics Conference, and the Hong Kong Institute of Acoustics.
"Whale hearing may be the most fascinating marine mammal sense," says Paul Nachtigall, a biologist at the University of Hawaii who started his career studying otter vision, but soon switched to whale and dolphin echolocation. Earlier experiments by Nachtigall and his colleagues suggested that whales can actively shield their hearing from loud outgoing echolocation clicks, which can reach sound levels equivalent to a rifle fired right next to the ear. The scientists wondered if the animals could similarly protect their ears from incoming loud noises.
The team repeatedly played a short warning sound followed by a loud sound to a false killer whale working in the laboratory in a floating facility off Coconut Island at the Hawaii Institute of Marine Biology. They measured the animal's hearing sensitivity by placing suction-cup sensors on the skin of the whale's head and recording the frequency of its brainwaves. Initial results indicate that the whale significantly reduces its hearing sensitivity when warned that a very loud noise is about to arrive.
"It appears as though the whales learn this pairing of warning signal and loud sound rapidly through classical conditioning," says Nachtigall. Many human activities such as oil exploration and the use of ships' sonar create loud noises in the ocean. If wild whales could quickly learn the meaning of a short warning sound, the technique might help lessen the impact of anthropogenic noise on marine mammals, Nachtigall notes.
Whales' auditory systems are similar to other mammals' in structure, but larger in size, most likely because they have evolved for the special task of echolocation. Right now scientists aren't certain how the whales are able to adjust their hearing sensitivity. Bats, which also use sound to navigate, automatically contract muscles in their ears to reduce hearing sensitivity while they produce loud outgoing echolocation calls. Nachtigall says that whales may do something similar, but that understanding the mechanism will take considerably more work. "We think -- based on much of our echolocation work -- that it is much more than a simple reflex," he says.
So far, the team has only tested the warning signals with one false killer whale. In the future they would like to test additional species of marine mammals. "We are also very interested in how long it takes a naïve animal to learn to lower its sensitivity when warned," Nachtigall says.