London: Sound plays a key role in the world of whales and dolphins – they hunt and navigate by listening for echoes.
Navigating in this way requires super-sensitive hearing. And scientists have found that, for some whales, this sense is adjustable.
Researchers in Hawaii measured the hearing of a female false killer whale, and found that she could fine-tune her most crucial sense.
The whale would “turn down” her hearing when she anticipated a loud noise.
Dr Paul Nachtigall from the University of Hawaii led the research, working with Kina, a trained false killer whale.
He and his colleague, Prof Alexander Supin from the Russian Academy of Sciences, first noticed five years ago that Kina might have the ability “to control the level of her hearing”.
The scientists were monitoring Kina’s hearing as she hunted.
False killer whales belong to a group of species known as “toothed whales, which includes dolphins, sperm whales and killer whales. These mammals hunt using echolocation - producing high-frequency buzzing or clicking sounds and decoding the echoes they produce to locate prey.
Echolocating clicks pass through a fatty structure at the front of their skulls called the melon. It is this structure that forms a visible bulge on the animal’s head and researchers say it acts as an adjustable acoustic lens, focusing the sound into a beam and altering the size of that beam.
Other non-echolocating marine mammals have different tricks for finding their way underwater. Seals, for example, have super-sensitive whiskers, which can detect the fattest fish by sensing the trail they leave behind
What lives in the open ocean?
To study Kina’s hearing, the researchers needed an insight into what was happening inside her head.
“Her whole head is an ear. There are many paths for sound to travel up to her actual ears,” explained Dr Nachtigall.
He and Prof Supin placed sensors contained within soft latex suction cups on Kina’s body to measure the electrical activity in Kina’s brain as it responded to sound.
“Louder sounds make big brain waves, quieter sounds make smaller waves. [And] if she does not hear the sound we do not see the pattern,” said Dr Nachtigall.
The researchers played Kina a “neutral tone” - an innocuous bleep - then followed that with a five-second pulse of 170 decibels. That is approximately equivalent in intensity to the sound of a rifle being fired one metre away.
Over time, Kina learned that this neutral tone was a warning signal and turned down her hearing sensitivity when she heard it, so in subsequent experiments, the sensors recorded a smaller signal from a noise of the same loudness.
Dr Nachtigall explained that echolocating marine mammals might have evolved this rapidly adjustable hearing to protect themselves from their own clicks and buzzes.
“They sounds they produce are very loud - they can be over 230-decibel pulses, and then must listen immediately for very quiet echoes,” Dr Nachtigall told BBC Nature.
The team hopes that their findings will eventually be applied to the protection of wild marine mammals.
There is evidence that whales and dolphins are disturbed or damaged by man-made undersea noise, such as naval sonar and the loud seismic airguns used in oil and gas exploration.
“[This] makes us very optimistic that many echolocating porpoises dolphins and whales will be able to change their hearing to protect it if they are properly warned. We want to define the proper way to warn them,” said Dr Nachtigall.
The researchers presented their findings at the Acoustics 2012 meeting in Hong Kong.