Instead of wearing earplugs at a rock concert, imagine you could simply tune a dial inside your
ears to lower the volume-and protect your hearing. Four species of whales and dolphins can do
this naturally, new research reveals. This could potentially allow the animals to shield themselves
from the cacophony of Navy sonar and oil drilling, which has been linked to at least 500 marine
mammal deaths since 1963.
"The finding...is groundbreaking and will open up numerous lines of research in this field," says
zoologist Maria Morell of the Institute for Neurosciences of Montpellier in France, who was not
involved with the study.
Many species of whales and dolphins have supersensitive hearing because they use sound to
navigate, a process known as echolocation. They make clicks that they're able to hear bounce off
objects as small as a ping pong ball 20 meters away. Some hear high-pitch frequencies up to 100
kilohertz (kHz), which is about 80 kHz higher than the upper limit of human hearing.
This sensitive hearing makes them particularly susceptible to loud blasts of sound in the ocean.
For example, the U.S. Navy uses underwater sonar to find enemy submarines, underwater mines,
and determine water depth. The sonar pulses can be so loud that they cause temporary hearing
loss in some marine mammals, which may cause them to strand themselves on beaches and
In recent years, after being sued by environmental groups, the U.S. Navy agreed to curtail training
activities in important whale and dolphin habitats. Seismic surveys that use loud air guns to
search for oil and gas are also prohibitied from these areas, whereas elsewhere the seismic ships
are required to slowly increase the intensity of the air guns so animals in the area have time to
leave or adapt. But the effectiveness of these mitigation techniques has never been proved, and
the regulations are fickle, vacillating between prioritizing the environment, national security, and
the oil and gas industries.
In 2008, researchers at the University of Hawaii in Honolulu began to suspect some marine
mammals could protect their hearing naturally, at least in modest ways. the team used suction
cup electrodes to study marine mammal brain activity during echolocation. Their captive false
killer whale, a species of large dolphin, heard her outgoing clicks at a quieter level than equivalent
signals presented right in front of her, showing she could adjust her hearing sensitivity when
her trainers asked her to find something far away.
The team collaborated with scientists from Russia and the Netherlands to look for this effect in a
bottlenose dolphin, a harbor porpoise, and a beluga whale, in addition to extending the study on
the false killer whale. The scientists measured the animals' brain activity while hearing sounds
loud enough to evoke a response, but below the threshold of causing temporary hearing loss.
Each of the trained captive animals learned to reduce its hearing sensitivity by 10 to 20 decibels
when the scientists played a warning signal before producing the loud sound, the researchers
report in a paper in press at Integrative Zoology.
"That's similar to a human putting in foam earplugs," says team leader Paul Nachtigall, a marine
biologist and director emeritus of the Marine Mammal Research Program at the Hawaii Institute of
Marine Biology in Kailua. "It's really fascinating to be able to have that switch inside of your head."
Echolocating bats can dampen their hearing with a mescle reflex that happens independently of
sound frequency. But the researchers suspect marine mammals control auditory signals in the
brain by regulating the activity of receptor cells and thus sensitivity to the sound, rather than
relying on a muscle reflex.
This natural muffling ability suggests that military researchers and petroleum exploration
companies could produce warning signals at sea before setting off their test explosions, the team
suggests. But is this approach likely to be more effective than the current technique of slowly
raising the volume? That's "the empirical question," Nachtigall says. Another open question is
whether the work will translate to animals in the wild.
"Any efforts to come up with ways to mitigate impacts of human-caused noise is both needed and
welcomed," says Robin Baird, a biologist at the Cascadia Research Collective in Olympia, who
was not involved with the work. But he's skeptical that a warning signal would be an effective
mitigation technique, because whales and dolphins are typically transient species, except in very
specific circumstances. "I think there are subsets [of marine mammals] where this may be relevant,
but I don't see it as a panacea to solve the ocean noise problems."
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