Dec 2017 Science
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 die.
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 prohibited 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 prioritising 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 she knew the impending sound would be loud. The dolphin also increased 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 harbour 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. “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 muscle 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.