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HEAR ME NOW? A trained bottle-nosed dolphin takes a hearing test as sensors pick up changes in the animal's brain waves that indicate sound detection. Hovering near recordings of sonar pings caused a temporary impairment in the sounds the animal could hear.A. Mooney

Exposing a bottle-nosed dolphin to a couple of minutes of nearby sonar pinging could cause some temporary hearing impairment, a new test finds.

What sonar does to marine mammals has become a hotly debated topic in recent years, as beaked whales and other species have stranded in shallow water and died after naval training exercises in the area.

For the first time, researchers have played recordings of actual naval sonar to a marine mammal and tested its hearing after progressive step-ups in intensity over a couple of months, says Aran Mooney of the Woods Hole Oceanographic Institution in Massachusetts.

Working with a bottle-nosed dolphin trained in the protocols of hearing tests, Mooney and his colleagues played a series of recordings of sonar pings, building up to an intensity that mimicked a ship some 40 meters away. (The bottle-nosed dolphin was free to swim farther away from the noise. It didn’t, but then again researchers were offering it fish.)

After hearing 15 pings at 203 decibels, the animal’s threshold for detecting sounds had shifted up some six decibels, Mooney and his colleagues report online April 7 in Biology Letters.

“It’s a rock-concert effect,” Mooney says. After listening to blasting music, fans may notice a cottony quality to their hearing, as if something’s blocking their ears. The effect diminishes in time. And the bottle-nosed dolphin’s hearing also returned to normal typically after about 20 minutes, Mooney reports.

That’s a mild threshold shift, and physiological changes caused by sonar may not be as important as the animals’ behavioral reactions, comments veterinarian and stranding specialist Paul Jepson of the Institute of Zoology in London. Startled animals may panic and surface so fast that they get decompression sickness, which would explain some of the damage Jepson has seen.

The conditions causing the shift in this study would also be “pretty unlikely” in the wild, Jepson says. Animals probably wouldn’t linger so close to a ship if the noise bothered them.

The test results fit with other hearing tests on marine mammals, says Brandon Southall, who directs the National Oceanic and Atmospheric Administration Ocean Acoustics Program, based in Silver Spring, Md. Earlier tests found threshold shifts too, but the new experiment is “unique in using real sound signals that we know animals in the wild have experienced,” Southall says.

Mooney and his colleagues report that the test animal didn’t try to get away from noise but did show an increase in breathing and seemed to swim slower afterward. But Southall cautions that it’s “difficult and preliminary to interpret possible behavioral responses” from this test since the bottle-nosed dolphin was a pro at hearing experiments.

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