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August 18, 1997

Hearing: The din of the ocean


Rocky, a California sea lion, listens for tones through a pair of headphones

Pinnipeds may not talk, but they growl, bellow, snort, whine, and caterwaul with abandon. All of this helps make the ocean and onshore breeding grounds very noisy places indeed. Add the cacophony of civilization and industry, and one might wonder how marine mammals hear what they need to hear.

Schusterman's group wondered that, too. So, led by Kastak, the team undertook a methodical effort to gauge the hearing abilities of its four charges, both in air and in water. They devised an experimental setup that is analogous to the classic hearing test for humans: Wear headphones, raise your hand when you hear a tone. A child might get a lollipop for performing well on this test, but Burnyce and her gang get fish.

For the aerial tests, the group directs tones into the animals' ears via headphones held on by nylon straps. Under water, sounds emanate from a speaker. Each subject is trained to touch a ball with its nose if it hears--or thinks it hears--a sound. "False alarms," when the animal touches the ball even though no sound was played, help the researchers spot overeager cheating.

The team tested each animal at different frequencies of sound, from low- to high-pitched, to create baseline "audiograms." The results, compared across species, were remarkably reflective of the animals' evolutionary histories:

In ongoing work, the team is finding that exposure to loud sounds can induce "temporary threshold shifts," or slight but transient hearing losses, in their animals. Just as our ears feel cottony after a rock concert, it appears that marine mammals lose some sensitivity if they venture near a tanker or a seismic survey explosion. For a short time, they might not hear the grunting erotica of a potential mate or the plaintive "Mom?" of a pup.

Schusterman first measured evidence of this effect in Sprouts when workers cleaned a nearby tank at Long Marine Lab with a sand blaster. Now, under controlled conditions, the researchers are showing that even moderate sounds can induce threshold shifts for a few hours. For ethical reasons, the team will not study permanent hearing loss, although it probably does occur in the ocean.

"Manmade noise, especially shipping traffic, is far more of a pollutant than anything else humans have put into the water," said Kastak. "It's not innocuous. If we can quantify the impacts of these sounds on our animals, we might be able to make predictions about the environmental effects."

Other current research, coordinated by graduate student Brandon Southall, is probing the "masking" effects of continuous sounds in our increasingly noisy seas. A persistent sound, even if not loud enough to have a physiological impact, might prevent marine mammals from hearing what they need to hear at a given moment. Then again, they may have evolved the ability to pick out key signals, just as we can isolate one conversation from a cocktail-party buzz.

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