Thursday, July 9, 1998
Web posted at 11:01 AM ET

By Environmental News Network staff

All the humming males together sound like a huge hive of bees or a squadron of motor boats.

Female midshipmen fish seeking a mate home in on the hums of homely males who have excavated cavity-like nests under rocks on the seafloor, according to biologists at Cornell University.

A team of researchers has been curious to discover how the rather ugly looking midshipmen fish find a mate. To do so, they examined the structure and function of midshipman brains.

Their discovery reveals that the auditory portion of the midbrain uses the acoustic qualities of all the noise to isolate one signal it is programmed to recognize as potentially interesting.

While the research applies only to midshipman fish, it could, the researchers say, also be relevant to people.

"Neuroscientists call this auditory scene analysis," says Andrew H. Bass, Cornell professor of neurobiology and behavior. "It's really very similar to the cocktail party effect."

Only some of the male midshipmen fish have the ability to hum, and those fish hide under rocks. All the humming males together sound like a huge hive of bees or a squadron of motor boats, and a female midshipman fish has to choose one nest in which to deposit her eggs.

When a humming male succeeds in attracting a female, he fertilizes her eggs, which adhere to the rocky ceiling of his nest. The female leaves forever, and the male resumes humming in hopes of attracting another female with more eggs.

Each female deposits all her eggs for that season in one nest and swims away. Hoping to lure more females to the nest, the male resumes singing, all the while remaining on guard until the offspring hatch and mature.

From earlier studies, Bass knew that a part of the midshipman male brain, called the hindbrain, contains neurons that constitute a kind of vocal pacemaker. Like a rhythm generator, the pacemaker tells the sound-generating muscles on the male's swim bladder to contract rhythmically and produce a hum averaging 100 Hz in frequency.

Midshipman females have neurons in their midbrains that respond to the 100-Hz hum. Humans, like midshipmen, have midbrains and hindbrains too.

Whenever the hums of two neighboring and competing males overlap the sounds form what is known as an acoustic beat, the biologists observed. Using computer synthesizers, the biologists reproduced the acoustic beat and were able to study the fish.

"Just as we expected, two or more synthesized fish hums played together produce the rhythmic, acoustic beats," Bass said. "And sure enough, the females were able to directly localize one of the humming speakers. Their midbrain neurons form a code of the beats that helps in their calculations to locate the hum of interest from all the rest."

Bass and research associate Deana Bodnar recently reported their findings in the Journal of Neuroscience. The research effort is being continued along the California and Washington state coasts as well as at Cornell.

"Midshipman are regarded as some of the ugliest fish in the sea and a nuisance because they hum almost incessantly," Bass said. "But they have thrived for hundreds of thousands of years, so they must be doing something right. We'd like to find out what."

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