Margaret Marchaterre/Cornell University

Midshipman fish at 2-3 weeks of age. Later, as male midshipman develop sexually, the enzyme aromatase will affect the ration of testosterone to estrogen in their brains and turn some males into "singers" and others into "sneakers."

A sexually maturing male midshipman fish can look forward to becoming either of two very different kinds of guy: A talented "singer" whose courtship songs lure females to his love nest, or a "sneaker male" who can't carry a tune but horns in on the mating action anyway.

Now a study by biologists at Cornell University and the University of California at Los Angeles (UCLA) has found a chemical difference between the singers and the sneakers. High levels of an enzyme called aromatase chemically convert testosterone into estrogen and may prompt profound changes in the brain circuitry and behavior of nonsingers.

The biologists report their findings in the current Proceedings of the Royal Society of London Series B (Vol. 265, Issue 1415).

"The brain of every species of vertebrate has some level of aromatase, although fish have a lot more than most others, including humans, and we knew from others' studies that brain aromatase had something to do with developing masculine neural structure and function," says Andrew H. Bass, Cornell professor of neurobiology and behavior. "When we find three- to five-fold higher levels of aromatase in adult fish that turned out to be sneaker males, this tells us that the enzyme may be important for the development of different brain circuits and behavior among members of one sex of any species, including our own."

However, the reason for differing levels of enzyme activity is still a mystery.

Midshipman fish (Porichthys notatus) are useful models for observing intrasexual differences, says the Cornell biologist, who has conducted field and laboratory studies of the species since 1984. Compared with the singing males (which actually hum; see "Humming Fish Facts," attached), the nonsinging sneaker males have a notably different architecture in the vocal control centers of the brain. The sneakers lack some of the vocal "pacemaker" hardware that

allows the singers to continue their courtship humming for hours. And differences in brain circuitry are reflected in easily observable differences in behavior.

Another intrasexual difference is the size of the testes in the two types of male midshipman fishes. Sneaker males' testes occupy up to 15 percent of their body, by weight, compared to only 1 percent for singing males. The researchers were able to show that larger gonads are not, however, responsible for differences in levels of aromatase.

Because both sneaker males and female midshipman fish have comparably high levels of aromatase activity -- and because females do not sing -- the researchers believe the enzyme could prevent testosterone-induced maturation of the vocal system. Experiments now under way in Bass' laboratory at Cornell aim to discover exactly when in the lives of maturing juvenile fish the aromatase starts to take effect.

But answering the why question will be tougher, Bass concedes. "We think we've found the biochemical switch, the magic bullet, that converts potential singing males into sneaker males. That chemical cascade could still be triggered early in development by environmental or social factors," such as crowding and competition, he says.

Perhaps, the biologist speculates, the maturing male brain assesses its environment and says to itself, "In a sea full of singers maybe I'd be better off as a sneaker." Then chemicals do the rest.

Bass is chair of the Section of Neurobiology and Behavior at Cornell. His field studies are conducted at the University of California's Bodega Marine Laboratory. Other authors of the Proceedings report include Barney A. Schlinger, an associate professor of physiology at UCLA who has studied aromatase in songbirds, and Christina Greco, a UCLA undergraduate at the time of the research. The studies were supported by the National Science Foundation.