Sudden change in social status triggers genetic response in male fish, study finds

lopment in all vertebrates, including people. As the male fish ascends toward dominance, these brain cells grow eight times bigger in volume and begin producing large amounts of the hormone. As a result, the fish becomes more aggressive, his appearance changes dramatically and his gonads mature. When the male descends in status, the opposite occurs--the hormone-producing brain cells decrease in size, the ostentatious colors and stripes disappear, and the testes shrink and the male becomes infertile.

"Previously we did an experiment in which we moved the subordinate male out of the tank and put him in a situation where he could rise and become dominant," Fernald says. "Because he was in a new situation, it took several days before he began to change physically and behaviorally."

But how would a subordinate male respond if the dominant fish were to suddenly disappear? To find out, Burmeister designed an experiment in which the dominant male was surreptitiously removed from the tank after dark. "Cichlids have great vision, but like all animals, they can't see in total darkness and basically do nothing," Fernald explains, "In some ways, this experiment was more like nature, where a predator comes in and picks off a dominant fish opening up the field for non-dominant animals."

Nighttime behavior

To observe nighttime behavior of the cichlids, Burmeister donned infrared night-vision goggles and kept close watch on the experimental fish tanks. She soon discovered that dominant males undergo a temporary change in the dark--their bright color turns pale, and their eyebar fades. "But when the lights come on, the dominant male slowly reactivates," she says. "He goes through a warm-up period in which his coloration and dominant behaviors return. Everyone in the tank accepts this process." Burmeister and her colleagues were the first to observe this diurnal transformation among dominants.

"During the experiment, I would rem

Contact: Mark Shwartz
Stanford University

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