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Research: removal of dominant rivals causes male cichlid fish to undergo remarkable transformation


In a new study of cichlid fish descended from others caught in East Africa’s Lake Tanganika, scientists have made some surprising observations about how those animals respond to changes in their environments known as "social opportunities."

Dr. Sabrina S. Burmeister, assistant professor of biology at the University of North Carolina at Chapel Hill’s College of Arts and Sciences, and colleagues found that subordinate male fish underwent a radical and rapid transformation when more dominant males were removed.

"When we took dominant cichlid males from an experimental tank, subordinate males started becoming dominant themselves in as few as two minutes," Burmeister said. "Their colors -- blue and yellow -- got much brighter, a black stripe we call an eye bar appeared near their eyes, and they became much more aggressive than they were before. The remaining males also quickly paid a lot more attention to females because for the first time, they had an opportunity to reproduce."

No one had any idea before that perceived changes in their social status could begin altering animals’ behavior and appearance so quickly, she said. Previous studies had shown the changes took as long as a week and were associated with increased fertility.

Burmeister’s report on her experiments, conducted at Stanford University, appears in the November issue of the scientific journal PloS Biology, which is being released today (Oct. 17). Co-authors are Drs. Erich D. Jarvis and Russell D. Fernald, neurobiologists at Duke University and Stanford, respectively.

The research is part of a larger effort to understand some of the most intriguing questions in all of biology -- how did brains evolve and how can the environment change an animal’s physiology through actions on its brain?

Such studies are relevant to humans since the hormones and genes involved are close to identical, she said. Obviously, such internal gene activity studies cannot be done directly in humans.

After observing the striking changes in appearance and personality, Burmeister and colleagues turned their attention to the inner workings of the fish’s gene-hormone interactions by analyzing brain tissue.

"The gene we focused on, egr-1, was a good candidate for study because it controls expression of other genes," Burmeister said. "We found that perception of social opportunity caused more egr-1 to be expressed in the hypothalamus, a region of the brain that controls fertility. We believe that in our fish, egr-1 turns on expression of a second gene, GnRH1, which produces a hormone necessary for reproduction."

The basic mechanisms that control reproduction in fish and in humans are the same and may be in all vertebrates, she said. The brain’s hypothalamus links the nervous system to hormonal systems.

"Reproductive physiology is often influenced by environmental factors, including social cues, the scientist said. "In humans, one of the best examples came from work by Dr. Martha McClintock showing that the menstrual cycles of women were influenced by olfactory cues from other women. Another group found that the timing of ovulation in women is influenced by olfactory cues from men."

In humans and many other mammals, olfactory cues -- various odors --provide important information about the social environment, Burmeister said. The situations are analogous -- social cues from the environment influence the reproductive system through GnRH neurons and create a "cascade" of molecular interactions that result in increased fertility.

The National Institutes of Health and the National Science Foundation supported the research.

David Williamson | EurekAlert!
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