Removal Of Dominant Rivals Causes Male Cichlid Fish To Undergo Remarkable Transformation

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

"Whenwe took dominant cichlid males from an experimental tank, subordinatemales 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 theybecame much more aggressive than they were before. The remaining malesalso quickly paid a lot more attention to females because for the firsttime, they had an opportunity to reproduce."

No one had any ideabefore that perceived changes in their social status could beginaltering animals’ behavior and appearance so quickly, she said.Previous studies had shown the changes took as long as a week and wereassociated with increased fertility.

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

Theresearch is part of a larger effort to understand some of the mostintriguing questions in all of biology -- how did brains evolve and howcan the environment change an animal’s physiology through actions onits brain?

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

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

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

Thebasic mechanisms that control reproduction in fish and in humans arethe same and may be in all vertebrates, she said. The brain’shypothalamus links the nervous system to hormonal systems.

"Reproductivephysiology is often influenced by environmental factors, includingsocial cues, the scientist said. "In humans, one of the best examplescame from work by Dr. Martha McClintock showing that the menstrualcycles of women were influenced by olfactory cues from other women.Another group found that the timing of ovulation in women is influencedby olfactory cues from men."

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

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