Stress appears to prevent all but the dominant males from achieving reproductive success.
Fernald said that since many species have evolved dominant and non-dominant males, the role of stress hormones in social systems may be widespread. Fernald is the Benjamin Scott Crocker Professor of Human Biology and director of the Human Biology program at Stanford. He is also a professor of psychology and a member of the Neurosciences program faculty. He and members of his laboratory already have shown that for cichlid fish, social position determines physiology. It's not so much that the biggest, brightest fish becomes dominant.