Publisher Summary Successful animals survive because they mode their behavior in response to the changes in their physical and social environments. Because reproduction is arguably the single most important aspect of an animal's life, reproductive behaviors offer a unique chance to study change. Reproduction requires exquisite coordination of physiological state and behavioral acts. Many aspects of reproductive behavior occur only under natural conditions, so it is imperative to analyze naturally occurring behaviors in real animals, preferably in the natural habitat. An African cichlid fish in natural and semi-natural conditions is studied in the chapter because the connection between physiology and behavior can be easily seen. Moreover, the consequence of social success can be traced directly to the changes in the brain, both in the short and long term. In this species, territorial males inhibit the sexual maturation of non territorial males during development. Even after a male becomes sexually mature and territorial, being defeated causes his gonads to regress rapidly. These changes are mediated by gonadotropin releasing hormone (GnRH) containing cells in the hypothalamus, whose size changes in response to altered social status in the male. This means that in males which become dominant, GnRH containing cells undergo hypertrophy and conversely, in males which become non territorial, the same cells undergo atrophy. In females, corresponding changes in GnRH containing cells occurs, but cell size reflects reproductive state, not social status. This striking difference in the regulation of the same elements brain–pituitary–gonadal axis offers a rare chance to understand the way this control is achieved.