Social regulation of the brain: sex, size and status.

Fish comprise the largest group of extant vertebrates with approximately 25,000 known species. Some of these species are exceptional among vertebrates because they can change sex as adults. This observation raises ultimate questions about what selective forces led to the evolution of sex-changing ability and raises proximate questions about what mechanisms could account for this process. Sex change can be either from female to male (protogyny) or the reverse (protandry). In either case, the actual process of sex reversal requires reorganization of many critically important physiological systems from transformation of the gonads to modification of the neural and hormonal control systems. All of these changes require an individual animal to initiate the process based on information gleaned from the social situation. This is all the more remarkable because the information could be as simple as size discrimination or as complex as detecting subtle behavioural signals. Although it is self-evident that the brain controls behaviour, clearly behaviour can also 'control' the brain. How does behaviour cause changes in the brain? The work described here links molecular events with organismal behaviour by using an African cichlid fish model system in which social behaviours regulate reproduction. These animals have a complex social system based on the behaviour of two distinct classes of males, those with territories and those without. Changes in social status produced by behavioural interactions cause changes in neurons and endocrine responses. Surprisingly, growth rate is also regulated by social status and prior social history. Discovering how relevant social information is transduced into physiological processes requiring cellular and molecular action presents a major challenge.