For many animals, possibly even for humans, mating success is determined by social status or dominance. A male's position in the "pecking order" - a term coined by the Norwegian Thorleif Schjelderup-Ebbe, who first proposed the concept of social dominance based on his work with chickens - can actually control his fertility. Social status also has other well-established, long-term physiological consequences. It can determine how big an animal grows, for example, or how it responds to stress. However, little is known about the neural mechanisms that link the social environment to the physiological changes associated with dominance.
In a paper published in the open-access journal PLoS Biology, Sabrina Burmeister, Erich Jarvis, and Russell Fernald demonstrate that subordinate male cichlid fish become dominant within minutes of an opportunity to do so, rapidly developing the bright coloration of dominant males and indulging in dominant behaviors. The authors simulated natural social upheavals by removing the dominant male an hour before daylight (cichlids rely primarily on visual cues to monitor their social position) from a social group consisting of several females, a dominant male, and a subordinate male - an approach designed to represent naturally occurring behavioral and neural responses. The authors then watched the erstwhile subordinate male for behavioral signs of dominance, and measured changes in the expression of a class of genes called immediate-early genes within the brain. The proteins encoded by these genes - which show behavior-specific expression in many animals - induce the expression of other genes in the brain that produce changes in the animal's physiology. Subordinate and dominant males whose position in the social hierarchy had not been experimentally manipulated were similarly examined.
At the moment of development of the bright coloration and behavior associated with dominant males, the expression of the immediate-early gene egr-1 increased in the brain region that links the nervous system to hormonal systems, in particular, gonadotropin-releasing hormone 1 (GnRH1) for subordinate males. On the basis of several experiments, the authors were able to show that the social opportunity itself rather than any behavior exhibited in response to the opportunity induced egr-1 expression.
The egr-1 gene encodes a transcription factor that is important for neural plasticity, and could regulate the expression of the GnRH1 gene. Thus, Burmeister et al. concluded, the social opportunity–induced expression of egr-1 in the anterior preoptic area is an early trigger in a molecular cascade that ultimately produces the physiological changes associated with dominance in cichlids. Given that the preoptic region and GnRH are highly conserved in vertebrates, a similar neural mechanism could link social status to sexual physiology in other animals.
Citation: Burmeister SS, Jarvis ED, Fernald RD (2005) Rapid behavioral and genomic responses to social opportunity. PLoS Biol 3(11): e363.
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