Rising StARs: behavioral, hormonal, and molecular responses to social challenge and opportunity.

Across taxa, individuals must respond to a dynamic social environment of challenges and opportunities on multiple biological levels, including behavior, hormone profiles, and gene expression. We investigated the response to a complex social environment including both territorial challenges and reproductive opportunities in the African cichlid fish Astatotilapia burtoni (Burton's mouthbrooder), a species well-known for its phenotypic plasticity. Male A. burtoni are either socially dominant or subordinate and can transition between the two phenotypes. We used this transition to simultaneously study changes in aggression, reproductive behavior, testosterone and estradiol levels, gonadal histology, and testes expression of three genes involved in testosterone synthesis. We have found that males immediately become aggressive and increase testosterone levels when they become dominant in this paradigm of challenge and opportunity. Reproductive behavior and estradiol increase slightly later but are also up-regulated within 24h. Increases in steroid hormone levels are accompanied by an increase in expression of steroidogenic acute regulatory protein (StAR), the rate-limiting enzyme during testosterone synthesis, as well as an increase in testis maturation as measured by histological organization. Reproductive behavior was found to correlate with female gravidity, suggesting that males were able to perceive reproductive opportunity. Our study demonstrates the rapid plasticity at multiple levels of biological organization that animals can display in response to changes in their complex social environment.