Sex differences in aggressive intensities and brain steroids during status resolution in a sex changing fish, Lythrypnus dalli.

For vertebrates living in social hierarchies, the neuroendocrine system regulates temporal aspects of aggressive interactions during status establishment. In teleost fishes, the sex steroids 17β-estradiol (E) and 11-ketotestosterone (KT), and the glucocorticoid, cortisol (CORT) are associated with aggression in distinct phases of their life history. Bluebanded gobies, Lythrypnus dalli, exhibit bidirectional sexual plasticity by responding to changes in their social structure by escalating aggression associated with neural changes that precede gonadal reorganization to the opposite sex. Here, we used a novel experimental design to investigate systemic (waterborne) and neural steroids associated with the earliest behavioral changes associated with feminization and masculinization during protandrous and protogynous sex change respectively. In stable social groups of wild-caught L. dalli comprising of one male and two females, we disrupted hierarchy by adding or removing a male, providing a social context for intrasexual aggression. Within only 30 min, males exhibited high rates of physical aggression inside the nest to maintain their territory, while females exhibited high rates of chases outside the nest to reestablish social status. During this period of instability, while waterborne steroids were not affected, brain E was higher in all fish and CORT was lower in male brains. Brain KT was higher in males who emerged as dominant compared to dominant females. Overall, a combination of differences in brain E, CORT, and KT were important in the regulation of hierarchy re-establishment and maintenance. Rapid responses during conspecific aggressive encounters are likely mediated by neural steroid synthesis that precede changes in systemic steroids.