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 17 beta-estradiol (E2) and 11-ketotestos-terone (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 reorga-nization 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 E2 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 E2, 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.

Automated summary

The neuroendocrine system regulates aggressive interactions during status establishment in vertebrates living in social hierarchies. Sex steroids (E2 and KT) and cortisol are associated with aggression in different phases of teleost fishes' life history. Brain E2, CORT, and KT play important roles in the regulation of hierarchy re-establishment and maintenance during protandrous and protogynous sex change in bluebanded gobies.