Behavioral and neural correlates of social hierarchy formation in a sex-changing fish

Social hierarchies in sex-changing fish determine which fish will change sex, yet little is known about how these hierarchies are formed at the neurobehavioral level. Here, we investigate the formation of social hierarchies within groups of the New Zealand spotty wrasse (spotty), integrating both behavioral observations of social activity with neural activation patterns upon social disruption. We find that dominance hierarchies form rapidly and linearly based on size, with larger fish displaying more aggressive or dominant behaviors (such as monopolizing the shelter) and smaller fish displaying more submissive behaviors (such as escape behavior). Disruption of the social hierarchy induced rapid behavioral changes, particularly in the second-ranked fish, highlighting that second ranked spotty will opportunistically adopt a dominant position in the hierarchy upon removal of the most dominant fish. Neural activation patterns reveal that the social decision-making network is deeply involved in the establishment of dominance, with the fish attaining dominance showing significant differences to all other ranked fish. This study underscores the complexity of social relationships and their neural underpinnings in the spotty wrasse, providing a foundation for further research into the cellular and molecular mechanisms of socially-controlled sex change. Overall, we demonstrate that disruption of social hierarchy triggers rapid changes in behavior and brain regions associated with social decision-making.