Behavioural 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 the complexities of hierarchy formation at the neurobehavioural level are still being unravelled. Here, we investigate the formation of social hierarchies within groups of New Zealand spotty wrasse, integrating behavioural observations with neural activation patterns upon social disruption. We find that dominance hierarchies form linearly based on size, with larger fish displaying more dominant behaviours and smaller fish displaying more submissive behaviours. Disruption of the social hierarchy induced rapid behavioural changes, particularly in second-ranked fish, highlighting that second-ranked fish will opportunistically adopt a dominant position. Analysis of neural activation patterns reveals 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. Overall, 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, and demonstrates that disruption of the social hierarchy triggers rapid changes in both behaviour and the social decision-making regions of the brain.