Repeated competition influences neural response, but not neurogenesis or physiology, in the fighting fish Betta...

Territorial competition has costs such as wounds received, energy spent and elevated stress. How does Betta splendens, a species that has been artificially selected for enhanced territorial aggression, maintain high levels of metabolically costly displays (gill flaring) over repeated days of territorial competition? Here, we conducted two experiments. The first exposed male B. splendens to a size-matched unresolvable competition (a mirror) daily for 3 weeks. We found that B. splendens indeed maintained gill flaring throughout the competition schedule and exhibited no baseline stress impacts (measured via a scototaxis assay). We then exposed B. splendens to 1, 2 or 3 weeks of competition and quantified cortisol levels, neurogenesis and the neural response in brain regions associated with social processing. We again found that B. splendens showed consistently elevated gill flaring and no impact on stress (measured via waterborne cortisol). Competition-exposed fish displayed an increased neural response in the caudal supracommissural nucleus of the ventral telencephalon (Vs), a region that is homologous to the mammalian medial amygdala/bed nucleus of the stria terminalis and a central mediator of social decision making. The neural response in this same region positively correlated with individual variation in gill flaring. Competition did not influence neurogenesis (doublecortin expression). Lastly, a measure of reproductive investment (gonadosomatic index) did not correlate to behavior but did correlate to cortisol levels, suggesting a cohesive physiological state related to reproduction in this species. Taken together, this work details the mechanisms that are relevant to the maintenance of costly aggressive behavior.