Behavioral and neuroendocrine correlates of displaced aggression in trout

In humans and other primates, violent actions performed by victims of aggression are often directed toward an individual or object that is not the source of provocation. This psychological phenomenon is often called displaced aggression. We demonstrate that displaced aggression is either rooted in evolutionarily conserved behavioral and neuroendocrine mechanisms, or represent a convergent pattern that has arisen independently in fish and mammals. Rainbow trout that briefly encountered large, aggressive fish reacted with increased aggression toward smaller individuals. There was a strong negative correlation between received aggression and behavioral change: Individuals subjected to intense aggression were subdued, while moderate assaults induced strong agitation. Patterns of forebrain serotonin turnover and plasma cortisol suggest that the presence of socially subordinate fish had an inhibitory effect on neuroendocrine stress responses. Thus, subordinate individuals may serve as stress-reducing means of aggressive outlet, and displaced aggression toward such individuals appears to be a behavioral stress coping strategy in fishes.