Frustrative reward omission increases aggressive behaviour of inferior fighters

Animals use aggressive behaviour to gain access to resources, and individuals
adjust their behaviour relative to resource value and own resource holding
potential (RHP). Normally, smaller individuals have inferior fighting abilities
compared with larger conspecifics. Affective and cognitive processes can
alter contest dynamics, but the interaction between such effects and that of differing RHPs has not been adjudged. We investigated effects of omission of
expected reward (OER) on competing individuals with contrasting RHPs.
Small and large rainbow trout (Oncorhynchus mykiss) were conditioned to
associate a light with reward. Thereafter, the reward was omitted for half of
the fish prior to a contest between individuals possessing a 36–40% difference
in RHP. Small control individuals displayed submissive behaviour and
virtually no aggression. By contrast, small OER individuals were more aggressive,
and twoout of 11 became socially dominant. Increased aggression insmall
OER individuals was accompanied by increased serotonin levels in the
dorsomedial pallium (proposed amygdala homologue), but no changes in
limbic dopamine neurochemistry were observed in OER-exposed individuals.
The behavioural and physiological response to OER in fish indicates that
frustration is an evolutionarily conserved affective state. Moreover, our results
indicate that aggressive motivation to reward unpredictability affects low RHP
individuals strongest