Reasons for aggression may be deduced from the situations preceding aggressive behaviour. This we may call the retrospective approach. In addition to results from this conventional procedure the present paper investigates the predictability of aggressive behaviour. In this so-called anticipatory approach, particular non-agonistic events were used as predictors of aggression. These events had in common that they were intuitively judged to be unpleasant for one of the chimpanzees. This individual was observed during the next 10 min in order to establish if its inclination to exhibit aggression had increased shortly after that seemingly frustrating event. The studies reported here were carried out on a colony of 20 semi-free-living chimpanzees Pan troglodytes in Arnhem Zoo. The data show that: (1) half of all aggressive actions arose in agonistic context: thus aggression was contagious; (2) adult males performed apparent spontaneous aggression remarkably often: this result is suggested to reflect dominance-rivalry among males; (3) aggressive behaviour was predictable on the basis of intuition, but attempts to account for the observer's intuitive knowledge were not entirely successful
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