Most species engage in aggressive behavior in some circumstances and at certain times, and the fact that it is virtually ubiquitous suggests that it must be adaptive despite its hazards. Aggressive behavior has been defined by psychologists as activity that seems intended to convey a noxious stimulus or to destroy another organism, and classifications have included such categories as frustration aggression, fear-induced aggression, and irritable aggression, all based on their supposed underlying motivation. A more function-based classification is now widely accepted in animal behavior, in which aggression is regarded as behavior by which an individual actively attempts to exclude rivals from essential resources such as mates, food, water, and shelter. There are two broad types of aggression: interspecific aggression, largely comprising predatory and antipredatory aggression, and intraspecific aggression, involving competition between members of the same species. The latter can be subdivided into territorial, dominance, sexual, parental, and parent-offspring aggression (Wilson, 1975; Moyer, 1976). Predatory aggression, which is motivated by hunger and is part of a different motivational system (Scott, 1972), is not associated with conflicting aggressive and submissive tendencies or the compromises between them. As Lorenz pointed out, a cat killing a mouse may appear as calm as a cow grazing in a pasture; but this in not the case for antipredatory aggression shown by the prey. Intraspecific conflicts, in contrast, typically involve a mixture of aggressive and submissive tendencies, as illustrated by the ambivalent facial expressions and postures of cats (Chapter 2). The terms agonism and agonistic behavior were therefore coined to describe the entire spectrum of both aggressive and submissive behaviors. Agonistic displays almost always reflect this ambivalence and involve elements of both “fight and flight”.
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