Aggression: Appetite or aversion?—An ethologist's viewpoint

The term “aggression” has been used to describe practically all situations in which an organism threatens, damages, or kills another-with the exception of certain predator-prey relationships. As a result, a great deal of confusion and controversy has arisen as to the causal factors and functions of aggression. The main ethological hypotheses concerning the nature of aggression are put forward. In order to illustrate the divergence in causal factors and functions, differences in motivational, physiological, and ecological bases of aggression in two closely related fish species are described. A basic division of aggression into “self-defensive” and “property-protective” behavior is suggested, together with the parameters separating these categories on behavioral, causal, and functional bases. Mechanisms which enable motivational changes from one category to another to take place during a single encounter are discussed, these being pain stimuli and physical or psychological “cornering.” The importance of learning effects on fighting behavior are emphasized, especially their negative and positive reinforcing effects in relation to self and property defense. The biological significance of aggressive conditioning for the animal and the role of aggression in species maintenance are discussed.