Varieties of Animal Aggression | While the term "aggression" can often be used without misunderstanding in ordinary conversation, it has proved to have so many different meanings, and to be so difficult to conceptualize, that Benjamin (1985) selected it (along with "intelligence" and "self-esteem") as a particularly egregious example with which to demonstrate the difficulties of concept analysis. Analyses from the animal literature suggest one important reason that a single, and universally accepted, scientific definition of aggression has been difficult: Several different phenomena are encompassed by the term.

Offensive Attack | The distinction between "offensive" and "defensive" forms of adult aggression has received a good deal of systematic attention (e.g. Blanchard and Blanchard, 1977; Blanchard et al., 1984), with both of these typically being differentiated from play fighting (Pellis, 1988) and from predation. Offensive aggression occurs in the context of a resource (including territory) or dominance dispute and its successful outcome is the termination of the dispute through "victory," manifested as flight or defeat-related behaviors by the opponent. Dominants (consistent victors) gain resources, including access to females (a particularly important "resource" from an evolutionary perspective) and food (Flannelly and Lore, 1977; Blanchard et al., 1984). Offensive attack involves a set of species-typical behaviors that enable the aggressive animal to contact particular body sites on the opponent where bites or blows are delivered. Offensive attack can thus be differentiated from other forms of aggression on the basis of the specific target sites for attack, as well as the behaviors by which these sites are reached.

Defensive Attack | Defensive attack is seen only when the subject is defending its own body, not when it is attacking another animal to "defend" a disputed resource. It includes a salient threat component not seen in offensive attack, with loud vocalizations and display of weapons such as teeth or claws. The bites or blows delivered tend to be made on different body sites on the opponent than those contacted in offensive aggression. The successful outcome of defensive aggression is discouragement of the body-threatening conspecific or predator and discontinuation of its attack. This can occur prior to the defensive attack, as the result of defensive threat, or following the delivery of a bite or blow, particularly to the sensitive eye/snout sites that are the targets of defensive attack. While relatively little field research has been done on the effectiveness of defensive threat and attack, the strong inhibitory effect of fear on predation suggests that defensive threat may serve as a considerable deterrent (Pellis et al., 1988).

Play Fighting | Play fighting is common among the young of many mammal species, dropping off in frequency after sexual maturity is attained (Pellis and Pellis, 1991b). The behaviors involved in play fighting have considerable structural similarity to those of adult attack and defense, in that a species-typical attack pattern is used to approach and make contact with a specific site on the body of the opponent, while the defender utilizes species-typical behaviors to make that body site unavailable to the attacker. In addition, across-species studies suggest that, as in adult fighting, attack and defense in play fighting are motivationally distinct behavior patterns (Pellis and Pellis, 1991a). However, the transition from juvenile play fights to adult fighting does not appear to involve a continuity in individual levels of attack tendency from one to the other: Males that show the highest attack rates during play fighting tend to become subordinates rather than dominants (Pellis and Pellis, 1992a; Smith et al., 1996). Also, play fighting, at least for males, may be more linked to adult sexual behavior than to adult fighting, as the play fighting attack target in a spar between males may correspond to an important contact target on the female, utilized in adult male sexual behavior—a view that is supported by the finding that deprivation of play fighting has more of a deleterious effect on sexual behavior than on fighting skill (Pellis et al., 1992). In addition, play fighting defenses in juvenile females may involve some of the responses that later become useful in fending off the sexual advances of males.

Predation | Phenomena related to predation are also often subsumed under the rubric of aggression. In the laboratory, many rats kill and eat mice, a behavior that, due in part to the similarities of rats and mice, suggests conspecific aggression. In fact, predatory (as opposed to conspecific) attack can occur in species even more closely related and more similar than rats and mice: Grasshopper mice kill and eat laboratory mice. However, both the target sites for attack and the behaviors typical of the attack pattern are different from conspecific attacks by grasshopper mice (Pellis and Pellis, 1992b). These findings indicate that predatory attack can and should be differentiated from conspecific attack, even when the combatants involved are closely related animals. In addition to differences in stimuli, response patterns, target sites for contact, and outcome of these various behavior patterns, recent work on the anatomic and neurochemical systems associated with some of these strongly suggests that the physiology of these systems is also different (e.g. Bandler and Shipley, 1994). Most of the work on the neurobiology of "aggressive" behaviors has actually involved defensive threat and attack, with a relatively substantial literature also on "quiet biting attack," which likely corresponds to predation. The pharmacology of offensive and defensive aggression appear to be different, with the former (Olivier, et al., 1991) but not the latter (Blanchard et al., 1985) responding dramatically to a class of "serenics" with effects at various serotonin receptor subtypes. Motivational variables also produce different effects on these behaviors, with fear reducing offensive attack (Blanchard et al., 1988) and predation (Pellis et al., 1988) but not altering defensive attack (Blanchard et al., 1980).

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