Brain Mechanisms

A detailed survey of intraspecific aggressive behavior in muroid rodents has led me to propose that there are three motivational systems involved: offense, defense, and submission (Adams, submitted for publication). It has become quite common in the last few years to make a distinction between offense and the other motivational systems. The distinction has been made for the cat on the basis of behavioral observation (Leyhausen 1956), and for the rat on the basis of pharmacological (Miczek 1974), behavioral (Blanchard et al. 1975; Adams 1976), and neurological data (Adams 1971). Offense is shown, for example, by the resident male rat; its motor patterns include approach, an offensive sideways posture, piloerection, and a bite-and-kick attack. Other behaviors are shown by an intruder male rat; its motor patterns include fleeing, freezing, a full submissive posture, squealing, and ultrasound vocalization.

A further distinction is also necessary between defense and submission, and this distinction has not been made in most of the literature. Defense is the behavior shown by wild animals or laboratory animals with forebrain lesions, and it may be quite damaging. It includes the lunge-and-bite attack directed at the face or protruding parts of the body of the opponent. It also includes various other defense motor patterns, including squealing, upright posture, fleeing, freezing, and various types of warning noises and vocalizations. Submission is the behavior usually observed in laboratory animals under attack by conspecifics. It consists of a less damaging response. including many of the same motor patterns of defense, such as squealing, upright posture, fleeing, and freezing, but also including specific submissive behaviors such as the full submissive posture (lying on the back) and ultrasound vocalizations that have been shown to inhibit conspecific offense (Lehman & Adams 1977).

A further set of evidence for the distinctions among offense, defense, and submission derives from the different effects of hormones on muroid rodent social behavior. Offense is dependent upon gonadal hormones; in most species males show offense more than females, and the effect may be manipulated by castration or administration of testosterone (shown by many investigators). Maternal aggression is enhanced by prolactin (Wise & Pryor 1977), and an analysis of the postures of lactating rats in our laboratory indicates that maternal aggression includes both offense and defense. The effect of the prolactin upon defense is probably due to the suppression of the hypothetical "consociate modulator" and release of defense from its inhibitory influence. Corticosteroids, which enhance submission (Moyer & Leshner 1976), probably exert their effects by facilitation of the hypothesized consociate modulator.

The differences between offense, defense, and submission may be understood on another level, in terms of their communicative function and their evolutionary histories. Defense, it may be assumed, evolved under the pressure of attack by predators. As such, its primary purpose is not communication, at least not in relatively small and weakly-armed animals such as the muroid rodents. Instead, freezing enables the animal to avoid detection or to avoid releasing a predatory attack, fleeing brings the animal to the safety of a burrow or tree, and attack is a "last-resort" behavior that depends for its effectiveness upon infliction of pain or damage. In a predator such as the cat, defense may have more of a communicative function, since the cat is a relatively well-armed animal, and other carnivores (such as the dog) may respond to threat by desisting from an attack. Offense and submission, on the other hand, are primarily systems of communication. Offense, in the cat or rat, consists of threat and an attack, which is ritualized to such an extent that it does not usually produce serious damage. For example, the bite-and-kick attack of the rat inflicts superficial wounds on the flank of the opponent, a part of the body least vulnerable to serious damage. In species for which the land is partitioned into territories or domains (Brown 1975), such threat and ritualized attack enables an individual territory holder to maintain an arrangement of "loyal opposition" with its neighbor, in which each animal is dominant on its own territory but knows and respects the others. Since the neighbors do not kill each other, they remain familiar opponents and do not have to reestablish their relationship, often with potentially damaging combats. Submission may be seen as the opposite, complementary side of this communicative system. Use of the upright posture and the full submissive posture, which protect the flank from the bite-and-kick attack, and use of ultrasound, which inhibits offense, enables an animal to acknowledge the momentary dominance of its opponent without receiving a wound. Another aspect of ritualization is the exaggeration of threat postures and (in the cat) vocalizations which enable the opponent to escape before fighting occurs (Eibl-Eibesfeldt 1970). As a result, in both offense and defense, motor patterns tend to occur in a graded hierarchy from low-intensity threat to high-intensity attack or escape. The complex evolution of threat among muroid rodent species has been reviewed elsewhere (Adams, submitted for publication).

In the following pages the data on the brain mechanisms of intraspecific aggression, obtained primarily from the cat and rat, will be considered in terms of the three proposed motivational systems: defense, submission, and offense. The neural organization of the motivational mechanism, sensory filters for motivating stimuli, sensory filters for releasing and directing stimuli, and motor patterning mechanisms will be considered in that order. An extension of the discussion to other mammals, in particular, primitive mammals (the opossum) and primates, will be made at the conclusion of the other analyses.

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