Brain Mechanisms

Although most available data are from studies on cats and rats, it is useful from the standpoint of an evolutionary analysis to examine data from the opossum and from primates as well. The opossum can tell us something of the general evolution of brain mechanisms of aggression in mammals as a whole, since it is similar to primitive mammals who evolved tens of millions of years ago. The primates can tell us something of the evolution of brain mechanisms for aggression in humans. In general, as will be noted, there are more points of similarity than difference among all these animals. The interpretation of data from the opossum and from primates is hampered by lack of an ethological analysis that could distinguish motivational systems of offense, defense and submission.

Midbrain mechanisms have been studied in the rhesus macaque. A study, completed over 40 years ago by Magoun et al. (1937) showed clearly that the midbrain pathways for defensive vocalizations and facial expressions are the same in the monkey as in the cat. Stimulation of the central gray has also been carried out in freely-moving macaques in a social situation, in which case it elicits a full range of defensive (and offensive?) motor patterns including chasing, jumping, biting, and fighting (Delgado 1963). Although midbrain studies, as such, have not been performed in opossums, it was the tentative conclusion of Bergquist (1970) that the main output pathway from the hypothalamus concerned with defense in the opossum is the periventricular fiber system to the midbrain central gray, just as it is in the cat.

There is a major forebrain pathway for defense in the opossum and in primates that may be activated by electrical stimulation of the lateral hypothalamus in preoptic and perifornical regions. This has been demonstrated in the opossum (Roberts et al. 1967; Bergquist 1970), the rhesus macaque (Robinson et al. 1969; Alexander & Perachio 1973), and the squirrel monkey (Renfrew 1969). The anatomical sites for stimulation of defense are similar to those in the cat and the rat, and the elicited behaviors appear to be homologous to biting defense in the rat and affective defense in the cat.

As in the cat and rat, there are two separate but adjacent zones of the hypothalamus and amygdala of the rhesus monkey from which defense (including attack) and submission (including only escape) can be obtained by electrical stimulation (Perachio & Alexander 1975). This suggests that there may be a consociate modulator in primates as well, and that it may switch the animal from defense to submission.

The taming effects of amygdala lesions, mentioned earlier with regard to the cat and rat, were originally described in primates (Kluver & Bucy 1939). The effect has been carefully studied in primate group social interactions (Rosvold et al. 1954). It has also been reported for the opossum (Hara & Myers 1973).

Septal lesions, which sometimes increase defense in cats and rats, have not increased defense and submission in primates and opossums. Defense reactions to tactile stimulation in opossums remain unchanged after septal lesions, while defense behavior due to visual and auditory stimulation is temporarily decreased (Hara & Myers 1973). Emotional behavior of squirrel monkeys (Buddington et at 1967) and macaques (Votaw 1960) is not changed by septal lesions. As noted earlier, the effects of such lesions in cats and muroid rodents are also variable from species to species and experiment to experiment.

Thalamocortical lesions in primates yield effects similar to such lesions in the cat and the rat. Lesions of the cingulum decrease defense (Glees et al. 1950), just as they do in the cat (Koridze & Oniani 1972). Lesions of the orbital frontal cortex of rhesus monkeys increase submission (called "aversive reactions" by the authors) and decrease defense (called "aggressive reactions") in response to a doll or a snake (Butter et al. 1970).

One report on primates is difficult to compare with data from the cat and rat: lesions of the globus pa1lidus abolish the penile display of squirrel monkeys, which has been interpreted as an "aggressive" motor pattern (MacLean 1978). One would presume that this would correspond to offense in the cat or rat, although the relevant comparative ethological analysis has not yet been undertaken. There are no comparable data on pallidal or other striatal lesions abolishing offense in other mammals, although there are suggestive data in the finding that dopamine metabolism in the mouse changes as a function of fighting (Hutchins et al. 1975).

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