Evolution.

Society for Experimental Biology

Date:: 1953

Catalogue details

Licence: Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0)

Credit: Evolution. Source: Wellcome Collection.

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SOCIAL BEHAVIOUR AND PRIMATE EVOLUTION 433 mediate the control of emotion; a reduction in size and relative importance having taken place in the nuclei with olfactory origin and connexions (cortico-medial) (Crosby & Humphrey, 1941). It is equally significant for our understanding of the structure of the control of autonomic function that the anatomical basis exists for equili- bratory circuits to provide the mechanism of emotional control. 'Papez (1937) traced a path from the gyrus cinguli, through the hippocampus and fornix, to the mammillary body and thence, via the mammilothalamic tract, to the anterior nuclei of the thalamus, and back to the starting point in the gyrus cinguli.' Moreover, another 'feedback' circuit from the mammillary body (concerned with sympathetic efferent discharges) to the hippocampus, and thence back to the mammillary nucleus, enlarges as we trace the changes in it from reptile through the mammal to the primate (Le Gros Clark, 1952). The greatest enlargement occurs in the retroactive circuit which feeds back impulses leaving the mammillary body to the hippocampus. This is evidence that the anatomical basis for sympathetic control is most highly developed in the group we are discussing, and that the characteristic trend of their evolution represents in this respect, as in their enlargement of the neocortex, a continuation of those changes which brought the mammals into existence and first enlarged the cortex in the tetrapods. We have postulated that the enlargement of the neocortex is an anato¬ mical adaptation to the circumstances requiring an equilibrational response, and that, therefore, the neocortex facilitates equilibration. In one of the few instances in which social behaviour has been studied following brain lesions, the subsequent behaviour of the animal is of very great interest. Ward (1948 <2) has given the following description of changes in social behaviour of macaques following frontal lobe operation. Immediately following either unilateral or bilateral subpial resection of the rostral cingular gyrus in the monkey, there is an obvious change in personality (Ward, 19496). In a large cage with other monkeys of the same size it (the monkey) showed no grooming or acts of affection towards its companions. In fact, it behaved as if they were inanimate, it would walk over them, walk on them if they happened to be in the way, and would even sit on them. It would openly take food from its companions, and appeared surprised when they retaliated, yet this never led to a fight for it was neither pugnacious nor even aggressive, seeming merely to have lost its 'social conscience' (Ward, 1948 ß). It is thus evident that, following removal of the anterior limbic area, such monkeys lose some of the social fear and anxiety which normally EBS VIT 28