The influence of social structure on social isolation in amphetamine-treated Java monkeys (Macaca fascicularis)

Amphetamine-induced social isolation in monkeys has often been considered a valid animal model for certain negative symptoms of schizophrenia. However, there appear to be many ambiguities in relation to the exact nature of the isolation. Therefore, the effect of orally administered amphetamine (AMP) on the occurrence of social isolation in Java monkeys was studied. In part I the rank dependency of the effects of AMP (0.5mg/kg) was investigated in four alpha-males and three beta-males. AMP increased 'proximity' and 'passive groom', and decreased 'active allogroom' in alpha-males. In contrast, AMP decreased all three behavioural elements to a certain extent in beta-males. It is concluded that AMP induces social isolation in beta-males, but not in alpha-males. In part II of this study the AMP-induced behaviour of the treated monkey and the simultaneously occurring changes in the non-treated monkeys were investigated in a detailed study of a single social group. AMP significantly reduced the frequency of 'exploration', 'locomotion', 'self-groom', 'swing', 'active groom', 'inspect', 'approach' and originally-present stereotypies. Thus AMP apparently reduces the ability to initiate behaviour which is characteristic for the adult animal. AMP did not affect the frequency of 'present' and 'play' and enhanced that of 'aggression' and 'fear' in the beta-male; it also elicited various juvenile-like behaviours in both alpha- and beta-males, suggesting that AMP induces a behavioural regression. Furthermore, the behaviour of the non-treated monkeys of the group was decisive for the occurrence of social isolation of the treated monkey. Thus, the effects of AMP on the social behaviour of Java monkeys depend on the individual sensitivity, the social position which the subject occupies in its group, and the behaviour of the partners of the treated subject.