The Effects of Strangers On Rhesus Monkey Groups

Observations were made of behaviour in each of 3 different laboratory groups of rhesus monkey adult females and infants for 5-6 replications of conditions in which either: (l) a group was undisturbed for at least a week, (2) a stranger adult male was present for 2 weeks, or a stranger adult female was present for 1/3 day in either (3) the group witltout a stranger male, or (4) the group with a stranger male. Groups behaviourally discriminated between their own members and strangers and between strangers of different sex. Strangers discriminated between other strangers and core group members. The presence of a stranger affected behaviour among core animals, the type of effect differing with the stranger's sex, and a stranger also affected behaviour between core animals and another stranger. These consequences of the presence of strangers were complex, but all could be explained by assuming that: (I) when more than two adult monkeys are together they are likely to have or attempt to form alliances, with high rates of affiliative and aggressive behaviour within an alliance, and cooperative aggression directed at non-alliance members, and that (2) females tend to prefer as alliance partners, in order, males most, then familiar females, and unfamiliar females least, while males tend to prefer less familiar over more familiar females. The latter preference may be a reason for inter-troop transfer by males in free-living troops. There was decreased tolerance among females, especially unfamiliar ones, in the presence of males. Such a mechanism might be a contriltitor to the stability of troop affiliation in free-living females. Various effects seen led to the viewpoint that while behaviour is determined by both individual characteristics and by the overall context of the group, these factors are quite interrelated, in that each contributes to the other. Group context "effects" may account for behavioural differences between the groups studied.