Synergistic effects of kinship, sex and rank in the behavioural interactions of captive stump-tailed macaques

In almost all non-human primate species kinship, sex and rank bias the patterning of interactions among subjects, seemingly in a way that favours the individual's fitness. A number of studies have been done in order to study separately the effects of these variables, concluding that an adequate model of interactions cannot be developed assuming preponderance of a single variable. The 'willingness' of one animal to interact with another can be explained as the result of assessing the possible outcomes (proximate and/or ultimate) of the interaction. The interrelationship of variables such as kinship, sex and rank, among others, provides the actors with a basis for deciding their rates of social interactions. The following study was done to assess whether kinship, rank and sex exerted independent, additive effects in the within-group distributions of eight social behaviour categories of captive groups of stump-tailed macaques. We found that the three variables synergistically affected the patterning of all behavioural categories. This suggests that the pay-offs of social interactions do not vary linearly, being intrinsically correlated with the actors' attributes so that uncertainty in the choice of a potential partner is minimized.