Group size, contest competition, and social structure in Cayo Santiago rhesus macaques

Abstract

In non-human primate systems, the Ecological Model of Female Social Relationships (EMFSR) views variation in female social structure as adaptations to variation in recent past and/or current ecological conditions. Group size may be a major additional demographic factor affecting social structure through its effects on resource competition. In particular, in Resident Nepotistic Despotic (RND) societies, when resources are uniformly clumped, larger groups are expected to face higher levels of within-group contest competition (WGC) than smaller groups and to respond by showing more highly despotic social relationships than smaller groups. On the other hand, smaller groups are expected to face higher levels of between-group contest competition (BGC) and hence, show greater levels of intragroup tolerance (or lower despotism). While formal models do not explicitly predict this kind of variation within species, many researchers have investigated such intraspecific variation. Thus, we tested these predictions across three groups of free-ranging rhesus macaques (Macaca mulatta) of varying sizes on Cayo Santiago, Puerto Rico, where we were able to control for variation in resource-patch contestability and predation risk. Subjects were adult females. For each group, we collected data on (1) exposure and (2) behavioural reactions to WGC and BGC at two independent sets of clumped resources (feeding corrals and drinking stations) and (3) agonistic and affiliative social behaviour, using focal animal, all occurrences, and ad-libitum sampling methods. As predicted, our largest group showed the greatest levels of exposure and behavioural responses to WGC contexts. Likewise, our smallest group showed greatest levels of exposure and responses to BGC contexts. However, aspects of social structure did not vary across groups in directions predicted by the EMFSR. These findings are broadly consistent with several previous comparative tests of the model on other primate taxa that found strong links between group size, ecological factors and contest competitive regimes, but little or no evidence of links with social structure. Our study adds strength to these conclusions given our unique ability to control for several variables on Cayo Santiago. We suggest that our findings may be generalizable to several wild rhesus populations in North India, given that they have lived in similarly dense populations and have been frequently provisioned by humans for thousands of years.