Grooming interchange for resource tolerance: biological markets principles within a group of free-ranging rhesus macaques

Abstract

In group-living animals, allogrooming is a common, heterogeneously distributed affiliative behaviour. Among non-human primates, Barrett et al. (1999) predicted ways in which Biological Markets principles interact with competitive regimes to influence grooming reciprocity and interchange. Most tests of these predictions, done at a group level, have produced inconsistent results. Here we take a novel approach by testing these predictions across individuals within a group. This is based on the premise that in groups facing moderate-to-high within-group-competition, individuals vary in their abilities to access resources based on their competitive abilities, causing them to pursue different grooming exchange strategies. We examine evidence for grooming reciprocity and interchange for tolerance at drinking sources among adult females within a group of rhesus macaques (Macaca mulatta) on Cayo Santiago. We test the above premise by assessing hierarchical steepness, and the relationship between individuals’ David’s scores (DS) and access to drinking sources. Finally, we examine the relationship of DS with grooming reciprocity and interchange to see whether they are consistent with the operation of market forces among individuals. Social network comparisons revealed that giving grooming was strongly predicted by both receiving drinking tolerance (interchange) and receiving grooming (reciprocity), despite strong associations with proximity and maternal kinship. The group showed a moderately steep hierarchy, and negative correlations between individuals’ David’s scores and difficulties in accessing drinking stations. Finally, we found partial support for a market-based explanation. Individuals with relatively low David’s scores were more likely to interchange grooming with drinking tolerance. However, grooming reciprocity wasn’t greater among individuals with higher David’s scores. Our findings suggest that multiple explanatory frameworks — reciprocity, market-based interchange, and/or proximity-mediated interchange/social bond investment — may all shape rhesus grooming exchange patterns. Future directions include examining evidence for additional forms of grooming interchange, and the influence of between-group-competition and stress-indicators on grooming reciprocity.