Class structure in a rhesus monkey group: the interplay between dominance and tolerance

Tests of competitive dominance are usually designed to eliminate social tolerance and variations in motivation. The present study applied a more naturalistic test paradigm to a large captive group of rhesus monkeys (Macaca mulatta). The monkeys were deprived of water for only 3 h, after which a shareable resource was presented. Twenty such tests were videotaped over a 6-month period. Drinking priority and the nature and duration of interactions around the water basin were compared to the group's clear-cut formal hierarchy, as defined by the direction of submissive teeth-baring. The adult hierarchy could be subdivided into two classes: precedence in drinking tests was as expected from formal dominance between classes but was not clear-cut within classes. This seemed due to variations in tolerance. Kin always belonged to the same class and showed high tolerance. Tolerant behaviour was also more common among unrelated adult females of the same class than of different classes. An alternative hypothesis to class structure, explaining the differences in tolerance on the basis of rank-distance effects, was rejected. The class division may be maintained through special intolerance between females ranking at the class borders.