Comparison of the physical characteristics of grooming in two species of macaques (Macaca nemestrina and M. radiata).

Examined social grooming in groups of bonnet and pigtail macaques to test hypothesis that the physical aspects of grooming (body sites, postures, methods) evolved in more aggressive species of primates to serve social functions (proximity maintenance and tension reduction). Both species used social presents to direct grooming to particular sites, and used grooming to other sites to terminate interactions, thus regulating proximity with grooming to certain body sites. The 2 species differed in method of grooming. Pigtails primarily stroked (method used for tension reduction); bonnets picked or pick-stroked (method used for hygiene). Pigtail but not bonnet grooming method regulates tension reduction. Pigtails groomed in a social context, whereas bonnets groomed in solitary context. The pigtail results are similar to those found with rhesus monkeys. Different aspects of grooming serve different functions.