Nature of Proximate Mechanisms Underlying Primate Social Systems: Simplicity and Redundancy SALLY P. MENDOZA, DEEANN M. REEDER, AND WILLIAM A. MASON Given the complexity of primate so- cial systems, it is not surprising that the search for proximate mechanisms is a daunting task. We have long known that the types of social groups that primates form and maintain are characteristic of a given species. 1 It is also clear that social systems emerge from interactions between individu- als. 2 Understanding proximate mech- anisms contributing to social systems, therefore, requires a focus on the ten- dencies of individual members of a species to interact with one another in particular ways that lead to the pro- duction of the species-typical group- ing pattern. At a gross level of analysis, primate social systems can be categorized by the number and sex ratio of adults typically found in the groups. Under- lying the most common modal group- ing pattern, the multimale-multife- male group, is the tendency for adults to tolerate and even seek like-sex com- panionship. Other types of primate social groups depend in some part on categorical exclusion of conspecifics. 3 For example, in single-male or harem groups males are intolerant of the close proximity of other males, and in monogamous primates both males and females are intolerant of unre- lated like-sex adults. In each of these systems, the degree of tolerance of like-sex companions is dependent on circumstances or age. Young males in species characterized by unimale groups often leave their natal groups and form bachelor groups until the opportunity to secure a harem arises. Clearly, the intolerance displayed by males of these species for like-sex companionship is provisional. Toler- ance of conspecifics may also be man- ifest between groups. In some cases, extensive interactions between ani- mals of neighboring groups lead to elaborate multigroup coordination of activity. 4 This is seen in extreme form in the Hamadryas baboon (Papio hamadryas), which lives in harem groups but shows coordination of sleeping and travel patterns among the scores of harems in a community. 5 Male-female relationships are also variable, giving rise to distinct quali- ties of sociality. Squirrel monkeys (Saimiri sciureus), for example, live in large multimale-multifemale groups, but males and females do not interact extensively. In contrast, the male-fe- male relationship in the monogamous titi monkey (Callicebus moloch) is characterized by a close emotional bond. 6 Understanding the types and quali- ties of long-term associations, both sought and avoided, is a critical step in elucidating the proximate mecha- nisms underlying primate social sys- tems. 7,8 For a complete understand- ing, we must also take into account the need of group members to coordi- nate many other activities, such as travel patterns, feeding schedules, and selection of sleeping sites. Individual activity patterns, the manner in which individuals respond to social and non- social events, and the use of the spa- tial environment must also be consid- ered. Where do we begin the search for mechanisms? Given the persistence of the species-typical form of social groups across environments and gen- The authors are at the Department of Psy- chology and California Regional Primate Research Center, University of California at Davis, Davis, CA 95616. Grant sponsor: National Institutes of Health; Grant number: RR00169; Grant sponsor: National Institute of Mental Health; Grant number: 1F31 MH11502- 01A1. *Correspondence to: Sally P. Mendoza, California Regional Primate Research Center, University of California, Davis, CA 95616. E-mail: spmendoza@ucdavis.edu Key words: group composition; social dynamics; titi monkey; monogamy; territoriality; neurobiological systems; modelling; components of sociality Evolutionary Anthropology, Suppl 1:112–116 (2002) DOI 10.1002/evan.10071 Published online in Wiley InterScience (www.interscience.wiley.com). © 2002 Wiley-Liss, Inc. Understanding the types and qualities of long-term associations, both sought and avoided, is a critical step in elucidating the proximate mechanisms underlying primate social systems. For a complete understanding, we must also take into account the need of group members to coordinate many other activities, such as travel patterns, feeding schedules, and selection of sleeping sites. Individual activity patterns, the manner in which individuals respond to social and nonsocial events, and use of the spatial environment must also be considered. Here, we will illustrate some of the proximate mechanisms contributing to the monogamous and territorial social system of the titi monkey. We will then present a model showing how many relatively simple, error-prone mechanisms can collectively yield stereo- typic expressions at the group level. 112 Primate Behavior, Ecology, and Conservation