Abstract
Multilevel (or modular) societies are a distinct type of primate social system whose key features are single-male–multifemale, core units nested within larger social bands. They are not equivalent to fission–fusion societies, with the latter referring to routine variability in associations, either on an individual or subunit level. The purpose of this review is to characterize and operationalize multilevel societies and to outline their putative evolutionary origins. Multilevel societies are prevalent in three primate clades: papionins, Asian colobines, and hominins. For each clade, we portray the most parsimonious phylogenetic pathway leading to a modular system and then review and discuss likely socioecological conditions promoting the establishment and maintenance of these societies. The multilevel system in colobines (most notably Rhinopithecus and Nasalis) has likely evolved as single-male harem systems coalesced, whereas the multilevel system of papionins (Papio hamadryas, Theropithecus gelada) and hominins most likely arose as multimale–multifemale groups split into smaller units. We hypothesize that, although ecological conditions acted as preconditions for the origin of multilevel systems in all three clades, a potentially important catalyst was intraspecific social threat, predominantly bachelor threat in colobines and female coercion/infanticide in papionins and humans. We emphasize that female transfers within bands or genetic relationships among leader males help to maintain modular societies by facilitating interunit tolerance. We still lack a good or even basic understanding of many facets of multilevel sociality. Key remaining questions are how the genetic structure of a multilevel society matches the observed social effort of its members, to what degree cooperation of males of different units is manifest and contributes to band cohesion, and how group coordination, communication, and decision making are achieved. Affiliative and cooperative interunit relations are a hallmark of human societies, and studying the precursors of intergroup pacification in other multilevel primates may provide insights into the evolution of human uniqueness.
Highlights
Among primates, multilevel social systems comprise several hierarchical tiers that are perhaps better referred to as modular or nested systems
Summary of the Papionin Pattern In sum, we give preferentiality to the social model for the evolution of multilevel societies in hamadryas baboons and possibly geladas, i.e., the transition from mm–mf to modular was shaped by intersexual factors to a substantial degree, while food dispersion and localized resources provided the ecological background in a process leading to multilevel societies
We have developed a theoretical framework based on socioecological theory to picture parsimonious scenarios for the evolution and maintenance of these most complex social systems in a variety of primate lineages
Summary
Multilevel social systems comprise several hierarchical tiers that are perhaps better referred to as modular or nested systems. The basal unit is typically a spatiotemporally cohesive one-male unit (OMU) with one adult male and one to several females. These core units aggregate at varying temporal scales and in that way form at least one second (the band) or even higher grouping level (the troop or herd) (Grueter and Zinner 2004). Interactions among individuals occur both within and between the social layers, but relationships are clearly much more close knit within the first tier, and core units tend to represent the reproductive units (Colmenares 2004; Dunbar and Dunbar 1975; Grueter et al in press-b; Stammbach 1987; Yeager and Kirkpatrick 1998; Zhang et al 2012). In multilevel societies (which on higher levels consist of multiple males and multiple females), subunits have stable membership over longer periods and bands are quite stable in
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