The theoretical framework for understanding primate grouping patterns was originally proposed by Wrangham (1980). In it, he set up the contrast between female-bonded groups, in which females remained with kin to defend food resources, and non-female-bonded groups, in which females disperse to avoid competing with kin for food resources. The main determinant of whether females should remain or disperse in their natal groups is the distribution of resources. When resources are clumped, females benefit from banding together to maintain access to those clumped resources and defend them in contests. However, when resources are dispersed across the landscape, females need to separate to forage, and scramble competition results in them indirectly competing against each other. Thus, females should disperse so that they are competing against mostly unrelated females.
This theoretical framework is the foundation of most of our current research on primate social organization. However, in the thirty-plus years since its publications, there have been additions, revisions, and challenges to this framework. For example, other factors that have been incorporated into socioecological theory include the role of predation and infanticide.As research accumulates on a greater range of primate species in different environments, we find more situations in which primates do not appear to follow the theoretical predictions. Thierry (2008) highlights some other factors that need to be considered. These include the role of males in resource defense, the selective pressures of pathogens and parasites on social groups, and the role of cognition in mediating social relationships and foraging decisions. Furthermore, he also highlights the role that phylogenetic inertia may play in determining social systems. Clutton-Brock and Janson (2012) also highlight some of the weakness in the socioecological model. For example, why do frugivorous female macaques and capuchin monkeys stay in their natal groups, whereas frugivorous spider monkeys and chimpanzees disperse? They suggest that we should also consider the role of life history and male coalitionary defense in female dispersal. For example, if females are reaching reproductive age within the group that still contains their fathers, uncles, or brothers, and those males prevent other males from accessing the group, females may need to disperse to gain access to mating opportunities.
For me, one of the greatest limitations of the Wrangham (1980) and others’ socieocological models is their predictions for female social relationships. These theories predict that females that disperse should have weak social relationships and undifferentiated dominance hierarchies. However, in species with female dispersal, we see a range of female relationship strength and dominance hierarchies. As we study more and more species that were originally characterized as “non-female-bonded,” we are increasingly seeing that female bonds are important.
The contrast between spider monkeys, chimpanzees, and bonobos is one that illustrates some of this variation. Chimps and bonobos are closely related, and share the same basic patterns of dispersal and social organization. However, as we’ve discussed, they differ greatly in some aspect of social behavior, particularly regarding female social relationships. Females have strong dominance hierachies, and exhibit strong bonds. Although female chimpanzees also have dominance relationships, these relationships are not as pronounced, and females spend greater amounts of time foraging in smaller parties or alone. As Symington (1990) described, spider monkeys follow a similar pattern to chimpanzees. Furthermore, the longest-running studies on female spider monkeys are only able to generally divided females into dominant and subordinate groups, without any further differentiation. Asensio (2008) suggest that these patterns are largely due to age and residence. Females that are well-established in the community are the dominant females, whereas new immigrants and natal subadults are low-ranking.
While we have some evidence indicating that availability of resources allows for increased female affiliative opportunities in bonobos and limits these opportunities in spider monkeys and chimps (White 1998, Symington 1990, Chapman et. al 1995), there is greater within-species variation than we realized. Even despite some of those ecological constraints, we nonetheless are now increasingly finding evidence for strong female affiliative relationships for chimpanzees and spider monkeys (Lehmann and Boesch 2008; Rodrigues 2013; Wakefield 2013). However, it appears as though these patterns may differ greatly across sites. Our next step is to try to determine what factors affect the pattern of these relationships, and variation between sites.
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