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Report on SAGA2/COE Symposium

"On human bonding: The search for a unified theory of ape evolutionary ecology and hunter-gatherer social organization."

Richard Wrangham

     Humans evolved from apes, which implies that human social organization might follow principles for explaining ape social organization. This conjecture has not been seriously tested, partly because until recently, principles explaining the great ape social systems have been uncertain. However, the comparative analysis of great ape evolutionary ecology is becoming stronger as a result of an improved understanding of the social effects of scramble competition, contest competition, the costs of motherhood, and infanticide pressure. In addition, the selective pressures influencing hunter-gatherer social ecology are becoming clearer, including the roles of hunting and cooking. Based on these developments, a unified theory may be possible, but it stumbles on an unresolved problem in the relationship between grouping and alliances within human groups. Among primates, sex differences in gregariousness tend to be correlated with sex differences in alliances: for example, among chimpanzees and spider monkeys, males are more gregarious than mothers, and form stronger intrasexual alliances. Among human foragers, however, mothers can be at least as gregarious as males, yet male-bonding is more important (has more important social consequences) than mother-bonding. This means that gregariousness is not a sufficient explanation for alliance formation in human foragers. Accordingly, it is necessary to reconsider the theory of male-bonding in other primates also.
     In this paper, as a result, I propose a new hypothesis for the pattern of intrasexual bonding in fission-fusion species. Fission-fusion species showing territorial defence include at least six groups of primates, which differ in unexplained ways in sex-specific bonding, philopatry and participation in territorial defence. Thus, among chimpanzees, humans, and spider monkeys, males are the philopatric sex, and males are also the primary defenders of the territory. In bonobos and muriqui, males are philopatric but both sexes can be involved in territorial defence. In ruffed lemurs, by contrast, females are both philopatric and the principal territorial defenders.
     To account for these patterns, I suggest that the sex that tends to become the defenders of the territory is whichever most often meets rivals of the same sex. Thus, I concur with previous analyses that the costs of parenthood cause scramble competition to be more intense for mothers than for males. In addition, however, I note that scramble competition not only causes mothers to travel in smaller parties compared to males, but also causes mothers to travel for shorter daily distances. This leads in most species to mothers having smaller home ranges than males, and therefore to males meeting rivals from neighbouring groups more often than mothers do. Male rivals from neighboring groups engage in contest competition over territorial space, and accordingly, males benefit from having reliable allies, and therefore from a philopatric residence system that offers male kin as allies. According to this "big-range" hypothesis, the female philopatry of ruffed lemurs can now be explained, because this is the only species of primate in which males take primary responsibility for guarding the young. Male ruffed lemurs therefore have higher costs of parenthood than mothers, which suggests that they have shorter day-ranges than mothers, and therefore that females are the sex that encounter neighbors most often. Similarly, the participation of both females and males in territorial defence among muriqui and bonobos (and some populations of chimpanzees, such as at Ta?) is explicable by these populations have relatively low costs of scramble competition, which reduce the ranging costs of motherhood. Finally, various carnivores such as spotted hyenas and wolves exhibit female-biased or bisexual philopatry. Carnivores provide dens for their young, and denning duties are shared by both sexes. This again means that the costs of motherhood are reduced or eliminated, allowing females to range as far as males, and therefore giving females the opportunity to encounter rivals from neighbouring groups. In sum, the "big-range" hypothesis appears satisfactory for explaining patterns of philopatry and territorial defence in non-human primates and carnivores. The "big-range" hypothesis also seems applicable to human foragers, but in a slightly different way because foragers present a unique combination of central-place foraging and sexually differentiated foraging strategies. Among some such populations, scramble competition appears relaxed for mothers, a result of mothers foraging for relatively low-quality foods, which they find close to camp. Essentially, the requirements of central-place foraging mean that the costs of motherhood reduce women's travel distance more than they reduce women's foraging group size. Men, by contrast, forage for high-quality foods, which induces more intense scramble competition, and smaller parties. However, because they are freed from the costs of parenthood, they can travel long distances. Since men are able to travel further than women, they have larger home ranges, and encounter invaders more often than women do. As in chimpanzees, it therefore pays them to respond to the potential for contest competition by forming coalitionary bonds. These bonds are then available for use within the social group, including for political domination of women. Variations in the strength of male-bonding among recent human foragers are then explicable as results of varying intensities of territoriality: these may themselves arise from either ecological or political sources.
     How long has male-bonding in the context of territorial defense occurred? Central-place foraging and sexually differentiated foraging strategies both follow from the adoption of cooking. This theory therefore suggests that male-bonding has been an important component of the human social system since the adoption of cooking, which was certainly in place by 0.25 mya and may have begun about 1.9 mya. In most habitats, territoriality would have been an important pressure, as it is routinely among chimpanzees and other fission-fusion species of non-human primates and carnivores. Accordingly, and in contrast to the conventional wisdom in social anthropology and archeology, our hunter-gatherer past should be viewed as a time when humans typically lived in male-bonded groups, in which males used their bonds to compete aggressively against members of neighbouring groups. This view suggests that our evolutionary past has contributed importantly to the contemporary psychology of male intergroup aggression.


1999 R.W. Wrangham. Why are male chimpanzees more gregarious than mothers? A scramble competition hypothesis. In P. Kappeler (ed.) Male Primates. Cambridge University Press, p. 248-258.
1999. R.W. Wrangham, J. H. Jones, G. Laden, D. Pilbeam and N.L. Conklin-Brittain. The raw and the stolen: cooking and the ecology of human origins. Current Anthropology 40: 567-594.
1999 in press. R.W. Wrangham. The evolution of coalitionary killing: the imbalance-of-power hypothesis. Yearbook of Physical Anthropology.
2000 in press. R.W. Wrangham. Out of the Pan, into the fire: how our ancestors' evolution depended on what they ate. In F.B.M. de Waal (ed.) Tree of Origin. Harvard University Press.


Copyright (C) 1999- COE International Symposium