"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.
References.
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.