A course in behavior and evolution, taught by chimps [ NYT ]
Very interesting summary of modern knowledge of chimp behavior in the wild. Disturbing violence and war-party behavior in a species that is 99.2% genetically identical to us [99.5% in "important genes" ]. There is a strong similarity to major "cultural" behaviors of primitive people.
But take heart from the chimp subgroup of peaceful, female-dominated, sex-mad bononbos.
Make love not war.
Doc
Nicholas Wade
Thursday, November 27, 2003
NEW YORK Fossil bones record the history of the human form, but they say little about behavior. A richer source of the way human social behavior evolved may be chimpanzees, with whom people shared a common ancestor as recently as 5 or 6 million years ago. From knowledge of chimp behavior, biologists can plausibly infer the social behavior of the shared human-chimp ancestor, and from that they can reconstruct the evolutionary history of human social behavior.
Such reconstructions are subject to much uncertainty and debate, especially when they imply a genetic basis of human behaviors like living in communities based on male kinship or conducting lethal campaigns against neighbors. But the goal is to shed light on the full sweep of human social behavior, tracing its evolution from an apelike community with separate male and female hierarchies 5 million years ago to the family-based societies of today.
A principal assumption is that chimps, unlike people, have changed little and so their social behavior is a good guide to that of the common ancestor. One support for this idea is that the earliest fossils on the human side after the split are very chimplike. Another is that the chimps of western and eastern Africa are hard to tell apart, despite 1.5 million years of separate evolution.
After 40 years of arduous study, biologists have put together a coherent, though not yet complete, picture of chimpanzee societies. From observations at several sites in Africa, there is an "emerging consensus regarding chimpanzee social structure, territory characteristics and intergroup interactions," two primatologists, Michael Wilson of the University of Minnesota and Richard Wrangham of Harvard, write in The Annual Review of Anthropology.
Chimps are not easy to observe. It can take five years for them to get used to watchers. They must be followed, through hilly country, for 15 hours at a stretch. They can be dangerous. Humphrey, a male chimp at the Gombe National Park in Tanzania, nearly killed Wrangham by hurling a large rock at his head. "He was just showing off," Wrangham says indulgently. A female chimpanzee at Mahale, also in Tanzania, attacked and severely wounded two researchers she considered to be allies of males that had killed her infant.
A major surprise has been that chimps turn out to live in territories whose borders are aggressively defended by roving parties of males. Jane Goodall, who pioneered long-term studies of chimps at Gombe, at first believed she was watching a single peaceful community. But as researchers started to follow animals throughout the day and watch their interaction with others, they found that groups of male chimps went out on border patrols, ready to attack and kill the males of neighboring communities.
The males in each community are related to one another because they spend their lives where they were born, whereas the females usually migrate to neighboring communities soon after reaching puberty, a practice that avoids inbreeding. This patrilocal system, of a community based on male kin bonding, is unusual, but familiar to anthropologists because it is practiced by most hunter-gatherer societies.
The males' operational strategy seems to be to defend a territory as large as possible so as to improve the community's food supply, which is principally fruit, and thereby their reproductive success. Anne Pusey of the University of Minnesota has found that the larger the female chimp's home feeding area, the shorter the interval between births.
In two known cases, a chimp community has wiped out all of a neighbor's males. Though the females may be absorbed into the victor's community, the goal seems to be getting rid of a rival rather than capturing females, since males often attack strange females.
Very few species live in male-kin-bonded communities with female dispersal. And only two practice lethal raids into neighbors' territory to kill off vulnerable enemies. "This suite of behaviors in known only among chimpanzees and humans," Wrangham and Dale Peterson write in their book, "Demonic Males."
Many chimp behaviors are ones people can recognize. Much of their body language - a mother holding her baby, the expression of an orphaned infant - is instantly readable. About 19 varieties of tool use by chimps have been noted, though each community has its own cultural subset. Chimps dose themselves with medicinal plants when they are sick. But where they differ most from human societies is in their sexual arrangements.
Males and females do not associate in families but in separate hierarchies. Males make females defer to them, with violence whenever necessary, and every female is subordinate to every male. A female chimp advertises her fertile period with a visible swelling and is then so pestered by males that she may get to eat only at night. But the great advantage of mating with every male in the community in a public orgy is that it confuses paternity, significant insurance given that males are liable to kill infants they know are not their own.
Males sometimes get females to leave the group and give them exclusive matings during a cycle. These consortships are sometimes forced on the female, who may recognize the risk to any infant conceived by a known father.
An intriguing variation on the chimpanzee social system is that of bonobos, which split from chimps about 1.8 million years ago. With bonobos, who live south of the Congo River, the female hierarchy is dominant, and males do not patrol the borders to kill neighbors. Though bonobos are almost as aggressive as chimps, they have developed a potent reconciliation technique - the use of sex on any and all occasions, between all ages and sexes, to abate tension and make nice.
Assuming the common ancestor of people and chimps had social behavior that was essentially chimplike, how much of that behavior has been inherited by people? The unusual behavioral suite of male kin bonding and lethal territorial aggression may look as if it has been inherited with little change. Among the Yanomamo, a South American tribe, the number of males who die from aggression is about 30 percent, the same rate found among Gombe chimps.
Not everyone believes that chimp social behavior is a good guide to human evolution. "All these things are suggestive and point tantalizingly to things we want to know," said Ian Tattersall, a paleoanthropologist at the American Museum of Natural History. "We just have to bear in mind that none of this is demonstrable in any highly convincing way."
But Robert Foley, an evolutionary anthropologist at the University of Cambridge in England, says a lot can be learned from ape sociality about the evolution of human social institutions. A community size of 80 to 100 people, typical among chimps and hunter-gatherers, is one feature inherited from the common ancestor. Another is a society formed on the basis of male kin bonding.
The New York Times
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