humans and chimpanzees.
A second unusual feature of a chimp community, but one that chimps also share with people, is a propensity to conduct murderous raids on neighbors. Male chimps not only defend their territory but conduct regular, often lethal, attacks on neighboring communities. This discovery came as a considerable surprise to many biologists and sociologists who had assumed that warfare was a uniquely human phenomenon. Why do chimps hold and defend territories in the first place? Why do they kill each other? Chimp researchers believe they have been able to unlock the basic logic of chimp social structure, at least in general outline. Chimp society turns out to be matched to the nature of the food supply, which is principally fruit. The trees come into fruit sporadically. They tend to be scattered and do not supply enough fruit for large parties of chimpanzees. Female chimps, needing to sustain themselves and their young, find it more efficient to feed by themselves. They live in home areas, a few square kilometers in size, which they rarely leave. The size of these areas is very important. Females have shorter intervals between births—in other words bear children faster—when their territory is larger, according to an analysis of Gombe chimps by Jennifer Williams and Anne Pusey.
Considering strategies for the male chimps, each could try to achieve reproductive success by guarding one female. But it seems to be more efficient for the males to band together and defend territory that includes a larger number of females. One reason that this makes sense is that the males tend to be related to each other, because of the patrilocal system, and therefore in defending a group of females they are assisting their male relatives' reproductive efforts as well as their own. An individual's kin carry many of the same genes as he does. As the evolutionary biologist William Hamilton pointed out in his doctrine of inclusive fitness, for a person to help get an equivalent number of his kin's genes into the next generation is about as good as propagating his own. This is why genes favoring altruistic behavior have evolved in kin-based societies. The same logic underlies the cohesiveness of ant and bee societies, whose workers, by a special quirk of insect genetics, are more closely related to their sisters than to any daughters they might have. Because of this relationship, the workers have forsaken their own chance to raise children entirely and are content to live as sterile nurses for the queen's children. In chimpanzee society, males and females do not generally spend much time together except for the purpose of mating. The members of each sex are organized in separate hierarchies. Every adult male demands deference from every female, resorting to immediate violence if a submissive response is not forthcoming. Differences notwithstanding, chimp and human societies serve the same purpose, that of providing males and females appropriate ways of securing their individual reproductive advantage.
At the head of the male hierarchy is the alpha male, who maintains his position by physical strength and, just as importantly, by building alliances with other males. "A dominant male is constantly at risk from opportunistic coalitions formed by lower-ranking individuals and must continually assert his dominance through agonistic display," write John Mitani and 171
colleagues. These tests of leadership, which primatologists sometimes refer to ironically as elections, can occur at any time. Losing an election in chimp society is not a good idea. The loser's defeat may take the form of having personal parts torn off of him and being left for dead. Long rule does not guarantee a peaceful retirement. Ntologi was alpha male at Mahale for 16 years before he was overthrown by a rival coalition and killed. What is the upside of being alpha male if life is a daily gamble on retaining power, with violent death the only retirement plan on offer? Whether or not chimps ponder this question, evolution has provided the answer: high position in a chimp male hierarchy guarantees that a male will have more matings and more progeny.
This outcome was at first far from obvious to researchers. When females enter their fertile period, they advertise the fact with melon-sized pink swellings on their rear end. They become very gregarious and do their best to mate with every male in the community, with an average of 6 to 8 couplings a day. One female observed by Goodall achieved 50 copulations in one 172
day. The females' purpose, biologists believe, is to confuse paternity. If a male chimp believes there is a chance a baby is his, he is less likely to kill it.
Given this seemingly chaotic mating system, how do high ranking males in fact reap their due rewards of office? First, they do secure more matings, even though rarely exclusive ones. Second, there is the phenomenon of sperm competition. Because of the chimps' multiple mating system, advantage will accrue to the male who can deliver the most sperm and flood out the competition. Hence evolution has favored male chimps with very large testes for their body size. But whether or not the senior males reaped the rewards of rank was unclear until the advent of DNA paternity testing. Julie Constable and colleagues recently reported the results from a 20-year study of chimps at the Kasakela community in Gombe. They found that despite appearances, the system works. The reigning alpha male accounted for 36% of all conceptions, and for 45% if one excludes his close female relatives, with whom conceptions 173
would be avoided. Another 50% of matings were scored by high ranking males. Usually at Gombe there is the alpha male and then two or more other males who count as high ranking. Most of these conceptions studied by Constable occurred during general free-for-all sexual romps, or "opportunistic matings," as the primatologists call them, suggesting that the alpha males owe of a lot of their fatherhoods to victory in the sperm competition wars.
Like males, female chimps have a hierarchy. It is less discernible, because females spend much of their time feeding alone in their core areas and are not in constant interaction as the males are, but it bears significantly on the females' 174
reproductive success. Low ranking females lose more of their babies than do socially ascendant females. This is partly because socially superior females will sometimes kill the infants of lowly females. The high ranking Passion and her daughter Pom snatched and ate the babies of their neighbors at Gombe, perhaps to discourage trespass on their feeding areas. What makes one female dominant over another is not yet clear, but in general terms rank in chimpanzee society seems to depend a lot on one's mother's status. Flo, a high ranking and sexually attractive female, was the mother of Figan, who was alpha male in his Gombe community for 10 years (his reign date was 1971-1981), as well as of Fifi, who became dominant female. Fifi helped her firstborn son Freud take his first steps to power by intervening on his side when, as an adolescent, he started to establish dominance over the females. Freud was alpha male from 1994 to 1998, when he fell sick with mange and was deposed by his younger brother Frodo. Historians attribute dynastic wars among people to all kinds of complex motives, from glory to territorial gain to spread of religion. Chimpanzees' intentions, unobscured by such rationalizations, can be judged strictly by their results. It's all about reproductive advantage. Each player acts so as to get as many descendants as possible into the next generation. The males try to ascend the male dominance hierarchy so as to mate with as many females as possible. The females seek out the best feeding areas so as to bear as many surviving children as possible. The ultimate objective is simple, but in a complex society each individual must act in many intricate ways to achieve it.
Presumably chimps' social behavior is genetically shaped, but like human societies they have culture too, in the sense of learned behavior that varies from one chimp community to another. In a recent survey of seven long term chimp studies, Andrew Whiten and colleagues identified 39 behaviors that differed from one community to another without obvious
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