Hunting is a cooperative endeavor. Cooperation is essential for making the larger kills—and also for avoiding their dangers, such as an enraged bush pig charging, tusks first, to save its young. The hunters exhibit real teamwork. One chimp may softly call to another when it has detected prey in the underbrush. They smile to one another. The victim is flushed out of its cover toward other chimps who are lying in wait. Escape routes are blocked off. Ambushes are refined. Plays are called. The chimps—so passionate after the kill—were coolly planning it all out beforehand.
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In densely forested habitats, the territory controlled by a given chimpanzee group is only a few kilometers wide. In sparsely wooded regions, it can be as much as thirty kilometers across. These are the territories that a chimp group considers its turf, its home, its fatherland or motherland, to which something like patriotic sentiments are owed. It is not to be trespassed by strangers. It’s a jungle out there. The typical day range of a chimp combat patrol is a few kilometers. So if they live in heavy forest, they can fairly readily patrol a good portion of the border in a single day. But if the vegetation and food supply are more sparse and their territory accordingly larger, it may be a few days’ journey from one end to the other, and longer if they go around the perimeter.A patrol is typified by cautious, silent travel during which the members of the party tend to move in a compact group. There are many pauses as the chimpanzees gaze around and listen. Sometimes they climb tall trees and sit quietly for an hour or more, gazing out over the “unsafe” area of a neighboring community. They are very tense and at a sudden sound (a twig cracking in the undergrowth or the rustling of leaves) may grin and reach out to touch or embrace one another.During a patrol the males, and occasionally a female, may sniff the ground, treetrunks, or other vegetation. They may pick up and smell leaves, and pay particular attention to discarded food wadges, feces, or abandoned tools on termite heaps. If a fairly fresh sleeping nest is seen, one or more of the adult males may climb up to inspect it and then display around it so that the branches are pulled apart and it is partially or totally destroyed.Perhaps the most striking aspect of patrolling behavior is the silence of those taking part. They avoid treading on dry leaves and rustling the vegetation. On one occasion vocal silence was maintained for more than three hours … [When] patrolling chimpanzees return once more to familiar areas, there is often an outburst of loud calling, drumming displays, hurling of rocks, and even some chasing and mild aggression between individuals … Possibly this noisy and vigorous behavior serves as an outlet for the suppressed tension and social excitement engendered by journeying silently into unsafe areas.31
In this description by Jane Goodall of a patrol at Gombe, we are taken by the ability of the chimps to overcome their fear, to exercise self-control by inhibiting their usual noisy interchanges, but particularly by their deductive abilities. These chimps are tracking. They are weighing the evidence of branches, footprints, droppings, artifacts. We might expect that, when food is in short supply, group differences in tracking skills help determine who lives and who dies. Not just strength and aggressiveness are being selected here, but something akin to reasoning and quick-wittedness. And stealth. When one human who lived with a troop for a long time tried to accompany a patrol as it set out, they looked at him reprovingly. He was just too clumsy. He could not, as they do, slip silently through the forest.
So the long-range combat patrol wends its way toward the borders of their turf If it’s more than a day’s walk, they’ll set up camp at night and continue on their way tomorrow. What happens if they encounter members of another group, Strangers from the adjacent territory? If it’s just one or two intruders, they’ll attempt to attack and kill them. There’s much less disposition here toward threat displays and intimidation. But if two parties of roughly equal strength encounter one another, now there are a great many threat displays, rocks and sticks are thrown, trees are drummed. “Somebody hold me back, I’m gonna break his knees,” you can almost hear them saying. They practice threat assessment If the patrol senses an obviously larger number of Strangers, it is likely to beat a hasty retreat. At other times chimp patrols may penetrate enemy territory or even raid its populated core area—for many purposes, including copulating with unfamiliar females. The combination of tracking, stealth, danger, teamwork, fighting hated enemies, and the opportunity for sex with strange females is enormously attractive to the males.
The delight shown by the members of a patrol after having successfully returned from dangerous—perhaps enemy-held—territory is little different from what happens when chimps unexpectedly encounter a substantial cache of food. They screech, kiss, hug, hold hands, pat one another on the shoulders and the rump, and jump up and down. Their camaraderie is reminiscent of teammates in mutual embrace just after winning the national title At the start of a heavy rain, male chimps often perform a spectacular dance. On coming upon a stream or waterfall they display, seize vines, swing from one tree to another, and cavort high above the water in a breathtaking acrobatic performance that may last for ten minutes or more. Perhaps they are awed by the natural beauty or entranced by the white noise. Their evident joy sheds a revealing light on the eighteenth-century doctrine32 that humans are right to enslave other animals because we are unmatched in our capacity to be happy.
The prescription offered by Sewall Wright for a successful evolutionary response to a changing environment closely matches many aspects of chimp society. The species is divided into free-ranging groups, generally comprising between ten and one hundred individuals. They have different territorial ranges, so that if the environment alters the impact will be at least slightly different from group to group. A staple food at one end of a vast tropical forest may be a rare delicacy at the other. A blight or infestation that might result in serious malnutrition or famine for chimps in one part of the forest might have negligible consequences in another. Each territorial group is enough inbred that the gene frequencies differ systematically, group to group. And yet the pattern of inbreeding is relieved by exogamy (outbreeding). There are key sexual encounters with chimps from adjacent territories, initiated either when a patrol penetrates into alien territory or when a foreign female wanders over. These unions provide genetic communication, group to group, so that if in an adaptive crisis one group were more fit than the others, the adaptation would rapidly spread to the entire chimpanzee population through a sequence of sexual contacts—perhaps hundreds of copulations in a chain linking the remotest groups of a vast tropical forest. If there’s a modest environmental crisis, the chimpanzees are ready.
If this is indeed the explanation, at least in part, of the territoriality, ethnocentrism, xenophobia, and occasional exogamy that characterize chimpanzee society, we do not imagine that individual chimps understand the reasons for their behavior. They simply can’t stand the sight of strangers, find them hateful and deserving of attack—except, of course, for the chimps of the opposite sex, who are unaccountably exciting. The females occasionally run away with strange males, no matter what crimes they may earlier have committed against their land and kin. Perhaps they feel something of what Euripides makes Helen of Troy feeclass="underline" What was there in my heart, that I forgotMy home and land and all I loved, to run awayWith a strange man?…
Ah, husband still, how shall thy hand be bentTo slay me? Nay, if Right be come at last,What shalt thou bring but comfort for pains past,And harbour for a woman storm-driven:A woman borne away by violent men …33