Killer Apes

From Michael P. Ghiglieri's THE DARK SIDE OF MAN: Tracing the Origins of Male Violence

[pp. 172-173] One big reason for those chimps’ community to fission into smaller parties is ecology. Sixty percent of a chimp’s diet consists of ripe fruit. Yet fruit is often so hard to find that wild chimps are drastically underweight compared to captive ones. Nor do enough huge fruit trees exist for all fifty or so chimps of a community to travel together and still get enough to eat. In any tree, the least dominant chimps, females in particular, lose in competition over what little fruit exists. Here again, however, males place solidarity ahead of calories. Despite the importance of a square meal, when approaching big fruit trees, males at Gombe and Kibale—but not females—have been observed pant-hooting loudly and drumming tree buttresses with their feet in a wild tattoo resounding through the rain forest for up to a mile. This bedlam attracts other chimps, who share the food of the calling males. This cooperative “food calling” pays off in three selfish ways for the males who called: by facilitating mutual grooming to rid them of parasites, by adding more male companions for safer territorial patrols, and by being able to mate with a female arrival. It also pays off in inclusive fitness by helping all relatives within earshot to achieve better nutrition. All of this, incidentally, is gained at a low cost because males usually call at trees big enough to feed all comers. By contrast, a female would gain nothing by food calling, because males habitually usurp the best feeding spots. And, to add insult to injury, she would be cheated by arriving males, who would not groom her after she groomed them.

Chimps typically travel in groups of two to six adults, but scarcity of food often forces them to go it alone. That they travel together anyway whenever they can leads us to ask the biggest question in social behavior: why do they bother to be social at the cost of not getting enough to eat?

Pieces of this puzzle fell into place in the early 1970s. The process began after Jane Goodall stopped her eight-year program of giving Gombe chimps six hundred bananas a day to habituate them to human observers and to keep them nearby. Her study community split into two factions. The biggest, the Kasakela community of thirty-five apes, stayed in the north. The Kahama faction of fewer than fifteen chimps went south. Within a year or two, the Kasakela males forayed south to the Kahama Valley in sortie after sortie, during which they killed at least five of the seven Kahama. males (the last two vanished due to causes unknown). They likely also killed two of the old females. These gang killings were at least as brutal as the one described at the beginning of this section. Males stomped on, twisted, bit, yanked, dragged, gouged, pounded, dismembered, and threw boulders at their outnumbered opponents with such fierce and deliberately lethal aggression that Goodall admitted, “If they had had firearms and had been taught to use them, I suspect they would have used them to kill.”