learned which party was victorious, nor the cause of the war: but I felt for the rest of that day as if I had had my feelings excited and harrowed by witnessing the struggle, the ferocity and carnage, of a human battle before my door.” What is distinctive about ant battles and the deaths that ensue is that they are coordinated, with success driven by group size. As the biologist Eldridge Adams has demonstrated, bigger groups are more likely to win, more likely to kill a higher number of their smaller opponents, and less likely to incur any fatalities. Despite the similarities between ant and human battles, two differences undermine the usefulness of this analogy for understanding the evolution of lethal aggression in humans: ants are only a very distant evolutionary cousin, subject to extremely different pressures of social life, and their cooperative efforts are largely among individuals who are virtual genetic clones. When humans go to battle, cooperation is largely among unrelated individuals who are complete strangers. Of the small sample of species committing adulticide, chimpanzees are our best bet as they are closer evolutionary cousins and they join forces with kin and non-kin.
To get a sense of lethal aggression in chimpanzees, consider the following description by the anthropologist David Watts and his colleagues concerning an attack by males of the Ngogo community of Uganda (emphasized words are mine):
[Field Assistant] G. Mbabazi found 12 adult and three adolescent males, 10 of which had participated in
the boundary patrol 2 days before ... in the eastern part of the Ngogo chimpanzees’ territory. They
started another boundary patrol by quickly and quietly moving south and then east. At 0830 hr, they
moved east through a field of elephant grass (Pennisetum purpureum), then reentered the forest and went
toward the spot where BT, LO, and MO [three adult male chimpanzees] had been displaying 1 day
before and the area where Ngogo males had patrolled the day before that. As they reentered the forest,
the Ngogo chimpanzees met chimpanzees from another community. The neighboring chimpanzees were
feeding quietly on Pseudospondias microcarpa fruit in the same tree under which BT, LO, and MO had
displayed. G. Mbabazi could not ascertain the precise number of chimpanzees from the neighboring
group, but he saw at least two females with infants, one juvenile, and one adult male that immediately
fled northeast with the Ngogo chimpanzees in pursuit. The Ngogo chimpanzees caught up to the strange
adult male after chasing him for about 100 m and surrounded him. Adult Ngogo male EL began to
pummel the intruder, and adults BF, BRU, LO, and MO quickly joined him. The strange male tried to
escape down a small hill but could not elude these five Ngogo males and others that joined them. The
Ngogo males, led by EL, continued to beat, bite, and kick him for 20 min, and dragged him farther
down this hill into a small stream valley about 50 m away from the spot of his initial capture, where he
died during or shortly after the attack. All of the Ngogo males remained in the area after the stranger
was killed. Several circled his body and some sniffed it, while others sat nearby. ...Careful inspection
showed that the victim suffered wounds across his entire body ... including a deep gash to the bone on
the left humerus and a deep puncture on the left side of the thorax near the heart. The only missing
body part was the victim’s testes, which were recovered 50 m away, near where he was initially captured
(2006, p.g., 166).
Watts’ description captures several important features of adulticide in chimpanzees — both at the same field site and others throughout Africa — and in other species. The lethal attacks are explicitly proactive and planned. This is important because many cases of non-lethal aggression are reactive and impulsive, and studies of human and nonhuman aggression reveal different brain mechanisms underlying
these two forms of violence. When chimpanzees attack, they use stealth to sneak up on the victim, and
Hauser Chapter 1. Nature’s secrets 39
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