Primatologists have long known that chimpanzees can be demonic: Bands of males routinely head to the borders of their territory to seek, and sometimes destroy, foreign chimpanzees. But what triggers these patrols, and why do males of the troop - who compete fiercely with one another most of the time- seem to cooperate while on patrol? The answer, it seems, may be a mob mentality. In a study of a group of 150 chimpanzees at Ngogo in the Kibale National Park in Uganda, researchers found that chimpanzees went on patrol only after they had assembled enough members to have overwhelming force. Patrols require "safety in numbers" because attacking a foreign chimpanzee is dangerous, explains primatologist John Mitani of the University of Michigan, Ann Arbor, co-author of the study with primatol-ogist David Watts of Yale University.
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