characteristically at the disposal of primitive peoples." In most Yanomamo villages, members are on average related to each other more closely than half-cousinship.^9^ But nontribal societies are a lot larger, as if some new cohesive factor has to come into play if a community is to outgrow the organizational limits imposed by kinship. Recent human history, Chagnon writes, could be viewed as a struggle to overcome these limits: "Many general discussions of our social past as hunters and early cultivators allude to the 'magic' numbers of 50 to 100 as the general community size within which our recent cultural and biosocial evolution occurred, a maximal community size that was transcended only in the very recent past—within the last several thousand years."191

One principle that biologists think may help explain larger societies, both human and otherwise, is that of reciprocal altruism, the practice of helping even a nonrelated member of society because they may return the favor in future. A tit-for-tat behavioral strategy, where you cooperate with a new acquaintance, and thereafter follow his strategy toward you (retaliate if he retaliates, cooperate if he cooperates), turns out to be superior to all others in many circumstances. Such a behavior could therefore evolve, providing that a mechanism to detect and punish freeloaders evolves in parallel; otherwise freeloaders will be more successful and drive the conditional altruists to extinction.

Conditional or tit-for-tat altruism cannot evolve in just any species. It requires members to recognize each other and have long memories, so as to be able to keep tally. A species that provides a shining example of reciprocal altruism is none other than the vampire bat. The bats, found in South America, hang out in colonies of a dozen or so adult females with their children. They feed by biting a small incision in the skin of sleeping animals, nowadays mostly cattle or horses, and injecting a special anticoagulant named, naturally enough, draculin. But their blood collection drives are not always successful. On any given night a third of the young bats and 7% of the adults are unsuccessful, according to a study by Gerald W. Wilkinson of the University of Maryland.

This could pose a serious problem because vampire bats must feed every three days, or they die. The colony's solution, Wilkinson found, is that successful bats regurgitate blood to those who went hungry. Bats are particularly likely to donate blood to their friends, with whom they have grooming relationships, to those in dire need, and to those from whom they

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