Vampire bats (Desmodus rotundus) roost in colonies, a very simple social organization. They must feed every few nights to survive. Sometimes, they fail. The bat then “begs” its neighbor by nuzzling its throat. The second bat often regurgitates a small amount of blood, nourishing the first bat. That seems to contradict the selfishness we typically associate with natural selection. How can this happen? Well, if you continue to watch the bats on successive nights, you find that eventually the second bat faces the same dilemma. The circumstance is reversed, and the first bat generally reciprocates. Here is a case of enhanced survival through cooperation — notably social in nature.

—But suppose one bat cheats! She begs for meals, but never “repays the favor.” That individual would reap the benefit, while bearing no cost. The trait of cheating could proliferate in the population and the system of sharing would collapse. The bats, however, seem “wise” to cheaters. They can recognize individuals and remember past events. A bat that has not reciprocated does not get another free handout. Tit for tat. Bats who cheat ultimately do not benefit. The system of fair exchange is kept in check. Is this morality? We obviously don't know about the intent or motivations of vampire bats. Yet the social system clearly enforces fairness. —And that illustrates the importance of social organization in modifying individual behavior.

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