Reciprocal Altruism (quote)

 The biologist Robert Trivers developed a suggestion from George Williams on how another kind of altruism could evolve (where altruism, again, is defined as behavior that benefits another organism at a cost to the behaver). Dawkins explains it with a hypothetical example. Imagine a species of bird that suffers from a disease-carrying tick and must spend a good deal of time removing them with its beak. It can reach every part of its body but the top of its head. Every bird would benefit if some other bird groomed its head. If the birds in a group all responded to the sight of a head presented to them by grooming it, the group would prosper. But what would happen if a mutant presented its head for grooming but never groomed anyone else? These freeloaders would be parasite-free, and could use the time they saved not grooming others to look for food. With that advantage they would eventually dominate the population, even if it made the group more vulnerable to extinction. The psychologist Roger; Brown explains, "One can imagine a pathetic final act in which all birds on stage present to one another heads that none will groom."

But say a different, grudge-bearing mutant arose. This mutant groomed strangers, groomed birds that in the past had groomed it, but refused to groom birds that had refused to groom it. Once a few of them had gained a toehold, these grudgers could prosper, because they would groom one another and not pay the costs of grooming the cheaters. And once they were established, neither indiscriminate groomers nor cheaters could drive them out, though in some circumstances cheaters could lurk as a minority.

The example is hypothetical, illustrating how altruism among nonkin—
what Trivers called reciprocal altruism—can evolve.