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If group selection does not provide a strong explanation for the evolution of altruism, how can we explain this behavior? Reciprocal altruism, whereby individuals exchange favors, is one way that altruism can evolve. This idea is perhaps best summarized as “You scratch my back, and I’ll scratch yours.” For this kind of behavior to work, individuals have to be able to recognize each other and remember previous interactions. If I did you a favor, I need to be able to recall who you are so that I can be sure that I get the return I expect.

A famous example of reciprocal altruism comes from an unlikely source—vampire bats. Adult female vampire bats live in groups of 8 to 12, and often remain in these groups for years: In one study, two tagged females shared the same roost for 12 years. The female groups are occasionally joined by unrelated females, so they are not all closely related. Because they have long-term associations, there is opportunity for reciprocal interactions.

Feeding for these bats is a “boom or bust” activity—either a bat has found and successfully fed on a blood source (boom), or a bat has failed to feed (bust). As a result, bats are often either overfed or underfed. Underfed bats are at risk of dying by starvation. Bats returning from a successful feeding expedition often regurgitate blood to unsuccessful ones, on the expectation that the favor will be returned at a future date.