  Despite the attention paid to reciprocal altruism by theoreticians, clear-cut empirical examples in non-human animals are relatively few (Hammerstein 2003, Sachs et al. 2004, Taborsky 2013). This is probably because the pre-conditions for reciprocal altruism to evolve- multiple encounters and individual recognition—are not especially common. However, one possible example is provided by blood-sharing in vampire bats (Wilkinson 1984, 1990, Carter &amp; Wilkinson 2013). It is quite common for a vampire bat to fail to feed on a given night. This is potentially fatal, for bats die if they go without food for more than a couple of days. On any given night, bats donate blood (by regurgitation) to other members of their group who have failed to feed, thus saving them from starvation. Since vampire bats live in small groups and associate with each other over long periods of time, the preconditions for reciprocal altruism are likely to be met. Wilkinson and his colleagues' studies showed that bats tended to share food with their close associates, and were more likely to share with others that had recently shared with them. These findings appear to accord with reciprocal altruism theory.