Although many models have predicted reciprocal altruism, scientists had found evidence only of direct reciprocity ("if you help me, I'll help you") in non-human animals in previous studies. Direct reciprocity is intuitively appealing, yet requires that animals interact repeatedly with the same individuals and remember past interactions. By comparison, generalized reciprocity makes no such cognitive assumptions. In generalized reciprocity, animals are more likely to help a partner if they have been helped in the past, regardless of the past helper's identity.

For example, in humans, people who found a coin in the coin return of a telephone were more likely to help a stranger pick up dropped papers than control subjects who had not previously found money. In humans, this can be explained by cultural experience as well as by natural selection. But if similar reactions to anonymous experience can be found in non human animals, an evolutionary explanation would be far more likely.

In this study, Norway rats received help gaining food from a partner who pulled a stick to produce the food. Rats could therefore be grouped into two classes: those that had previously received help and those that had not. The rats who had previously been helped were then more likely to help another unknown partner receive food. This simple mechanism may promote the evolution of cooperation among unfamiliar non-relatives in many other animals.

Citation: Rutte C, Taborsky M (2007) Generalized reciprocity in rats. PLoS Biol 5(7): e196. doi:10.1371/journal.pbio.0050196.

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