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Explanations of cooperation between non-kin in animal societies often suggest that individuals exchange resources or services and that cooperation is maintained by reciprocity. But do cooperative interactions between unrelated individuals in non-human animals really resemble exchanges or are they a consequence of simpler mechanisms? Firm evidence of reciprocity in animal societies is rare and many examples of cooperation between non-kin probably represent cases of intra-specific mutualism or manipulation.
As Darwin appreciated, cooperative behaviour-actions adapted to assist others that involve costs to the fitness of participants-poses a fundamental problem to the traditional theory of natural selection, which rests on the assumption that individuals compete to survive and breed1. Early explanations of cooperative behaviour in animal societies (see Figs 1-4) often argued that it was maintained by benefits to groups or populations2,3, but most recent theoretical treatments are rooted in Hamilton's models of the evolution of cooperation based on the concept of inclusive fitness4 and it is now widely accepted that selection operating through benefits to non-descendant kin is commonly involved in maintaining cooperative behaviour (see Box 1).
Alternative paths to cooperation between non-kin
Although kin selection theory provides a satisfactory explanation of cooperation between kin, cooperation between unrelated individuals remains a problemand the evolutionary mechanisms that maintain it are still debated. The most frequent suggestion is that individuals exchange resources or services, suffering temporary net costs as a result of providing assistance, which are exceeded by subsequent benefits when they subsequently receive assistance from individuals that they have previously helped5,6. Explanations of this kind are derived from Trivers' concept of reciprocal altruism7, although they are now usually referred to as examples of 'direct' or 'cost-counting' reciprocity to avoid the implication that they involve altruism.
In his original description of reciprocal altruism, Trivers7 interpreted cooperative interactions between members of different species (as well as between members of the same species) as reciprocal exchanges of assistance, and pointed out their similarity to the Prisoner's Dilemma games of economists8. In these games, two individuals that are constrained to interacting with each other initially agree to cooperate and gain higher pay-offs if they do so than if they refuse, although the highest pay-off is gained by individuals that defect when their partners cooperate. In iterated versions, cooperation can persist...