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Evolutionary biologists have long been interested in cooperation between animals, especially cases in which one animal assists an unrelated animal at some cost to itself. Why should individual animals help non-kin? A common theory has been that such cooperative actions are reciprocated and represent exchanges of resources or services between individuals. In a review on page 51, zoologist Tim Clutton-Brock of the University of Cambridge, UK, examines a wide body of theoretical and empirical studies and concludes that simpler mechanisms are often involved. He tells Nature more.

Do unrelated animals often cooperate?

Empirical evidence suggests that they do, but not for the reason proposed by theoreticians. In non-human animals, most cooperative interactions involve relatives. Extensive cooperation between unrelated individuals is uncommon, and seldom involves activities likely to be of substantial cost to the one providing the favour. For example, unrelated animals rarely feed each other's offspring, a comparatively common activity among relatives. In this respect, human cooperation — which often involves non-kin — is unusual.

Why is it unlikely that exchange is involved in cooperation between unrelated animals?

There is a fundamental problem with the reciprocity explanation. Interpreting cooperative interactions as exchanges means that it would pay for individuals to 'cheat' and accept assistance but not provide it. Moreover, there's very limited empirical evidence that individuals exchange assistance in this way.

Why did you undertake this review?

Because of the disparity between theoretical and empirical studies of cooperation. Theoretical studies generally explain cooperation between non-kin as exchanges of assistance. However, empirical studies suggest that the cooperative participant either gains immediate net benefits or is coerced or manipulated by the beneficiary into providing assistance. It may be that the prevalence of cooperation between non-kin in humans, where reciprocity is common, has influenced theoreticians' thinking.

Why is this conclusion important?

It would be unfortunate if theoretical research on animal cooperation continued to focus on an approach that empirical studies suggest is of limited relevance. For our understanding of the evolution of animal cooperation to develop, theoreticians should devote as much effort to exploring mutualistic and manipulative interactions as they have to models of reciprocity.