Reciprocal Trading of Different Commodities in Norway Rats.

The prevalence of reciprocal cooperation in non-human animals is hotly debated [1, 2]. Part of this dispute rests on the assumption that reciprocity means paying like with like [3]. However, exchanges between social partners may involve different commodities and services. Hitherto, there is no experimental evidence that animals other than primates exchange different commodities among conspecifics based on the decision rules of direct reciprocity. Here, we show that Norway rats (Rattus norvegicus) apply direct reciprocity rules when exchanging two different social services: food provisioning and allogrooming. Focal rats were made to experience partners either cooperating or non-cooperating in one of the two commodities. Afterward, they had the opportunity to reciprocate favors by the alternative service. Test rats traded allogrooming against food provisioning, and vice versa, thereby acting by the rules of direct reciprocity. This might indicate that reciprocal altruism among non-human animals is much more widespread than currently assumed.