Reciprocity can explain cooperative behaviour among non-kin, where individuals help others depending on their experience in previous interactions. Norway rats (Rattus norvegicus) cooperate reciprocally according to direct and generalized reciprocity. In a sequence of four consecutive experiments, we show that odour cues from a cooperating conspecific are sufficient to induce altruistic help of rats in a food-exchange task. When rats were enabled to help a non-cooperative partner while receiving olfactory information from a rat helping a conspecific in a different room, they helped their non-cooperative partner as if it was a cooperative one. We further show that the cues inducing altruistic behaviour are released during the act of cooperation and do not depend on the identity of the cue provider. Remarkably, olfactory cues seem to be more important for cooperation decisions than experiencing a cooperative act per se. This suggests that rats signal their cooperation propensity to social partners, which increases their chances to receive help in return.

To test for reciprocal cooperation, we used wild-type Norway rats that were capable of producing food for a partner . In addition, we manipulated the availability of olfactory information by controlling the airflow between social partners. We performed a series of four consecutive experiments to stepwise clarify which information rats consider when deciding to provision a social partner with food