Norway rats recruit cooperation partners based on previous receipt of help while disregarding kinship.

Norway rats are known to liberate trapped conspecifics, which implies an empathic response to the deplorable situation of the captive. If this is an altruistic behavior reflecting an evolved decision rule, the requisite fitness enhancement to the actor may result either from close relatedness or the expectation of future returns. Neither potential effects of relatedness nor of reciprocal returns have yet been examined. Our two-stage experiment revealed that wild-type Norway rats preferably collaborated with partners that had previously freed them from a trap and subsequently cooperated with each other, indicating that expected future benefits may underlie the deliverance of trapped companions. Relatedness had no effect on their cooperative propensity. These results show that rats recruit partners to coordinate cooperation by direct reciprocity but not kin discrimination, suggesting that the evolutionary mechanism responsible for the altruistic liberation behavior of Norway rats may be reciprocal altruism rather than kin selection.