Reciprocal altruism in Norway rats

Reciprocal altruism has been proposed to generate evolutionarily stable levels of cooperation, but empirical evidence in non-human animals is contentious. A series of experimental studies on Norway rats revealed that these animals reciprocate received help by using decision rules characterising direct and generalized reciprocity. The direct reciprocity decision rule states that individuals should help someone who previously helped them, whereas the generalized reciprocity decision rule implies that individuals should help anyone after having received help from someone. Here, we present results from meta-analyses assessing if Norway rats generally help partners according to the direct and generalized reciprocity decision rules by giving more help sooner and matching help received either to previously cooperative partners or to anyone after having received help from a conspecific. We also compared the rats' performance when applying the direct or generalized reciprocity decision rules. Results from these meta-analyses clearly show that female and male focal rats help partners according to the direct reciprocity decision rule. Rats did not adjust the amount of help they gave by matching the help they received according to both the direct and generalized reciprocity decision rules; however, they may adjust the duration to the first help they gave by matching the help they received according to the direct reciprocity decision rule. Female, but not male, focal rats help partners according to the generalized reciprocity decision rule. There was no apparent difference in the help given by female rats to partners between the direct and generalized reciprocity decision rules. These results (i) demonstrate, across a wide range of studies, the reciprocal cooperation of Norway rats by use of decision rules denoting direct and generalized reciprocity, and (ii) suggest that rats may frequently apply the cognitively less demanding decision rule of generalized reciprocity, unless they need to distinguish between individuals differing in helpfulness.