Reciprocal cooperation may evolve if the costs of help are reliably compensated for by delayed returns provided in future interactions. The associated probabilities and cost–benefit ratios may vary systematically between the sexes, which often display different dispersal strategies and interaction patterns. Whereas female Norway rats, Rattus norvegicus, are known to apply direct and generalized decision rules of reciprocal cooperation, the rules according to which males reciprocate favours are less well understood. Therefore, we investigated the cooperation propensity of male wild-type Norway rats. Male test rats experienced cooperating partners that provided food to them, or defecting partners that refused to provide help. Afterwards, test rats could donate food to previously experienced or unknown partners, resembling direct and generalized reciprocity paradigms, respectively. Male rats cooperated according to direct reciprocity, suggesting that this decision rule is similarly important for both sexes. However, whereas females additionally help according to generalized reciprocity, males did not apply this rule. These results suggest a sex difference in reciprocal decision rules, highlighting the potential importance of different interaction patterns and cost–benefit ratios between the sexes.

Schweinfurth, M.K., Aeschbacher, J., Santi, M. & Taborsky, M. (2019) Male Norway rats cooperate according to direct reciprocity but not to generalised reciprocity. Animal Behaviour 152: 93-101.