Reciprocal cooperation has been observed in a wide range of taxa, but the proximate mechanisms underlying the exchange of help are yet unclear. Norway rats reciprocate help received from partners in an iterated Prisoner’s Dilemma game. For donors, this involves accepting own costs to the benefit of a partner, without obtaining immediate benefits in return. We studied whether such altruistic acts are conditional on the communication of the recipient’s need. Our results show that in a 2-player mutual food-provisioning task, prospective recipients show a behavioral cascade reflecting increasing intensity. First, prospective receivers reach out for the food themselves, then they emit ultrasonic calls toward their partner, before finally showing noisy attention-grabbing behaviors. Food-deprived individuals communicate need more intensively than satiated ones. In return, donors provide help corresponding to the intensity of the recipients’ communication. This indicates that rats communicate their need, which changes the helping propensity of potential donors. Communication of need and corresponding adjustment of cooperation may be a widespread proximate mechanism explaining the mutual exchange of services between animals.

Schweinfurth, M.K. & Taborsky, M. (2018) Norway rats (Rattus norvegicus) communicate need, which elicits donation of food. Journal of Comparative Psychology 132(2): 119-129.