Constraints on the Evolution of Reciprocity: An Experimental Test with Zebra Finches

Reciprocity is apparently uncommon in animal societies because it requires, to evolve, specialized cognitive abilities that most species would not possess. In particular, memory would be important because cooperation can emerge and be maintained only if the same opponents interact repeatedly and adjust their decisions to their opponent's previous move. To investigate how these constraints influence reciprocity, we first conducted a preliminary foraging experiment with zebra finches (Taeniapygia guttata) to insure that corticosterone elevations impaired the birds' event memory capabilities. Then, we performed a reciprocity experiment with established pairs that were tested in a two-choice apparatus under two different conditions: once with an empty implant and once with an implant of corticosterone. We found that the birds were capable of sustained cooperation in the Prisoner's Dilemma treatment but only when they had an empty implant. Thus, our results support the hypothesis that limitations in event memory constraint the frequency of reciprocity and hence would make this form of cooperation difficult for non-human animals. In addition, as levels of stress hormones can differ greatly among individuals, our findings might also contribute explaining individual variation in the propensity to cooperate.