Pigeons' choice between shared and unshared feeding sites in game situations.

Cooperative behavior in nonhuman animals has been studied within the framework of game theory, typically by using the prisoner’s dilemma game. Previous studies on cooperation by pigeons using this game have revealed that, under these conditions, the animals did not learn the tit-for-tat strategy played by their opponents. In many cases, animals fail to choose cooperation and in so doing do not maximize their gains. The present experiment examined pigeons’ cooperative choices in the prisoner’s dilemma game situation by using a different type of apparatus than that used in previous studies: Subjects moved to choose one of two feeding sites, one of which was shared by another, stooge, pigeon whose choices were controlled by a computer and the other of which was not shared by other pigeons. In this choice situation, the presence of the stooge pigeon increased the subjects’ choices of the shared feeding site significantly. Further, the pigeons learned the other player’s choice strategy (tit-for-tat and random), showing that choice proportions for the shared feeding site were significantly higher in the tit-for-tat condition than in the random condition. These results suggest that the presence of a conspecific at the feeding site is a reinforcer for choosing it.