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Adequate models for the evolution of cooperation have centered around the so-called prisoner's dilemma, a group of problems characterized by situations in which two individuals may cooperate or not. In the classical version of the dilemma two prisoners, ignorant of each other's response, are questioned. If one denies guilt and the other admits it, the one admitting it is severely punished, and the one denying goes free. If both deny it, they are both punished but less severely so, and if both admit it, they are punished minimally. Thus, if they cooperate, it is to their combined advantage for both to admit the crime, but for either individual, the strategy of denying the crime is better if he believes the other person will act cooperatively. A rational solution to this problem arises when there are repeated opportunities for cooperation. This is called the iterated prisoner's dilemma. In this situation, deciding whether or not to cooperate on the basis of the history of cooperation of the other individual becomes a rational strategy. Under such circumstances, one would predict, on the basis of analyses of cooperation in the iterated prisoner dilemma situation, that individuals would stop cooperating with others who have a history of not doing so.
The territorial behavior of female African lions provides an ideal opportunity to study these phenomena. The lions cooperatively protect against intruders. However, if one lion of the group is more active in this protection, attacking the intruder earlier, she is at much greater risk of injury and death. In cleverly designed experiments in which prides of lions were “attacked” by recordings of strange lions roaring, Heinsohn and Packer observed that some of the lions were uncooperative, lagging behind, and that this style of response was stable to the individual. Yet although the lead lion was clearly aware of the laggardness (looking over her shoulder for the companion), the style of interaction of the lead and laggard lion