Where is the Prisoner's Dilemma found in nature?

In the last decade, the amount of game-theoretical literature on various aspects of the IPD has increased considerably. For most scientists the IPD has become something like a 'theoreticians's playground' with at times very little relation to empirical observations. Evidence of the mere existence of an IPD in biological societies is scarce, not only due to the difficulties in measuring the essential parameters but, as will be shown, also owing to the limited scope of the IPD. In this section I will try to relate some empirical work from different taxa to the theoretical findings described so far.