The excuse principle can maintain cooperation through forgivable defection in the Prisoner's Dilemma game

Abstract

Reciprocal altruism describes a situation in which an organism acts in a manner that temporarily reduces its fitness while increasing another organism's fitness, but there is an ultimate fitness benefit based on an expectation that the other organism will act in a similar manner at a later time. It creates the obvious dilemma in which there is always a short-term benefit to cheating, therefore cooperating individuals must avoid being exploited by non-cooperating cheaters. This is achieved by following various decision rules, usually variants of the tit-for-tat (TFT) strategy. The strength of TFT, however, is also its weakness-mistakes in implementation or interpretation of moves, or the inability to cooperate, lead to a permanent breakdown in cooperation. We show that pied flycatchers (Ficedula hypoleuca) use a TFT with an embedded 'excuse principle' to forgive the neighbours that were perceived as unable to cooperate during mobbing of predators. The excuse principle dramatically increases the stability of TFT-like behavioural strategies within the Prisoner's Dilemma game.

Keywords: Prisoner's Dilemma; birds; cooperation; reciprocal altruism; tit-for-tat.

Figure 1.

A male pied flycatcher mobs a stuffed tawny owl. The mobbing flycatcher appears as a shadow. (Online version in colour.)

Figure 2.

Number of pied flycatcher pairs that assisted their neighbours (unfilled bars) and those that remained at their own nests (filled bars) during the second trial in the absent-neighbour group and the defecting-neighbour group.