title: 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.
authors.affiliation: Institute of Ecology and Earth Sciences, University of Tartu, , 51014 Tartu, Estonia. indrikis.krams@ut.ee
authors.forename: Indrikis
authors.initials: I
authors.lastname: Krams
authors.forename: Hanna
authors.initials: H
authors.lastname: Kokko
authors.forename: Jolanta
authors.initials: J
authors.lastname: Vrublevska
authors.forename: Mikus
authors.initials: M
authors.lastname: Abolins-Abols
authors.forename: Tatjana
authors.initials: T
authors.lastname: Krama
authors.forename: Markus J
authors.initials: MJ
authors.lastname: Rantala
mesh_terms: D000533:Altruism; D000818:Animals; D001522:Behavior, Animal; D003299:Cooperative Behavior; D061347:Forgiveness; D005716:Game Theory; D046109:Passeriformes / Q000502:physiology*; D011235:Predatory Behavior
keywords: Prisoner's Dilemma; birds; cooperation; reciprocal altruism; tit-for-tat