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Reciprocal altruism in primates

While struggling to understand the evolution of cooperation throughout the different taxa, one of the most intricate tasks is to study cooperation in primates. Packer (1977) was the first to report reciprocal altruism among male olive baboons (Papio anuhis). The formation of short-term coalitions among adult male P. anuhis during aggressive interaction against a single adversary is initiated by one male enlisting a partner and completed by subsequently fighting the opponent. Packer found considerable evidence that individuals who join an enlisting baboon have previously succeeded in enlisting that very same individual, suggesting cooperation may be partly based on reciprocity (Packer 1977). Determining costs and benefits is extremely difficult, since the payoff matrix may vary according to the different circumstances under which coalitions are formed (e.g. the strength of the opponent specifies the costs, while the cause of the aggression specifies the benefits). However, assuming that most of the coalitions were formed to take over an oestrous female and that the wounds received were non-fatal, the resulting sired offspring would be a greater benefit than the less costly wounds received in a number of consecutive 'plays' if both partners reciprocated in aiding each other (Packer 1977).

But since only one of the cooperating partners (usually the enlisting male) obtains the female at issue, two coalitions make a 'game'. This case is at best the alternating variant of the simultaneous IPD, recently investigated by Nowak and Sigmund (1994). Noë (1990) argued that this form of delayed reciprocity is better explained with a 'Veto game' that includes information exchange, partner-choice and partner competition. The ability of primates in trading different beneficial acts for other behaviour supporting the benefactor, further complicates the matter. Seyfarth and Cheney (1984) looked at coalition-forming in female vervet monkeys (Cercopithecus aethiops) in relation to preceded grooming. Their data seem to support the view that vervets are more apt to help an enlisting, unrelated individual if they have been groomed by those individuals in the recent past (Seyfarth and Cheney 1984). However, due to the complexity of exchanging acts in various currencies (especially if more than two behaviours are involved) and due to the difference in hierarchical rank that breaks the symmetry of the game, quantification of costs and benefits seems unlikely to be possible (Noë 1990). Therefore, all results obtained in studies as the two described above will be persistently disputed (e.g. Noë 1990, Hemelrijk 1991, Seyfarth 1991).

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