A MODEL OF COALITION-FORMATION IN ANIMALS

A simple three-player model is presented for the evolution of coalitio ns. The model demonstrates that, under certain conditions,'winner' and 'loser' effects both favour coalition formation. Winner effects are d efined as an increased probability of winning at time T+1, given a vic tory at time T, whereas loser effects entail an increased probability of losing at time T+1, given a loss at time T. Increasing the strength of loser effects or winner effects, or the strength of an individual' s position in the hierarchy, makes coalition formation in general more likely, whereas increasing the costs of giving aid does the opposite. The model does not assume any form of reciprocity but rather examines whether some form of reciprocity or pseudoreciprocity emerges from th e model itself. When either winner or loser effects exist, reciprocal coalition formation (e.g. i helps j against k, and j helps i against k ) between beta (second-ranked individual) and alpha (highest-ranked in dividual) or between alpha and gamma (lowest-ranked individual) was po ssible, but reciprocal aid-giving between gamma and beta was never fav oured. Thus, we have the counterintuitive result that although a coali tion between the two lowest members of a hierarchy against the dominan t individual is possible (as selection may favour gamma aiding beta ag ainst alpha), such a coalition is not predicted to be reciprocal in ki nd. Interpopulational comparisons examining winner-loser effects and c oalition formation would allow for a test of many of the model's most basic predictions. Unfortunately, most work on coalitions has been und ertaken in primates, whereas work on winner and loser effects has focu sed on rodents, and more recently, in fish and birds. Hopefully, the m odel presented here will spur future work that will look at all of the se factors simultaneously in many taxa.