Coalition formation in animals and the nature of winner and loser effects

Coalition formation has been documented in a diverse array of taxa, yet the re has been little formal analysis of polyadic interactions such as coaliti ons. Here, we develop an optimality model which examines the role of winner and loser effects in shaping coalition formation. We demonstrate that the predicted patterns of alliances are strongly dependent on the way in which winner and loser effects change with contestant strength. When winner and l oser effects decrease with the resource-holding power (RHP) of the combatan ts, coalitions will be favoured between the strongest members of a group, b ut not between the weakest. If, in contrast, winner and loser effects incre ase with RHP, exactly the opposite predictions emerge. All other things bei ng equal, intervention is more likely to prove worthwhile when the benefici ary of the aid is weaker (and its opponent is stronger), because the benefi ciary is then less likely to win without help. Consequently, intervention i s more probable when the impact of victory on the subsequent performance of a combatant increases with that individual's strength because this selects for intervention in favour of weaker combatants. The published literature on hierarchy formation does not reveal how winner and loser effects actuall y change with contestant strength and we therefore hope that our model will spur others to collect such data; in this light we suggest an experiment w hich will help to elucidate the nature of winner and loser effects and thei r impact on coalition formation in animals.