Spatial Ultimatum Games, collaborations and the evolution of fairness

It is often the case that individuals in a social group can perform certain tasks (such as hunting, for example) more efficiently if they collaborate with other individuals than if they act alone. In such situations one is ne cessarily faced with the problem of how the resource obtained as the result of such a collaboration should be divided among the collaborating individu als. If one of the individuals in the collaboration is in a position (throu gh its dominance rank, for example) to impose a particular division of the resource on the other members of the collaboration then we show that an evo lutionary dilemma arises which prevents such collaborations being evolution arily stable. This dilemma, which is closely related to the well-known Ulti matum Game, results from the fact that in such situations natural selection favours individuals who, if dominant, offer smaller and smaller shares of the resource to the others and, if subdominant, will accept lower and lower offers. We also show, however, that this dilemma is naturally resolved in a spatially structured population with selection favouring the evolution of a fair division of the resource and consequently ensuring the evolutionary stability of collaborations of this type.