The evolution of cooperation in mobile organisms

. Current game theory models of cooperation based on reciprocity do not take into account the active switching of partners made possible by mobility. Since such situations cannot be understood by means of a repeated Prisoner's Dilemma game, new theory is developed which considers both a current partner and the access to other partners. It is shown that mobility seriously restricts the evoluton of cooperation: an efficient free rider could move rapidly through a population of cooperative individuals, searching out victims to exploit. Properties of the social environment, such as population size and density, influence the search time for a free rider and thus the possibilities for the evolution of cooperation. Behavioural adaptations, such as initial suspiciousness towards strangers, and gossiping, may to some extent counteract the effects of mobility and favour cooperation. The possible importance of search time is illustrated by comparative data on the relationship between nest sharing and size of nest aggregations in sphecid wasps.