Adaptive indirect effects: the fitness of burying beetles with and without their phoretic mites

Summary Any behavior that equally affects a group of organisms cannot be selected by the evolutionary forces operating within that group. The evolution of such behaviors requires a population structure consisting of many groups that vary in their genetic and/or species composition. In this paper we present evidence for the evolution of behaviors with shared consequences in phoretic mites that utilizeNicrophorus beetles (Silphidae) for transport. Eighteen experiments, totalling over 1500 beetle broods, demonstrate that the mites (1) have no negative effects on the beetles at normal densities, (2) occasionally have short-term beneficial effects, (3) appear to have long-term beneficial effects that require a period of time to manifest themselves, and (4) themselves have negative effects at abnormal densities. A survey of other phoretic associations indicates a similar mix of commensalism and mutualism. We conclude that most phoretic associations have evolved to eliminate their own negative effects on the carrier, and also have evolved positive effects when the ecological situation permits.