A field assessment of optimal foraging in ants: trail patterns and seed retrieval by the European harvester ant Messor barbarus

The ant Messor barbarus is a major seed predator on annual grasslands of th e Mediterranean area. This paper is an attempt to relate: the foraging ecol ogy of this species to resource availability and to address several predict ions of optimal foraging theory under natural conditions of seed harvesting . Spatial patterns of foraging trails tended to maximise acquisition of food resources, as trails led the ants to areas where seeds were more abundant l ocally, Moreover, harvesting activity concentrated an highly frequented tra ils, on which seeds were brought into the nest in larger numbers and more e fficiently, at a higher mean rate per worker. The predictions of optimal foraging theory that ants should be more selecti ve in both more resource-rich and more distant patches were tested in the n ative seed background. We confirm that selectivity of ants is positively re lated to trail length and thus to distance from the nest of foraged seeds. Conversely, we fail to find a consistent relationship between selectivity a nd density or species diversity of seed patches. We discuss how selectivity assessed at the colony level may depend on factors other than hitherto rep orted behavioural changes in seed choice by individual foragers.