Foraging behaviour of Atta cephalotes (leaf-cutting ants): an examination of two predictions for load selection

Two mechanisms have been proposed to explain how colony-level foraging perf ormance of leaf-cutting ants can be maximized when workers harvest leaf fra gments of a size that does not maximize their individual performance. Each mechanism predicts that ants will adjust the size of leaf fragments between starting a foraging bout and establishing full traffic between the nest an d foraging site, but the two models predict shifts in opposite directions. I examined fragment sizes at the start of daily foraging in five field colo nies of Atta cephalotes in Costa Rica and detected an obvious shift in only one case. More shifts were detected when the small and large ends of the w orker body size range were considered separately, but the direction was inc onsistent among colonies. I also examined the role of returning laden worke rs in recruitment of nestmates by intercepting all laden workers for the fi rst 2 h of foraging, and measuring the effect on the arrival of recruits at the foraging site. In two cases, the flow of recruits was not diminished b y the interception of returning workers. The results suggest that neither m echanism correctly and consistently accounts for load size selection by lea f-cutting ants. (C) 2000 The Association for the Study of Animal Behaviour.