Waste management in the leaf-cutting ant Atta colombica

Abstract

Unlike most leaf-cutting ants, which have underground waste dumps, the leaf-cutting ant Atta colombica dumps waste in a heap outside the nest. Waste is hazardous, as it is contaminated with pathogens. We investigated the organization of the workforce involved in outside-nest tasks (foraging, waste disposal) and quantified task switching and heap location to test hypotheses that these tasks are organized to minimize contact between the heap and foraging entrances and trails. Waste management is an important task: 11% of externally working ants were either transporting waste or manipulating waste on the heap, and the other 89% were foragers. There is strict division of labor between foragers and waste workers, with no task switching. Waste management also has division of labor and is undertaken by transporters that carry waste to the heap margins and heap workers that manage the heap. Waste heaps are always located downhill from nest entrances. The distance to the waste heap is positively related to colony size and negatively related to slope. Foraging trails avoid the heap, with 92% of trails going away from the heap. This avoidance behavior is costly, increasing foraging trail length by at least 6%. Waste management in A. colombica is a sophisticated system that encompasses both work and spatial organization. This organization is probably adaptive in reducing disease transmission. Division of labor separates waste management from foraging, reducing the likelihood of foragers becoming contaminated with waste. The downhill location of heaps reduces waste entering entrances during rain. The orientation of foraging trails reduces the possibility of foragers becoming accidentally contaminated with waste.