Department of Biology, Carleton College, Northfield, MN, USA, 55057
Abstract. Atta cephalotes is one of a small number of leaf-cutting ants in which the smallest caste of workers, called minims, display a hitchhiking behavior, wherein the minims ride on leaf fragments as they are carried toward the nest by larger workers. A number of hypotheses have been put forth to explain this behavior; however, its function is still poorly understood. The leaf decontamination hypothesis proposes that hitchhiking minims are cleaning the leaves in order to rid them of microbes that might be harmful to the ants’ fungal gardens. We investigated this hypothesis at the La Selva Biological Field Station, Heredia Province, Costa Rica. At 8 individual A. cephalotes nests, leaf fragments from three points along a foraging trail were sampled (cutting site, midpoint, and nest). The relative levels of potential contaminants on each leaf fragment was determined by bathing leaf fragments in a sterile, pH buffered solution and then plating this solution onto general nutrient agar plates. Because leaves closer to the nest will have had more exposure to hitchhikers than those closer to the cutting site, we expected to observe fewer microbial colonies from leaf fragments sampled near the nest than from fragments sampled near the cutting site. Microbial colony counts were not different at different positions along the foraging trail. Leaf fragments collected at night had fewer microbial colonies than those sampled during the day, and minims were more common on foraging trails at night than during the day. While our data do not provide evidence that hitchhiking minims clean leaf fragments, the contrasts between day and night suggest that minims may still clean the leaves, though probably not while they hitchhike.