Leaf density and a trade-off between load-size selection and recruitment behavior in the ant Atta cephalotes.

This study considers the interplay between individual load-size selection and recruitment behavior in the leaf-cutting ant Atta cephalotes. Foraging workers anchor themselves on the leaf edge by their hind legs and pivot around them while cutting arcs from leaves. Since workers not only cut leaves but also lay chemical trails to recruit nestmates, we investigated whether there is conflict of motivation affecting the workers' decision either to quickly inform nestmates about a newly discovered food source, or instead to cut full-load leaf fragments, which could delay recruitment. Workers were presented with leaves of privet of three different grades of toughness (measured as leaf density=mass/area) as sources of different quality, and load-size selection and recruitment behavior by harvesting-satiated and harvesting-deprived workers were measured. The following results were obtained. (1) Leaf density affected individual load-size selection: both harvesting-satiated and harvesting-deprived workers were found to cut smaller leaf fragments from the denser leaves. (2) Harvesting-deprived workers cut smaller fragments than harvesting-satiated workers, and therefore saved cutting time. The fragments cut were smaller only during the initial phases of the recruitment process, when information about the discovery needed to be transferred. (3) Harvesting-deprived workers showed higher recruitment rates than harvesting-satiated workers. A considerable number of ants were observed to return to the nest unladen. During the initial phases, the ratio of laden/unladen workers was lower than that for harvesting-satiated workers, and increased with the development of the tograging process. (4) Scout workers confronted with familiar leaves ran back to the nest laying chemical trails without even contacting the leaves. They relied on olfactory cues to start recruiting nestmates, and leaf density played no role in their decisions. (5) When confronted with unfamiliar leaves, on the other hand, they assessed leaf quality by probing bites at the leaf edge, although no actual cuts occurred. In this situation, the resulting recruitment rates depended on physical leaf traits, being higher for the tenderer leaves. (6) Workers foraging on unfamiliar leaves cut smaller fragments than workers cutting familiar leaves, and most of them displayed trail-laying behavior when returning to the nest. The results support the hypothesis of a trade-off between time spent collecting and that invested to recruit nestmates. During the initial phases of exploitation of a newly discovered food source, workers reduced their individual carrying performance in order to return earlier to the colony for further recruitment.