When and how obstacle size and the number of foragers affect clearing a foraging trail in leaf-cutting ants

Abstract

Taking bad decisions to solve problems can negatively affect organism fitness, hence, the costs accrued by them should modulate decisions about when and how problems should be solved. We studied the problem of trail maintenance in leaf-cutting ants. We evaluated whether colonies have individuals exclusively dedicated to trail maintenance, and how obstacle size and ant forager flux influence the decision of removing obstacles, and the number of trail-clearing ants. We placed obstacles of different sizes, in low and high ant fluxes in different trails of Atta sexdens, and measured ant flux without and with obstacles, if ants removed obstacles, the number of trail-clearing ants and removal time. Obstacle cost was estimated as the proportion of ants that were blocked by the obstacle. We found that colonies lack exclusive trail-clearing ants, i.e., clearing ants also foraged and vice versa. The obstacle cost increased with obstacle size and ant flux. Removal probability increased with this cost. The number of trail-clearing ants increased with obstacle size but did not vary with ant flux, suggesting that the number of trail-clearing ants depends on intrinsic problem characteristics (i.e., higher resistance to traction in bigger obstacles), but do not depend on social context (i.e., interference with foragers). Regardless of the obstacle size, the removal time increased with the number of trail-clearing ants suggesting that interference among individuals is higher in larger working groups than in smaller ones. Our results suggest that individual capabilities as well as the coordination level among individuals influence the solving of a problem and the number of individuals involved in it. We discuss possible mechanisms behind results and propose a conceptual model about the costs and benefits of the removal task.