Clearing the way to the external world: do ants make optimal decisions when removing obstacles from their subterranean galleries?

Abstract

Transport tasks are simple tasks whose cost can be easily measured and that are thus well suited to test optimality hypotheses. Here we focus on a particular type of transport that occur when ants are clearing obstacles from their subterranean galleries. In the laboratory we studied how they extract an object from a gallery of various inclinations. We expected that if ants behave optimally, they should remove the object by the gallery extremity requiring the lower energetic effort. At the colony-level, we found that the obstacle was more often extracted by the lower end of the tube, even if this required a higher amount of mechanical work. At the individual level however, ants showed mechanically optimal pulling behaviours in 75% of cases. Our results suggest that individual ants take into account both the inclination of the gallery and the position of the obstacle in it to decide in which direction they pull. In addition, they seem to base their decision to release the obstacle on the relative effort they perceive while pulling. Using a simple simulation model, we argue that the suboptimal extraction bias observed at the colony-level can be explained by the sequential nature of the extraction task.