Tool use by the forest ant Aphaenogaster rudis: Ecology and task allocation

Debris dropping behavior by ants during foraging has been labeled alternately as tool use or a protective behavior. To address this controversy, we investigated the circumstances under which the common forest ant Aphaenogaster rudis drops and retrieves debris in the forests of Vermont, in the U.S.A. We tested the hypotheses, first, that debris dropping functions to protect workers from entanglement or drowning in liquids, and second, that debris dropping functions as part of foraging tool use. To determine how workers are allocated to the debris dropping and retrieval tasks, we studied individually marked foragers in the field and laboratory. Our results provide evidence that the debris dropping behavior of Aphaenogaster rudis deserves to be labeled as foraging tool use; A. rudis ants do not drop debris in non-food substances that present a hazard of entanglement or drowning to workers. We also found that potential tools represent a small, but non-negligible, percentage of the items that A. rudis foragers bring back to their colonies. Furthermore, debris dropping by A. rudis at baits discouraged colonization by other ant species. Finally, we provide the first evidence that tool use is a specialized task performed by a subset of A. rudis foragers within each colony at any given point in time. The execution of this task by a small proportion of workers may enhance the competitive ability of this ecologically dominant forest ant. Received 3 April 2006; revised 13 August 2006; accepted 1 September 2006.