Protective behavior or 'true' tool use? Scrutinizing the tool use behavior of ants

In the genus Aphaenogaster, workers use tools to transport liquid food to the colony. During this behavior, ants place or drop various kinds of debris into liquids or soft food, and then, they carry the food-soaked tools back to the nest. According to some authors, this behavior is not "true" tool use because it represents two separate processes: a defense response to cover the dangerous liquid and a transport of food. Here, we investigated the debris dropping and retrieving behavior of the ant Aphaenogaster subterranea to establish which of the two hypotheses is more probable by conducting manipulative experiments. We tested the responses of eight colonies (a) to liquid food (honey-water) and nonfood liquids (water) in different distances from the nest and (b) to nonthreatening liquids previously covered or presented as small droplets. We also tested whether the nutritional condition of colonies (i.e., starved or satiated) would affect the intensity and rate of debris dropping. Our results were consistent with the tool-using behavior hypothesis. Firstly, ants clearly differentiated between honey-water and water, and they directed more of their foraging effort toward liquids farther from the nest. Secondly, ants performed object dropping even into liquids that did not pose the danger of drowning or becoming entangled. Lastly, the nutritional condition of colonies had a significant effect on the intensity and rate of object dropping, but in the opposite direction than we expected. Our results suggest that the foraging behavior of A. subterranea is more complex than that predicted by the two-component behavior hypothesis and deserves to be considered as "true" tool use.