Protective behaviour or ‘true’ tool use? Scrutinizing the tool use behaviour of ants

In the genus Aphaenogaster, workers use tools to transport liquid food to
the colony. During this behaviour, ants place or drop various kinds of
debris into liquid or soft food and then they carry the food-soaked tools
back to the nest. According to some authors, this behaviour is not ‘true’
tool use because it represents two separate processes: a defence response
to cover the dangerous liquid and the transport of food. Here, we
investigated the debris dropping and retrieving behaviour 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 to liquid food (honey-water) and non-food liquids
(water) in different distances from the nest and to non-threatening
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 behaviour hypothesis. Firstly, ants clearly
differentiated between honey-water and water, and they directed more of
their foraging effort towards 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 an opposite direction than we expected. Our
results suggest that the foraging behaviour of A. subterranea is more
complex than that predicted by the two-component behaviour hypothesis and
deserves to be considered as ‘true’ tool-use.