Comparison of foraging tool use in two species of myrmicine ants (Hymenoptera: Formicidae)

Many ant species are known to exhibit foraging tool use, during which ants place various debris items (e.g., pieces of soil, leaves, pine needles, etc.) into liquid food, and then they carry the food-soaked tools back to the nest. In the present study, we compared the tool-using behavior in captive colonies of two closely related myrmicine ants with different feeding preferences: Aphaenogaster subterranea, an omnivorous species, and Messor structor, a mainly granivorous seed-harvester species. We supplied foraging ants with honey-water baits and six types of objects they could use as tools: sand grains, small soil grains, large soil grains, pine needles, leaves, and sponges. We found that the workers of A. subterranea both dropped more tools into honey-water baits and retrieved more of these tools than the workers of M. structor. While A. subterranea preferred smaller tools over larger ones, tool preferences for M. structor did not differ significantly from random. In addition, tool dropping was significantly faster in A. subterranea, and both the dropping and retrieving of tools began significantly earlier than in M. structor. For Aphaenogaster species that regularly utilize and compete for liquid food sources, the ability to efficiently transport liquid food via tools may be more important than it is for seed-harvester ants. Dropping tools into liquids, however, may still be useful for seed-harvester species as a means to supplement diet with liquid food during periods of seed shortage and also to serve as a means of getting rid of unwanted liquids close to the nest.