By Gábor Lőrinczi
Based on the research article “Comparison of foraging tool use in two species of myrmicine ants (Hymenoptera: Formicidae)” by Gábor Módra, István Maák, Ádám Lőrincz and Gábor Lőrinczi in Insectes Sociaux, 69, pages 5–12 (2022).
Some ants are real tool users. For example, some myrmicine ants (i.e., ants that belong to the highly diverse subfamily Myrmicinae) place or drop bits of leaf, wood, soil etc. into liquid food (e.g., rotten fruit pulp, body fluids of dead arthropods, droplets of honeydew released by aphids, etc.), and then they carry the food-soaked objects to the nest, where nestmates can feed on them. This behavior is similar to that shown by many primates who use leaves, mosses, paper, rags, etc. as “sponges” to soak up water and other liquids.
In this video, you can see a laboratory colony of Aphaenogaster subterranea using various tools to carry honey-water.
While this so-called foraging tool use is well documented and makes good intuitive sense in omnivorous ants like Aphaenogaster species, studies on granivorous ants (i.e., ants that primarily feed on seeds), which interestingly are also known to use tools when feeding on liquids, are still scarce.
So what about harvester ants? Do they have a different tool-using behavior as compared to omnivorous species? And why do they show this behavior in the first place? To answer these questions, we compared the foraging tool use in captive colonies of two closely related myrmicine ants with different diet, Aphaenogaster subterranea, an omnivorous species, and Messor structor, a mainly granivorous species.
In our experiments, we provided colonies with honey-water baits and a mixture of six types of objects (sand grains, small soil grains, large soil grains, pine needles, leaves and sponges) they could use as tools. During the observations, we recorded the type and number of tools placed into honey-water baits, and the type and number of food-soaked tools retrieved and transported to the nest.
As we have expected, the two species showed many differences in the nature of their tool-using behavior. Firstly, the foraging workers of A. subterranea both dropped more tools into honey-water baits and retrieved more of these tools than the workers of M. structor. Secondly, A. subterranea preferred smaller tools over larger ones, while M. structor showed no preference towards any specific tool type. Thirdly, tool dropping was much faster in A. subterranea, and both the dropping and retrieving of tools began much earlier than in M. structor.
We think that for Aphaenogaster species that regularly utilize and compete for liquid food sources, the ability to efficiently hide and transport edible liquids with the help of tools may be more important than it is for harvester ants like Messor species, which mainly feed on seeds. Using tools, however, may still be useful for harvester ants, for example, when local seed sources are not available but there are other opportunities to acquire food for the colony.