Broadly speaking, ants have two different feeding strategies. A large proportion of all species are "carnivorous," meaning that they are generalist predators feeding on other small animals or scavenging on their remains. Some, however, are "herbivorous". This is not to say that they only eat plants; rather, the bulk of their diets consist of plant-derived matter. For example, some forage on sticky fluids produced by plants to attract ants, called extra-floral nectar; others feed on the processed plant sap excreted by plant-sucking insects such as scale insects and aphids. Herbivorous ants are likely to be a highly under-estimated component of the global fauna as there are many tropical forest canopy specialists among them, and the forest canopy remains to this day surprisingly unexplored.
It has long been a mystery how herbivorous ant species gain all the nutrients they need. Their plant-derived diet comprises essentially water and sugars; it is deficient in protein and/or the nitrogen-based compounds that are the building blocks of proteins. Carnivorous ants face few such nutritional difficulties, as their diet tends to contain all the chemical compounds they require. Most ants are not renowned for being associated with microbesthe most famous suite of on-board microbial symbionts in insects is found in termites, whose guts harbor bacteria that facilitate the digestion of the woody material that constitutes the termite dietbut it has been recently hypothesized that herbivorous ants might host a set of indigenous symbionts that provide the missing components of the herbivorous ants' diets.
We tested this hypothesis by using molecular genetic techniques to look for the presence of microbes in 283 species of ants from 18 of the 21 ant subfamilies. We were able to classify each ant species as carnivorous or herbivorous based on the amount of heavy and light nitrogen (15N/14N) within the ants' tissues. By uniting the two datasets, we were the
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