Bacterial symbionts in plant-ants

Nitrogen isotope ratios for acacia-ants and other genera of ants from the same sites. Acacia-ants feed at a significantly lower trophic level than all other ants sampled.

Ants are the most numerous animals in many terrestrial ecosystems, comprising over 90% of arthropods in the rainforest canopy. This numerical dominance means that ants cannot all be predatory, as was once thought, but must instead feed on more nutrient poor, plant-based resources. Acacia-ants are exemplars of strictly herbivorous ants, living in acacia trees and feeding exclusively on resources provided by these plants. However, the nutritional content of the nectar provided by acacias is insufficient for colony growth and reproduction, leaving open the question of how these ants obtain the necessary nourishment. Many other animals, including humans, are now known to depend on bacteria living in their guts to help them extract nutrients from their food. We, therefore, compared the bacterial communities associated with acacia-ants to determine if microbes may be helping to enrich there diets. Although stable isotope analyses show that acacia-ants are remarkably strict in their plant-based diets, we find no evidence to suggest that specialized bacteria are contributing to their nutrition. Instead, these ants must be deriving all necessary nutrients from their host plants.