Nutritional and functional biology of exudate-feeding ants

Feeding extensively on plant exudates and honeydews, many tropical arboreal ant species exhibit delta N-15 values characteristic of herbivores. Consistent with hypothesized herbivory, these taxa behave in feeding assays as though more N-deprived than are strictly carnivorous ants. However, to an as yet uncertain degree, relationships with N-upgrading and/or recycling microsymbionts may lower isotopic ratios, making ants appear to be more herbivorous than they actually are. Nutritional (N) contributions from microsymbionts have been inferred for a variety of ant taxa based on intracellular or extracellular associations between ants and bacteria. However, stronger and more specific inferences are possible when variability in microsymbiont locations within the digestive system is considered in the context of taxonomic variability in ant diets and digestive anatomy. Diets of exudate feeders may vary predictably in ratios of usable carbohydrates (CHOs) to N, depending on the extent to which they tend melezitose-producing Homoptera. Status of the peritrophic membrane, proventricular structure, and number and placement of Malpighian tubules can be interpreted as traits contributing to supply of N and/or CHOs to microsymbionts. In general, a more integrative understanding of ant diets, digestive anatomy, and associated microsymbionts helps to set out specific hypotheses to be tested experimentally and (where possible) in a phylogenetic context.