Do ants feed plants? A 15N labelling study of nitrogen fluxes from ants to plants in the mutualism of Pheidole and Piper

1. Nutrient fluxes play a significant role in the interaction of myrmecophytic plants and their symbiotic ants. There is a clear flux from plants to ants via nectar or food bodies but nutrient fluxes from ants to plants are less obvious. 2. We report on a nitrogen flux from ants to plants in the association between Pheidole bicornis (Formicidae-Myrmicinae) and two myrmecophytic Piper species (P. fimbriulatum and P. obliquum, Piperaceae). Pulse experiments were performed by feeding ants with 15N-labelled glycine supplied in sucrose solution. Workers passed ingested label on to other ants and the brood by trophallaxis. 3. The distribution of label within the colony showed highest incorporation rates in larvae and in the working caste, while the reproductive caste received only a small amount of the 15N-labelled food. 4. Nutrient transfer from ants to plants occurred remarkably fast. Within 6 days, up to 25% of the nitrogen ingested by the ants was incorporated by the plants. 5. However, ant distribution within P. fimbriulatum plants did not correlate with the intra-plant uptake pattern of 15N, and ant-mediated nitrogen uptake by myrmecophytic P. fimbriulatum accounted for less than 1% of the plants' above-ground nitrogen demand.