A house in the tropics: full pension for ants in Piper plants / Una casa en el tropico:pensión completa para hormigas en plantas de Piper

On the southern Pacific slope of Costa Rica, several species of Piper plants (Piperaceae) live in an obligate mutualism with Pheidole bicornis ants (Fomicidae: Myrmicinae). These plants produce small single-celled food bodies (FBs) in the leaf domatia formed by the petiole bases and roofing leaf sheaths. FBs of four of the five Piper species known to live with the obligate ant mutualist Pheidole bicornis were analysed: P. cenocladum, P. fimbriulatum, P. obliquum and P. sagittifolium. FBs mainly consist of lipids (41% to 48% of dry mass (DM)) and proteins (17% to 24%) and are a high-energy food source (up to 23 kJ g-1 DM) for the inhabiting ants. By measuring the natural abundance of the stable (i.e. non-radioactive) isotopes of carbon (13C) and of nitrogen (15N) in FBs of Piper fimbriulatum and in ants it was shown that especially the larvae of the ants mainly feed on FBs. By feeding inhabiting ants with the 15N-labelled amino acid glycine, which was supplied in sucrose solution to the ants, nutrient transfer was demonstrated not only from plants to ants via FBs but also from ants to plants via faeces. 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. However, the provision of nitrogen by symbiotic ants to the Piper species accounted for a minimum daily input rate of 0.8% of the plant¿s above-ground nitrogen uptake, which is only of minor importance for the plant partner. On the other hand, only a minute part of the above-ground biomass is actually invested in food for ants. Hence, the energy and material investment in food bodies of Piper plants may be compensated by the ant-derived nitrogen.