Abstract Several species of Piper (Piperaceae) live in symbiosis with Pheidole bicornis (Formicidae-Myrmic- inae) on the southern Pacific slope of Costa Rica. These plants produce small single-celled food bodies (FBs) in leaf domatia, formed by the petiole bases and roofing leaf sheaths. In the present study the dependency of ants on FBs of Piper fimbriulatum as a food source was anal- ysed by comparing the natural abundance of 13 C and 15 N in ants and FBs. Both δ 13 C and δ 15 N values were very similar between FBs and Pheidole bicornis ants but dif- fered substantially between the plant and other ant spe- cies. Therefore we suggest that FBs are a main food source for Pheidole bicornis ants. To strengthen this sug- gestion, the chemical composition of FBs of four my- rmecophytic Piper species was analysed, with special emphasis on the nutritional requirements of inhabiting Pheidole bicornis ants. Standard chemical methods were modified and combined to a novel analysis scheme by which all major FB constituents could be quantified from minute [3–10 mg dry mass (DM)] quantities. Piper FBs mainly consisted of lipids (41–48% of DM) and proteins (17–24% of DM). Soluble carbohydrates and amino ac- ids proved to be quantitatively unimportant. N was pre- dominantly stored as soluble protein and, thus, was easi- ly available to the ants. FBs proved to be a high-energy food source (up to 23 kJ g –1 DM), with a chemical com- position that meets well the nutritional needs of the in- habiting ants. Keywords Ant-plant mutualism · Myrmecophyte · Nutrient provisioning · Nitrogen-15 stable isotopes · Carbon-13 stable isotopes Introduction Interactions between ants and plants are widespread and range from very loose associations to obligate symbioses (Hölldobler and Wilson 1990). While loose associations such as myrmecochory also exist in temperate habitats, symbiotic ant-plant interactions are exclusively found in the tropics, exhibiting the most distinct diversity in the Neotropics (Davidson and McKey 1993b). In such sym- bioses the species involved exhibit numerous adapta- tions. Benefits for the plants are not always obvious and may vary strongly. In some myrmecophytes (i.e. plants offering nesting space for ants) the inhabiting ants de- fend the plants against herbivores, encroaching vines and fungal pathogens (e.g. Janzen 1966; Schupp 1986; Vasconcelos 1991; Gaume et al. 1997; Heil et al. 2001a) or supply nutrients by debris accumulation or by defecation (Treseder et al. 1995). Advantages for inhabiting ants are more obvious: plants provide nesting space (“myrmecod- omatia”) and, often, food resources. Myrmecodomatia may be either of caulinary (i.e. hollow and swollen stems, Davidson and McKey 1993a; Brouat and McKey 2000) or of foliar origin (Svoma and Morawetz 1992). Myrmecophytes provide food sources either directly by extrafloral nectar and by food bodies (FBs) (Fiala and Maschwitz 1992), or indirectly, through sap-sucking ho- mopterans kept by ants inside the myrmecodomatia (Buckley 1987; Gaume et al. 1998, 2000). Although FBs have been shown to play an essential role in several plant-ant symbioses, such as those of Acacia and Pseu- domyrmex (Janzen 1966), Cecropia and Azteca (Janzen 1969) and Macaranga and Crematogaster (Fiala et al. 1989), studies on the chemical content of FBs are rare. During ultrastructural studies, Rickson (1971, 1975, 1976) found proteins and lipids in FBs of Acacia and phytoglycogen in the case of Cecropia. However, the on- ly thorough investigation on FB composition was recent- ly conducted on several Macaranga species (Heil et al. 1998). Carbohydrates and proteins were found in similar quantities, but lipids constituted the main component. It was suggested that FBs meet the specific nutritional re- R.C. Fischer · V. Mayer ( ✉ ) Institute of Botany, Division of Morphology, University of Vienna, Rennweg 14, 1030 Vienna, Austria e-mail: veronika.mayer@univie.ac.at Fax: +43-1-427754085 A. Richter · W. Wanek Institute of Ecology and Conservation Biology, Chemical Physiology of Plants, University of Vienna, Althanstrasse 14, 1090 Vienna, Austria Oecologia (2002) 133:186–192 DOI 10.1007/s00442-002-1000-y PLANT ANIMAL INTERACTIONS Renate C. Fischer · Andreas Richter Wolfgang Wanek · Veronika Mayer Plants feed ants: food bodies of myrmecophytic Piper and their significance for the interaction with Pheidole bicornis ants Received: 22 December 2001 / Accepted: 13 June 2002 / Published online: 23 August 2002 © Springer-Verlag 2002