Ant-fed plants: comparison between three geophytic myrmecophytes

In their association with myrmecophytes (i.e. plants that shelter a limited number of ant species in hollow structures), ants sometimes provide only poor biotic protection for their host plants, but may supply them with nutrients (myrmecotrophy). We studied three geophytic myrmecophytes growing in the understorey of Guianian rain forests. Allomerus ants build spongy-looking galleries rich in detritus and insect debris over the stems of their host plants [Cordia nodosa Lamark (Boraginaceae) and Hirtella physophora Martius & Zuccharini (Chrysobalanaceae)], while Pheidole minutula Mayr colonies deposit their waste in the leaf pouches of their host plant [Maieta guianensis Aublet (Melastomataceae)]. This waste is more nitrogen-rich than that found in the Allomerus galleries, themselves containing more nitrogen than the plant leaves. Using stable isotope analysis we noted a significant difference in delta(15)N between ant-occupied and unoccupied plants only for Maieta, for which 80% of the host plant nitrogen is derived from Pheidole waste. Experiments on all three plants using a N-15-supplemented solution of NH4Cl confirmed these results, with an increase in this isotope noted between control and experimental plants only for Maieta. The internal surfaces of Maieta leaf pouches bear protuberances whose likely role is to absorb nutrients from the Pheidole waste. The alternative hypothesis, that these protuberances play a role in provisioning ants, was rejected after comparing their structure with those of extrafloral nectaries and food bodies in a histological study. (C) 2004 The Linnean Society of London.