Several Central American species of Piper have developed obligate mutualisms with ants, in which plant partners provide nesting sites and food for ant partners. In turn, the plants receive some protection from herbivores and fungal infection. In addition to these obligate ant-plants, some species of Piper are found that have resident ants only sometimes (facultative), and still other Piper species are never found with ants. This continuum is found in Piper subg. Macrostachys, and the degree of ant association is closely correlated with plant morphology. This variation in morphology and ant presence has contributed to confounding evolutionary relationships. In obligate ant-plants, the sheathing petioles that are typical of some Piper become tightly rolled into a tube. The ants, Pheidole bicornis in most cases, move into this cavity, and in general, the stems become hollow as the plant increases in size with the colony eventually occupying the entire plant. The food bodies that the ants subsist on are produced on the adaxial surface of the petioles (i.e. inside the tube), thus available exclusively to the ants; furthermore they are only produced in the presence of the ants. While food bodies and hollow stems are only found in the obligate ant plants, variously closed petiole domatia are found in other Piper species; an increased degree of closure corresponds with increased density of ant residents. In these facultative ant plants, the association between ant and plant is less specific, and a number of ant species inhabit these plants. This continuum of ant mutualisms and plant adaptations is being examined in the context of an independently derived molecular phylogeny of Piper subg. Macrostachys, with the ultimate goal of understanding the evolutionary sequence of events and the key adaptations that resulted in obligate associations between ants and plants.

Key words: ant-plant mutualisms, evolution, phylogeny, Piper, Piperaceae, plant morphology