Effects of food rewards offered by ant–plantMacarangaon the colony size of ants

Abstract

Myrmecophytes (ant–plants) have special hollow structures (domatia) in which obligate ant partners nest. As the ants live only on the plants and feed exclusively on plant food bodies, sap‐sucking homopterans in the domatia, and/or the homopteran's honeydew, they are suitable for the study of colony size regulation by food. We examined factors regulating ant colony size in four myrmecophyticMacarangaspecies, which have strictly species‐specific association withCrematogastersymbiont ants. Intra‐ and interspecific comparison of the plants showed that the ant biomass per unit food biomass was constant irrespective of plant developmental stage and plant species, suggesting that the ant colony size is limited by food supply. The primary food offered by the plants to the ants was different amongMacarangaspecies. Ants inMacaranga beccarianaandMacaranga bancanarelied on homopterans rather than food bodies, and appeared to regulate the homopteran biomass and, as a consequence, regulate the ants' own biomass. In contrast, ants inMacaranga winkleriandMacaranga trachyphyllarelied primarily on food bodies rather than homopterans, and the plants appeared to manipulate the ant colony size. Per capita plant investment in ants (ant dry weight plant dry weight−1) was different among the fourMacarangaspecies. The homoptera‐dependentM. beccarianaandM. bancanaharbored lower biomass of ants than the food‐body dependentM. winkleri, suggesting that energy loss is involved in the homoptera‐interposing symbiotic system which has one additional trophic level. The plants' investment ratio to the ants generally decreased as plants grew. The evolution of the plant reward‐offering system in ant–plant–homopteran symbioses is discussed with an emphasis on the role of homopterans.