Temporary sterilization behavior of mutualistic partner ants in a Southeast Asian myrmecophyte

Obligate ant–plant interactions are known to be mutualistic but plant-ants that destroy flowers of their hosts have been reported. They were regarded as parasites in myrmecophytic systems. The mechanisms that lead to flower damage (sterilization) by plant-ants are not easy to understand as most sterilizing ants are actually regular colonizers of their plants and normally offer protection against herbivores and/or plant competition. It is difficult to find general patterns of ant or plant traits even in the few yet known associations of flower sterilization. We here present the first study from Southeast Asia where flower sterilizing occurs in the complex mutualistic Macaranga–Crematogaster system that differs from other cases. Flowers of M. hullettii in the Gombak Valley were destroyed by all three associated specific and otherwise protective Crematogaster species. The hypotheses that limitation of nesting space or food are main proximate factors for flower destruction were not strongly supported in our study system. Ants are even attracted to flowers by special food bodies produced by the plants. Only younger, not yet reproductive colonies were found to destroy flowers but not colonies with alates, indicating that flower sterilization behavior may only occur when the onset of host reproduction precedes ant reproduction, perhaps leading to a change in ant behavior. Fruit set always occurred in larger trees, and saplings for colonizing ant queens were therefore always present in the local population, stabilizing the association.