Maintaining an ant-plant symbiosis: secondary polygyny in the Macaranga triloba-Crematogaster sp. association.

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Ant-plant associations are common and important elements of tropical forest ecosystems. Their relationships vary from loosely facultative to obligate symbiosis (for an overview see Davidson and McKey 1993). In SE Asia many species of the pioneer tree genus Macaranga are myrmecophytes that provide their ant-partners with nutrients and nesting space in exchange for herbivoreprotection by the ants. The majority of the associated ant species belong to Decacrema, a subgenus of the myrmicine Crematogaster (Fiala et al. 1999) and they are obligatorily and specifically associated with their Macaranga plant-partners. Ants and plants are highly dependent on each other in this myrmecophytic complex. In most Macaranga species even very young saplings develop stem domatia, which are colonised by queens throughout the year. It is very rare to find more than one founding queen in one domatium but frequently several queens occupy different domatia of the same young plant. After claustral colony foundation and the emergence of the first workers, however, only one monogynous colony survives per host tree (Fiala and Maschwitz 1990). Generally it is supposed that the monogynous status is maintained throughout the entire existence of the symbiosis. The death of either partner – ant colony or host tree – can result in severe damage or even the death of the other partner. In the case of the death of the queen the colony dies out and the tree is highly endangered by herbivores (Fiala et al. 1989; Heil et al. 1997). Small trees may be colonised again by their specific ant-partners and then suffer only little damage during the development of the new colony which will soon be large enough to patrol the whole tree effectively. In mature trees, however, with their much larger leaf area, the establishment of a worker force would take too long. Do we find other mechanisms in ant colonies on mature trees that might enable the ants to further protect the tree against herbivoredamage as well as safeguard its territory and resources against competing ant species? Whereas much information now exists on the founding and growth of colonies on saplings (Fiala and Maschwitz 1990), no survey has ever been conducted on the development of mature ant colonies on larger Macaranga trees. We therefore studied queen numbers and colony development in maturing trees in the common myrmecophyte Macaranga triloba (Bl.) Muell. Arg., that is obligatorily associated with Crematogaster msp. 4 (Fiala et al. 1999).