HOW AGGRESSIVE ANT-GUARDS ASSIST SEED-SET IN ACACIA FLOWERS

The phenomenon of ant-guarding on Acacia trees is probably the best kn own case of a mutualism between plants and animals, the ants conferrin g biotic defence against herbivores and perhaps against encroaching ve getation(1-3). However, as with many defence mutualisms, sometimes the interests of the plant and its defender conflict: for example, when t hey are in newer the Acacia trees require the presence and service of other insects to effect cross-pollination. How is pollinator access ac hieved in the face of aggressive ant-guards? Here we report that ants are deterred from young flowers at the crucial stage of dehiscence, al lowing bees and other pollinators to visit and transfer pollen. This d eterrence appears to be a response to a volatile chemical signal from young flowers, perhaps from the pollen itself. Ants patrol the young ( undehisced) buds, and also return to the flowers after dehiscence, pro tecting the fertilized ovules and developing seeds. The outcome is a d irectly improved seed-set in the presence of ants (rather than an indi rect extra reproductive resource allocation due to decreased defoliati on(4-6)).