Sterilization and canopy modification of a swollen thorn acacia tree by a plant-ant

Obligate symbioses between specialized arboreal ants and plants have evolve d independently in many lineages(1,2). Ant-plants (myrmecophytes) typically provide hollow nest cavities and nutrition to the occupying ant colony(1,3 -6). In turn, resident plant-ants often protect their hosts from herbivory( 7-11) and/or overgrowth by surrounding vegetation(12,13). As individual pla nts are rarely occupied by more than one ant colon(14-17), co-occurring pla nt-ant species compete intensely for hosts(13,14,18,19). In such multispeci es systems, ecological interactions among potential partners may lead to th e evolution of cheating(20,21). Previous studies have revealed that some sp ecialized plant-ants are effectively parasites of their host-plants(8,18,22 ,23), but the selection pressures favouring such behaviours are poorly unde rstood. Here we describe host parasitism in an east African plant-ant that prunes and sterilizes its host-tree canopies, apparently to minimize contac t with competitively dominant ants occupying neighbouring trees. We propose that the high density of ant-trees and low diversity of tree species in th is savanna habitat have selected for induced, parasitic pruning of host tre es by this competitively subordinate ant species.