A CASTRATION PARASITE OF AN ANT-PLANT MUTUALISM

Exploring the factors governing the maintenance and breakdown of coope ration between mutualists is an intriguing and enduring problem for ev olutionary ecology, and symbioses between ants and plants can provide useful experimental models for such studies. Hundreds of tropical plan t species have evolved structures to house and feed ants, and these an t-plant symbioses have long been considered classic examples of mutual ism. Here, we report that the primary ant symbiont, Allomerus cf. deme rarae, of the most abundant ant-plant found in south-east Peru, Cordia nodosa Lam., castrates its host plant. Allomerus workers protect new leaves and their associated domatia from herbivory, but destroy flower s, reducing fruit production to zero in most host plants. Castrated pl ants occupied by Allomerus provide more domatia for their associated a nts than plants occupied by three species of Azteca ants that do not c astrate their hosts. Allomerus colonies in larger plants have higher f ecundity. As a consequence, Allomerus appears to benefit from its cast ration behaviour, to the detriment of C. nodosa. The C. nodosa-ant sys tem exhibits none of the retaliatory or filtering mechanisms shown to stabilize cheating in other cooperative systems, and appears to persis t because some of the plants, albeit a small;minority, are inhabited b y the three species of truly mutualistic Azteca ants.