Protection of Specific Ant Partners against Generalist Ants

Macaranga (Euphorbiaceae) is a large paleotropical pioneer tree genus consisting of approximately 280 species [18]. The majority of species are myrmecophilic, i.e., they offer nectar and food bodies on the plant surface to attract generalist ants, which provide protection against herbivory. Only 29 Macaranga species in southeast Asia are myrmecophytic, i.e., obligately associated with specialized plant-ants. These ants (belonging to the genera Crematogaster and Camponotus) exclusively nest in the hollow (or hollowed-out) stems of their host trees.

In 14 Macaranga ant-plant species, the stems are densely covered by blooms of epicuticular wax crystals. These waxy surfaces are very slippery for most generalist insects. A test of the climbing capacity of 17 generalist ant species on waxy Macaranga stems showed that ants often fell down from the plant or moved forward very slowly. This barrier effect of the wax crystal surfaces is only based on reduced physical adhesion; no evidence of chemical repellence was found [17]. In striking contrast to generalist ants, the specialized ant partners of waxy Macaranga hosts had no difficulty running on the slippery stems. Because of their exceptional wax running capacity, these ants escape predation and competition by generalist ants. In fact, Crematogaster (Decacrema) wax runners were clearly inferior to most other ants in direct encounters, so that their survival depended on the presence of vertical waxy stem sections preventing generalist ants from invading their nests [17]. The barrier effect of waxy stems is enhanced by the whorled growth form and the extreme epidermis longevity of many Macaranga ant-plants, which increase the length of slippery, waxy stem sections that have to be traversed to reach the upper parts of a tree [17].

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