COEVOLUTION OF MUTUALISM BETWEEN ANTS AND ACACIAS IN CENTRAL AMERICA

The purpose of this paper is to discuss the coevolution of one of the more thoroughly studied mutualistic systems in the New World tropics: the interdependency between the swollen-thorn acacias and their ant inhabitants. This system has recently been described in detail in respect to one species of plant, Acacia cornigera L. (Mimosoideae; Leguminosae), and one species of ant, Pseudomyrmex ferruginea F. Smith (Pseudomyrmecinae; Formicidae), and shown experimentally to be a case of mutualism (Janzen, 1966a). In this species pair, the ant is dependent upon the acacia for food and domicile, and the acacia is dependent upon the ant for protection from phytophagous insects and neighboring plants. The literature dealing only with the New World tropical acacias (Acacia spp.) and their ants (Pseudomyrmex spp.) has been re-evaluated by Janzen (1966a) and will not be discussed further in a review sense. The higher plants that commonly have ant colonies living in them have long been termed myrmecophytes. Ants living in plants range in habit from fortuitous usage of a plant cavity to highly complex interaction systems between the ant and the higher plant. The ant-acacia system represents this latter extreme, and the acacia is by any definition a myrmecophyte. A review paper on the subject of myrmecophytes is in preparation. In the present paper, plants with ants living in them will be called "ant-plants"; the ants will be called "plant-ants." The "swollen-thorn acacias" are those with 1) enlarged stipular thorns normally tenanted by ants, 2) enlarged foliar nectaries, 3) modified leaflet tips called Beltian bodies (eaten by the ants), and 4) nearly year-round leaf production and maintenance even in areas with a distinct dry season (Fig. 1). Swollen-thorn acacias have been shown experimentally to have a virtually obligate dependency on 1 Contribution No. 1308 from the Department of Entomology, The University of Kansas, Lawrence. This study was supported by National Science Foundation Grant GB-1428 to the University of California, Berkeley (Dr. R. F. Smith), University of California Associates in Tropical Biogeography Research Grants 66 and 81, and The University of Kansas General Research Grant 3173-5038, and also is a by-product of National Science Foundation Grant GB-91 (Dr. C. D. Michener).