Habitat selection by the queens of the leaf-cutting ant Atta sexdens L. in Brazil

Abstract

The founding of a new nest is the most crucial period in the life of an ant colony (Wilson 1971). Mechanisms that enhance colony survivorship during this phase are thus of great value. Of special importance is the search for a suitable habitat by the foundress queen, since established colonies of most ant species are unable to migrate over long distances (Wilson & Hunt 1966). That ant queens select the habitat in which they will found a new colony has been demonstrated for several temperate zone species (Brian 1952, Brian et al. 1966, Pontin 1960, Wilson & Hunt 1966) and for the Neotropical leaf-cutting ants Atta cephalotes L. and Acromyrmex octospinosus Reich (Cherrett 1968). In this paper, evidence that such a mechanism is operating among the foundress queens of another leaf-cutting ant species, Atta sexdens L., is presented. Furthermore, it is shown that, once in the preferred habitat, they search for the best microhabitat. Field work was done at 'Fazenda Dimona', a ranch located about 70 km north of Manaus, Brazil (600 4' W, 20 19 S). In this ranch two forest fragments of 1 ha, one of 10 ha and one of 100 ha were created during late 1984 to meet the requirements of the Minimum Critical Size of Ecosystem Project (a collaborative binational research project between the Instituto Nacional de Pesquisas da Amazonia and the World Wildlife Fund-US). A fifth fragment about 100 m wide and bordering a local stream, but not related to the MCSE Project, was created at the same time. Brachiaria grass was planted in the cleared area, but no efforts were made to maintain the pasture free of weed species. One year later, at the beginning of the study, stands of secondary vegetation 1-2 m tall, mainly Laetia procera (Poepp.) Eichl., Solanum rugosum Dun. and species of Cecropia, were present throughout the pasture. The vegetation of the fragments was a primary, terra-firme forest (as defined by Guillaumet 1987).