Ecological Relations of Three Atta Species in Panama

Abstract

Of the three Atta species of Panama (sexdens L., cephalotes isthmicola Weber and colonzbica tonsipes Santschi), sexdens is a grassland species as it is in South America, and the other two are forest or forest‐grassland ecotone species. While sexdens and tonsipes may nest in the same general grassy area, sexdens extends along the sandy Pacific shore, and the other two nest primarily in clay. A unique relationship in the heart of the city of Colon on the Atlantic side was a colony of isthnzicola surrounded by colonies of tonsipes. Their leaf‐bearing files were separate and they were mutually hostile. The main plaza of Panama City on the Pacific side has been occupied by sexdens despite the gradual extinguishing of foraging areas by buildings. Barro Colorado Island has isthmicola and tonsipes occupying similar habitats. Personal records since 1938 are noted. Usually the species of the leaf‐cutting ant, Atta, occupy well‐defined and separate areas (Fig. 1) throughout their range from Texas and Louisiana to Argentina and Uruguay. The gap between texana Buckley in Texas and the northernmost extent of mexicana F. Smith in Arizona is several hundred miles wide. In Uruguay sexdens L. is found to the north and on the rolling pampas while vollcnweideri Forel is found only close to the Rio Uruguay, according to Carbonnel (pers. commun.). The much closer relations of sexdens and cephalotes L. in Brazil have been described by Borgmeier (1959) and Goncalves (1960), and in Guyana and Venezuela in Weber 1946, and 1947. There is generally a well‐defined ecological difference, sexdens being found in grassland or the grassland‐forest ecotone and cephalotes occurring in the forest. In Guyana sexdens seemed more an ant of sandy soil and cephalotes an ant of clay, but this is not always consistent. The three species of Panama (Weber 1941, 1956, 1968) are sexdens, cephalotes isthmicola Weber and colombica tonsipes Santschi (Fig. 2). It should not be assumed that the ranges of any Atta are stabilized. All fluctuate with changes in climate and for more obscure reasons. In part it may be that the species and the genus have evolved in relatively recent times compared with such genera as Myrmicocrypta and Sericomyrmex. They clearly have more flexibility in meeting climatic change.