The Tocantins River basin, with an area of approximately 321,000 km2, is the largest hydrographic basin entirely within Brazilian territory. Of the five major vegetation types that form the "vegetational provinces" that cover the country, the Tocantins river basin has two: the Amazonian Dryland Forest or Umbrophylous Forest and the Savanna (Cerrado). Research efforts are primarily focused on the Cerrado region. Faunistic surveys in this region are scarce, being useful the lists of species related to environmental studies of hydroelectric projects (Tucuruí and Serra da Mesa). The project aims at the preliminary characterization of the fauna of small rodents and marsupials (Rodentia and Didelphimorphia) associated with the Tocantins basin. For terrestrial vertebrates of the Cerrado region, about 19% of species are unique to this biome, rising to 37% among rodents and 16% for birds. Literature reviews were conducted based primarily on the major zoological reference documents available. The completion of the literature survey indicated a great scarcity of records on mammal diversity in the Cerrado. So far, records of 78 species of marsupials and rodents have been found, among which the most frequent are Marmosa sp, Didelphis albiventris (Lund, 1840), Oryzomys sp. and Calomys sp. The fauna of the Cerrado, similar to that of the Caatinga, is characterized by a relatively small number of truly endemic forms; approximately 50% of the species reported are also common to the Atlantic Forest and 90% also occur in the Caatinga. Hershkovitz (1972) has previously noted the low degree of endemism of these species in the Cerrado. The fauna of the Cerrado is in general mainly composed of species that occur in open formations (Vanzolini, 1973).
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