The serras in the east have wet seasons that last for 6–8 months. The central body of the ecoregion in Mato Grosso, and the portions to the west, have wet seasons from 8–9 months. During the rainy season from December to May large areas are flooded. In the dry season the porous soil quickly drains the wetlands and pools, leaving large flats and patches of dry forest, savanna and moist forest. The Mato Grosso tropical dry forests ecoregion is a transitional zone between the moist forests of the Amazon basin to the north and the cerrado of the Brazilian Highlands to the south. The annual floods and periodic fires in the dry season form a complex mosaic of forest, grasslands and transitional vegetation. The dominant habitat is dry forest, but the ecoregion also contains savannas, gallery forests and areas of dense thicket. Much of the habitat is found in isolated patches within other ecoregions. The soil of the Serra do Cachimbo is white sand. It has an extensive region of campos rupestres, an open plant formation subject to drought growing on rocks that do not retain water. In appearance the campo ruprestre is similar to savanna, but the flora are different with species such as "Calea lutea" and others that are found in caatinga forest. The central and western areas hold tropical semi-deciduous seasonal forest. Forests are not as lush as those found further west, and the trees are shorter at . Flora includes "Piptadenia incurialis" and species of the Bowdichia, Hymenaea and Machaerium genera. Cerrado species include "Curatella american", Qualea species, and "Kielmeyera coriacea". The various seasonal habitats support diverse fauna with many endemic species including cayman lizards and colorful butterflies. In 1967–69 the British Royal Society and Royal Geographical Society undertook an expedition in the Mato Grosso (upper Xingu River). They found 46 genera of rodents including the large capybara ("Hydrochoerus hydrochaeris"), 52 genera of bats of which Artibeus and Molossus were most common, 3 of marsupials and 11 of edentates. They recorded 161 species of birds of which 60 occurred only in the cerrado, 33 in gallery forests and 25 in dry forest. The Serra do Cachimbo has an isolated population of caatinga antwren ("Herpsilochmus sellowi") and the endemic amphibian "Tropidurus insulanus".