The Brazilian Caatinga in South American Zoogeography: Tropical Mammals in a Dry Region

Abstract

Recent research in South American biogeography on groups other than mammals suggests that the semi-arid Caatinga of northeastern Brazil provided xeric refugia during mesic phases of the Pleistocene climatic cycles. If this supposition is correct, the Caatinga mammal fauna might be expected to contain numerous species showing a pronounced level of adaptation to aridity and the origins of a substantial segment of the fauna sthould be traceable to the diversification and eventual speciation that would be expected to have resulted from extended isolation in a xeric environment. An analysis of the extant mammal fauna fails to corroborate these predictions; only one endemic mammal species is found in the Caatinga, and the fauna lacks the expected physiological and morphological adaptations. Other vertebrate groups also exhibit low levels of endemism. The high degree of climatic unpredictability that is characteristic of the Caatinga may preclude the development of a unique fauna adapted to xeric conditions. It appears that the present inhabitants of the region avoid the environmental effects of aridity and climatic unpredictability during harsh periods by utilizing the numerous mesic enclaves scattered throughout the North- east. Indeed, even during periods of elevated precipitation, most mammal species reach their highest density in these relatively restricted areas. Although the Caatinga contains a xeric-adapted flora, it harbours an anomalous mammal fauna that is more characteristic of a mesic tropical biome in terms of both species composition and overall adaptation. The development of an assemblage of verte- brates whose adaptations do not parallel those of the dominant flora in a region is an unusual situation which should be of particular interest to palaeontologists attempting to reconstruct ancient environments.