De Mendoza CONICET


Aridlands were an important scenario in the evolution of the Southamerican mammalian biota. A large proportion of the dryland species (ie. several marsupials, edentates, caviomorph and murid rodents) are not found elsewhere in the New World. In part, this degree of endemism and adaptation to xeric existence is a consequence of evolution in highly heterogeneous and isolated mosaic of drylands in the southern part of the continent since the Oligocene. However, in comparisons with other deserts of the world (i.e.the North American deserts), some of the Southamerican drylands, such as the temperate Monte Desert of Argentina[1], were regarded, for more than two decades, as impoverished in terms of their small mammal species richness and in the repertoire of xeric adaptations when compared with ecomorphological traits such as bipedal locomotion, granivory, water balance and so on, regarded as paradigmatic features to desert existence. As a result, the desert biota that lacked the "kangaroo rat syndrome" were regarded as non-adapted to xeric existence. The "lack of adaptations" of the South American lineage of murid rodents (sigmodontine rodents) has been atributed, by some authors, to their relatively recent entrance to South America after the completion of the Panamian bridge, 3.5 million years ago, and to their short time of expossure to arid habitats. Recent studies, however, discard some of these previous notions pointing out the need to be careful when accepting paradigms constructed largely from one continent.

South America possesses a high diversity of tropical, highland, coastal and continental dryland biomes. In a finer scale, the small mammal (less than 500 g) assemblage varies widely (species richness and composition) between these major arid biomes (Caatinga, Brazil; Atacama desert , Peru, Chile; Altiplano, Peru, Chile, Bolivia and Argentina; Monte and Patagonia, Argentina). The Andean Altiplano desert or Puna, and the lowland Monte Desert possesses the highest species richness and endemisms. The Andean drylands (above 3,500 m) were a major scenario in the cladogenetic process of murid rodents. This is reflected in the number of endemics, which account for more than 50 % of the Altiplano rodents. Richness of small mammals decreases towards lowland aridlands in both sides of the Andes. This may be explained by low rainfall, little (scarce?) vegetation as well as climatic unpredictability. At a lower level (ie. the Monte Desert) the importance of aridlands have been noted when addresing Argentina´s diversity of mammals. At a regional and local scales, the temperate Monte Desert is a highly heterogeneous landscape, from very simple, live-sand dunes patches, to more complex habitats such as those composed of creossote bush, grasses, and mesquite (Fabacea: Prosopis) habitats. The small mammal assemblage of the Monte desert shows a marked spatial and temporal heterogeneity, a highly diversified trophic ecology and differential responses to habitat perturbations.

Here we review evidence on biogeographical, ecological, physiological, morphological and behavioral studies of small mammals of the South American drylands, and the temperate Monte Desert in particular, that portrays the heterogeneity of these biomes at different scales as well as the rich array of adaptations and convergence with counterparts from other deserts of the world.

The research program on the biology of desert small mammals has been supported by CONICET grants PIP 4684 and SETCyP PICT 03281 to R. Ojeda.This review was presented in the Rodens et Spatium Conference, Louvain-le- Neuve, Belgica, Julio 2002(Ricardo A. Ojeda, Gabriela B. Diaz, Stella M. Giannoni, Carlos E. Borghi, Claudia M. Campos, Valeria Corbalán, Solana Tabeni and Mariana Dacar).

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