Scale-dependent patterns in body size distributions of neotropical mammals

We report on the spatial scaling of body size distributions in the mammalia n fauna of South America, exclusive of fully aquatic or volant species. Alt hough the frequency distribution of body sizes for 720 species of South Ame rican mammals was qualitatively similar to that observed for 464 species of North American mammals, different patterns emerged at smaller spatial scal es in these two regions. At smaller scales in North America, body size freq uency distributions became progressively more uniform on a log axis, wherea s they remained modal with decreasing spatial scale in neotropical rain for est, and the mode shifted to larger body sizes. This unexpected scaling rel ationship appears to reflect the addition of a substantial third habitat di mension: the rain forest canopy. When rain forest mammals were separated by foraging stratum (terrestrial, scansorial, and arboreal), distributions of body sizes within strata exhibited scaling relationships similar to those observed in temperate areas of North and South America. These results are c onsistent with earlier interpretations of body size scaling across large ge ographic or taxonomic ranges, but they emphasize the pivotal role that habi tat structure and heterogeneity can play in the ecological and evolutionary development of faunas. Additionally, these results underscore the role of habitat complexity as a factor contributing to the latitudinal gradient in species richness.