Scale-Dependent Patterns in Body Size Distributions of Neotropical Mammals

We report on the spatial scaling of body size distributions in the mammalian fauna of South America, exclusive of fully aquatic or volant species. Although the frequency distribution of body sizes for 720 species of South American mammals was qualitatively similar to that observed for 464 species of North American mammals, different patterns emerged at smaller spatial scales in these two regions. At smaller scales in North America, body size frequency distributions became progressively more uniform on a log axis, whereas they remained modal with decreasing spatial scale in neotropical rain forest, and the mode shifted to larger body sizes. This unexpected scaling relationship appears to reflect the addition of a substantial third habitat dimension: 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 consistent with earlier interpretations of body size scaling across large geographic or taxonomic ranges, but they emphasize the pivotal role that habitat 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.