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.