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At the geographic scale, body mass has been analysed as a correlate of local abundance and area of range, or in terms of latitudinal variations in body mass, both intra- and interspecifically. One productive approach has been the analysis of the frequency distribution of body mass in animal assemblages at different spatial scales. In this paper we use such an approach to describe the geographic patterns of body-mass diversity in the Mexican mammalian fauna. We found contrastingpatterns for bats and non-volant species: Non-volant mammals in Mexico followed the already described pattern of a right-skewed distribution of body size at large scales and a more even distribution at smaller scales; bats, in contrast, showed skewed distributions at all scales. Statistical tests based on null models demonstrated that most assemblages of non-volant mammals present higher diversity of body massthan expected by chance, whereas chiropteran assemblages show the variation in body size that would be expected from randomly sampling the whole fauna of the country. Although we found an effect of latitudeon body-mass diversity, we also demonstrated that topographic features (peninsulas and mountain ranges) have a strong influence on the patterns of body mass at small scales. Using iSHEi analysis, a method to decompose the elements of diversity, we studied the scaling of body-mass diversity in south-eastern Mexico and documented subtle patterns that had not been observed in comparisons between biomes and local communities. In particular, we found that at intermediate scales the eveness of the distribution of body mass values remains constant, so diversity is determined chiefly by changes in the number of size classes found in the assemblages. In contrast, at the national level, diversity is comparatively low, despite the presence of all size classes, because of the low eveness, reflected in the highly skewed frequency distribution of body-mass values. Our results show that the body-mass [ABSTRACT FROM AUTHOR]