Body Mass Distribution and Body-Size Structure of Brazilian Terrestrial Mammals: A Reassessment of Conventional Size Classes

Body mass is a fundamental functional trait influencing mammalian biology, ecology, and conservation status. In Neotropical studies, species are conventionally classified into small (<1 kg), medium (1–44 kg), and large (>44 kg) mammals. However, whether these operational thresholds reflect natural discontinuities in body mass across regional faunas remains untested. We compiled body mass data for 529 Brazilian terrestrial mammal species across nine orders (excluding Chiroptera) to evaluate body-size distribution and test whether conventional categories align with empirical body-mass structure using Gaussian finite mixture modeling (GMM). Small mammals dominated the fauna (73.9%), followed by medium (24.8%) and large species (1.3%). A four-component Gaussian mixture model was retained as a biologically interpretable representation of body-mass structure. The estimated component means were approximately 22, 65, 298, and 1210 g. However, the observed body-mass ranges of species assigned to the components were not fully separated, particularly for the broad fourth component. These results suggest substantial structure within the lower portion of the body-mass distribution, while highlighting the limitations of interpreting probabilistic components as discrete body-size classes. We provide a standardized framework for body-size classification to enhance comparability across macroecological and conservation studies in the Neotropics.