Diversity in Relation to Body Size in Mammals: A Comparative Study

Abstract

A wide variety of higher taxa show right-skewed frequency distributions of species' body sizes, and a negative relationship is often found between the number of species within a taxonomic group and its mean size. These patterns may arise from essentially random cladogenesis and extinction, from the tendency of organisms to anagenetically track a relatively small optimal size, or from high rates of cladogenesis at relatively small sizes. We used body mass data for about 65% of the world's mammal species to test the predictions of these alternative mechanisms. Using phylogenetically independent contrasts to control for phylogeny and clade age, we found that although size does not appear to be a general correlate of diversity, large radiations of species do tend to be small bodied. This pattern seems most consistent with the hypothesis that the number of niches is potentially greatest for small-bodied taxa.

Keywords: body size; clade selection; comparative method; diversity; mammals; species sorting.