The inverse relationship between species diversity and body mass: do primates play by the “rules”?

Evolutionary biologists have long commented on a seemingly universal “rule” of nature—that in large taxonomic assemblages from groups as diverse as bacteria, plants, insects, marine invertebrates, fish, reptiles, amphibians, birds, and mammals, there exists a frequency distribution of body sizes among species that is highly skewed to the right (positive skewness). This distribution reflects the strong inverse, or negative, relationship often noted between mean body size of taxa and the number of species they contain—i.e., the observation that small body size is often associated with high species diversity (speciosity). This is sometimes “explained” by recourse to the idea that smaller-bodied taxa are able to subdivide their environments more finely than larger-bodied taxa. With but few exceptions, the applicability of this “rule” to the Order Primates has not been studied in any detail. In this study I address the following questions of (paleo)anthropological interest: (1) How speciose is the Order Primates? (2) Does this biological “rule” characterize the Order Primates (at any taxonomic level) in any meaningful way? (3) Does the association between speciosity and body mass within the Order Primates provide any useful models for interpreting and/or predicting speciosity in the fossil primate record? Using phylogenetically independent contrasts methods, I conclude that the answers to those three questions are: (1) not very; (2) no; and (3) not particularly (with the possible exception of larger-bodied taxa).