Association between climate and body size in rodents: A phylogenetic test of Bergmann's rule

The pattern of increased body size in colder climates, or Bergmann's rule, is common in endotherms. However, empirical support has been mixed among clades, and it varies among taxonomic levels. Taking advantage of greatly improved phylogenetic resolution among rodents, we tested Bergmann's rule using 19 bioclimatic variables, and body mass data, for 1315 species on a recent supermatrix phylogeny. We did not find the predicted negative relationship between body mass and temperature. Instead, phylogenetic generalized least squares (PGLS) analysis indicated that precipitation variables (especially variables associated with primary productivity) had a positive correlation with body mass, suggesting that rodent species tend to be larger in more productive regions. Multivariate regression between body mass and overall climate (based on 19 bioclimatic variables) found a significant relationship, that was robust to phylogenetic correction. Bergmann's rule was not detected in smaller and surface-dwelling rodents (despite their greater exposure to external climate), any more than in larger and subterranean rodents. We suggest that food availability, and not heat conservation, is the more important mechanism driving body size variation across rodent species at the order level.