Abstract
Bergmann's rule is a widely accepted biogeographic rule that individuals within a species are smaller in warmer environments. While there are many single-species studies and integrative reviews documenting this pattern, a data-intensive approach has not been used to determine the generality of this pattern. We assessed the strength and direction of the relationship between temperature and individual mass for almost 1,000 bird and mammal species. The majority of species did not have a strong relationship between temperature and mass. Most species had nonsignificant correlations with coefficients near zero. These results suggest that Bergmann's rule is not general and temperature is not a dominant driver of biogeographic variation in mass. Further understanding size variation will require integrating multiple processes that influence size. The lack of dominant temperature forcing weakens the justification for the hypothesis that global warming could result in widespread decreases in body size.