Bergmann's rule is the commonly accepted ecogeographic rule that, within a species, individuals will be smaller in warmer environments. While there are many studies documenting this pattern in single species and several recent reviews of these studies, a data-intensive approach has not yet been used to determine the prevalence of this pattern for many species. To determine if Bergmann's rule occurs broadly across endotherm species, we assessed the strength and direction of the relationship between temperature and individual mass for almost 1,000 endotherm species. This broad-scale data was obtained from Vertnet, a large compilation of global museum records. Each species had data for an average of 300 individuals spanning a broad spatiotemporal range.
Results/Conclusions
The majority of bird and mammal species did not have a strong relationship between temperature and individual mass. Most species had a correlation coefficient near zero, and less than a quarter of species had a statistically significant relationship. Of those species with a significant relationship, a third of them had an unexpected positive relationship. These weak relationships held regardless of taxonomic grouping, temporal lag of temperature, sample size/variation, or species characteristics. These results suggest that negative temperature-mass relationships are not widespread among species, as previously believed. Temperature is not a strong driver of mass variation and other factors that can affect mass, such as multiple abiotic factors or community context, need to be investigated. While the supposed prevalence of Bergmann's rule has led to suggestions that global increases in temperature will result in broad-scale body size decreases, our results imply that this is unlikely to occur.