Given future changes in climatic variables such as temperature and precipitation due to climate change conditions, it is important to know how traits such as intraspecific body size will respond to these changes. Body size variation in endotherm species is important in particular because of its impact on diverse aspects of ecological system, including resource use, interspecific competition, and ecosystem processes. The oldest and most well-supported pattern in body size variation due to a climatic variable is Bergmann's rule, or a negative intraspecific relationship between mass and temperature. As this pattern has not been examined in a data-intensive manner, we determined the relationship between annual temperature and mass in almost 1,000 global bird and mammal species. As a followup, we incorporated climatic variables in addition to temperature that have limited empirical evidence for an impact on intraspecific size variation. We looked at how temperature, precipitation, temperature seasonality, and precipitation seasonality affect intraspecific size variation of 150 bird and mammal species in North America, using higher resolution mass and climatic data. We additionally determined how these relationships were related to species niche characterizations, including niche centroids and breadths.
Results/Conclusions
We determined that Bergmann's rule is not as broadly common among bird and mammal species as previously determined. Of the 952 species in our dataset, 79% had relationships between intraspecific mass and annual temperature that were not significant, while 14% of the remaining species followed the predicted Bergmann's rule and 7% had the opposing positive temperature-mass relationship. Similarly, the relationships between intraspecific body size variation and the four climatic variables were weak. Though temperature was the climatic variable with the biggest effect size and, for over 80% of the species, at least one climatic variable had a significant impact on body size, all relationships were weak. Species with the coldest niche centroids tended to more commonly exhibit the negative temperature-mass relationship of Bergmann's rule, and species with drier niche centroids had stronger positive precipitation-mass relationships. In general, it appears that climatic factors including temperature are not dominant drivers of intraspecific body size variation. While additional abiotic factors should be investigated, the impacts of climate change on endothermic species size will not be strong and directional. It is likely that biotic factors such as community context have greater impact on the size of endothermic species.