Is Bergmann's Rule Valid for Mammals?

Bergmann's rule states that, within species of mammals, individuals tend to be larger in cooler environments. However, the validity of the rule has been debated. We examined the relationship between size and latitude as well as size and temperature within various species of mammals. We also tested the idea that smaller mammals follow Bergmann's rule more strongly than larger mammals, as expected if heat conservation is the cause of the rule. When all studies were included, the percentage of species showing a positive correlation between size and latitude was significantly >50% (78 of 110 species). Similarly, the percentage of species showing a negative correlation between size and temperature was significantly >50% (48 of 64). Analyses using only significant studies or only studies that sampled extensively also support Bergmann's rule. The size-latitude and size-temperature trends were consistent within all orders and most families for which data are available. We did not find support for the hypothesis that smaller mammals conform more strongly to Bergmann's rule than larger mammals. Thus, we found broad support for Bergmann's rule as a general trend for mammals; however, our analyses do not support heat conservation as the explanation.