Bergmann’s Rule in Ectotherms: A Test Using Freshwater Fishes

Understanding patterns of variation in body size within and among species is a central question in evolutionary ecology (Schlichting and Pigliucci 1998). The most well known pattern of variation in body size is Bergmann's rule (Bergmann 1847; Mayr 1956). The intraspecific version of Bergmann's rule holds that within endothermic species, body size increases with increasing latitude (or decreasing temperature; Blackburn et al. 1999; Ashton et al. 2000). In general, mammals conform to this rule (Ashton et al. 2000).