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

Abstract

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). Ray (1960) proposed that ectothermic organisms also follow Bergmann’s rule, on the basis of primarily one line of argument. Specifically, ectotherms reared at relatively lower temperatures typically mature later at larger sizes when compared with conspecifics reared at higher temperatures (Atkinson 1994; Sibly and Atkinson 1994). Ray (1960) reasoned that the negative slope of the reaction norm between size at maturity and rearing temperature combined with generally lower temperatures at higher latitudes would produce a cline of increasing body size with increasing latitude. Ray (1960) summarized field data from previous studies to add generality to his claim of ectotherms following Bergmann’s rule. In regard to fishes, he states that “the rule is obeyed by a great number of fishes as shown by numerous reports in the literature” (Ray 1960, p. 93), but he provides no citations related to variation in body size across latitude. All his citations regarding fish apply to meristic counts