Bergmann's rule encompasses mechanism: a reply to Olalla-Tarraga (2011)

The many definitions of Bergmann's rule have resulted in confusion and debate over how and in what organisms to test the original rule. Watt et al. published a paper in 2010, based directly on Bergmann's original paper, in the hopes of clarifying the rule and presenting direct translations to resolve uncertainties. Recently, Olalla-Tarraga has criticized our publication, stating that we assumed the rule was a causal law, which has narrowed our epistemological scope of the rule. We argue we did not assume the rule was a law and suggest that Olalla-Tarraga has only focused on the observed pattern and has ignored the proposed mechanism, which is inherent in the definition. We also discuss the proposed mechanism and describe why it cannot apply to ectotherms. Despite this, we encourage a thorough investigation of the mechanisms responsible for maintaining Bergmann's pattern in ectotherms and support Olalla-Tarraga's quest for a unifying mechanism to explain body size gradients in endotherms and ectotherms.