Bergmann's rule proposes that as latitude increases so will body size. This phenomenon is well documented in endotherms, particularly in mammals. There is also documentation that explains how this applies to ectotherms as well, however the authors of this study propose that those articles supporting the application of Bergmann's rule to ectotherms are inconclusive and, in fact, reach false conclusions. .
The authors begin by addressing those articles written previously and explaining how they may have been flawed. The first of these articles, written in 1960 by Ray, uses laboratory procedures to explain how ectotherms raised in the laboratory at relatively lower temperatures mature later and at larger sizes than their conspecifics raised at higher temperatures. The study, however, was conducted in the lab and makes no association to species found in their natural environments across latitudes. .
The second paper written by Lindsey in 1966, compiled data on the average size of ectotherms from high and low latitudes. This however was not taxa specific; Bergmann's rule, however, applies to the same species across latitudes. This paper has, according to the paper's authors, often been cited as evidence for Bergmann's rule, but it is not, even according to Lindsey, appropriate to do so.
The authors study was very brief. They compiled previously collected data on common large-bodied freshwater fishes from the northern hemisphere, specifically North America. They used the overall body length as the measure for body size. The pair had a rigorous set of rules for selecting which data they would utilize in their study and they were able to analyze the data with acceptable levels of possible error. .
The pair found that among the species that they used in the study, there was no evidence for body size among ectoderms with an increase in latitude. In fact, most of the species showed a trend that opposed Bergmann's rule.