The size of an organism is perhaps its most overt physical characteristic, and variation in body size has long been of interest to biologists. Bergmann's rule has been actively studied and debated for more than 150 years. Despite this long history, the generality and applicability of Bergmann's rule to ectothermic organisms generally, and to plethodontid salamanders specifically, has resulted in an extensive and conflicting literature. Regardless of mechanism, clinal variation in body size has been widely observed in plethodontid salamanders and other ectothermic vertebrates. In this study, we assessed the change in adult body size of four plethodontid salamanders (Desmognathus imitator, D. ocoee, D. wrighti, and Plethodon jordani) across a 1,350 m elevational gradient in Great Smoky Mountains National Park. Using 1,293 observations of salamanders at 25 sites, we found clear and significant patterns of increasing adult body size with elevation in all four species. Average rates of increase ranged from 1.09% to 3.98% per 100 m elevation gain. We found that elevation significantly covaried with maximum and mean temperature, as well as average annual precipitation. Our study reinforces previous research describing increases in plethodontid salamander body size with elevation, but also extends these findings to fully terrestrial, direct-developing species. However, the mechanisms underlying the observed pattern are still unclear and highlight an important area for future research. As a critical life history characteristic, an understanding of geographic variation in body size is important for assessing current population dynamics, as well as the potential effects of future climate changes.

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