Paper Reviewed
Angielczyk, K.D., Burroughs, R.W. and Feldman, C.R. 2015. Do turtles follow the Rules? Latitudinal gradients in species richness, body size, and geographic range area of the world's turtles. Journal of Experimental Zoology (Molecular and Developmental Evolution) 324B: 270-294.

It has long been believed, as Angielczyk et al. (2015) write, that species richness decreases with increasing latitude, that the body size of endotherms (warm-blooded animals) increases with increasing latitude (Bergman's rule) and that geographic range size increases with increasing latitude (Rapoport's rule). And if such is truly the case, global warming could dramatically alter what will be found where in a future potentially-warmed world, as well as how big those "whats" might grow to be. However, these three "rules" have long been debated; and so it was that the three researchers poured over the pertinent scientific literature to see how they may or may not apply to earth's many turtle species.

In the case of the first rule, the U.S. scientists found that just the opposite appears to be the case, at least as it applies to turtles, in that their wading through the literature on the subject revealed that "turtle diversity actually peaks at 25° north." In the case of the second rule, they say that "turtles also fail to follow Bergmann's Rule, and may show the converse (larger at lower latitudes), though trends are weak." And in the case of the third rule, they state that "turtles also show a complex relationship between latitude and range size that does not directly follow Rapport's rule."

In considering the totality of these findings, therefore, Angielzyk et al. conclude that "turtles do not strictly follow some classic biogeographical rules," which fact leads them to call for "further in-depth research to investigate potential causal mechanisms for these atypical patterns."