Evaluating Bergmann's and Allen's rules in the long bones of the eastern gray squirrel

Ecogeographical rules predict variation in body size and appendage proportions across climatic gradients, yet intraspecific tests using long bones from wild populations remain limited among small mammals. This study tests the hypothesis that eastern gray squirrels (Sciurus carolinensis) conform to Bergmann's rule, that is, body size increases with latitude, and to Allen's rule, whereby limb bone lengths relative to body size decrease with latitude. A total of 136 skeletally mature individuals were studied, spanning approximately 27°N-46°N latitude. Maximum lengths of the humerus, femur, and tibia were measured, and femoral head diameter was used as a proxy for body size whenever recorded body mass was unavailable. Linear regression analyses were used to evaluate relationships among body size, limb bone length, and latitude. First, we found that femoral head diameter was strongly correlated with recorded body mass (R² = 0.567, p< 0.0001), supporting its use as a body size proxy. Second, body size increased significantly with latitude (R² = 0.512, p< 0.0001), consistent with Bergmann's rule. Limb lengths standardized by body size were generally correlated with latitude with negative regression slopes (R² = 0.13-0.43), largely supporting the prediction of Allen's rule that appendage lengths decrease at higher latitudes, although these relationships were weaker than those observed for body size. These results suggest that climatic gradients are reflected in body size and, to a lesser extent, limb bone lengths within a single, widely distributed rodent species. The results highlight the influence of latitude and climate on skeletal morphology.