Geographical variations of the skull in the red fox Vulpes vulpes on the Japanese Islands: an exception to Bergmann's rule

In order to clarify the morphological differences between two subspecies of the red fox (Vulpes vulpes) on the Japanese Islands and test the validity of Bergmann's rule, we examined geographical variations in 25 cranial and 24 dental characters in V. v. schrencki from Hokkaido and V. v. japonica from the other main islands of Japan (Honshu, Shikoku, and Kyushu). Many skull measurements, including the male greatest length, condylobasal length, and the length of upper and lower tooth rows, were significantly larger for V. v. japonica than for V. v. schrencki, whereas most tooth measurements, especially the length of molars and premolars, in V. v. schrencki were larger than those in V. v. japonica. Although the two subspecies were morphologically well-differentiated from each other, the results did not support that they have evolved following Bergmann's rule of adaptation to cold climates. Based on consideration of the relatively large differences of their tooth sizes, which are not easily influenced by food abundance, and previous genetic research on the different migration histories of the two subspecies, the morphological differences detected in the present study may have resulted not only from the present ecological differences between the two subspecies, but also from the difference of migration history and evolutionary constraints.