Morphological Variation of the House Mouse Mus musculus and the Ryukyu Mouse M. caroli (Mammalia: Muridae) from Japan

Morphological variation in the house mouse Mus musculus was examined based on five external body measurements and 25 skull measurements of 166 specimens from Honshu, Kyushu, Kikaijima Island, Okinoerabujima Island, Okinawajima Island, Miyakojima Island (my1 and my2), and Yonagunijima Island (yn) in Japan, as well as 99 specimens of M. caroli from Okinawajima Island (ca). In skull measurements, yn and my2 specimens were overlapped and distinct from the other localities in principal component analysis (PCA). Specimens of yn and my2 had shorter mandible heights. Although yn and my2 specimens overlapped with ca specimens in PCA using mandibular measurements, yn specimens had an elongated angular process reaching the posterior tip of the coronoid process, different from ca specimens with a more robust and long coronoid process. In this study, distinct morphological features were found on Yonagunijima Island. Specimens of my1 and my2 from Miyakojima Island differed from each other; my2 specimens were more similar to Yonagunijima Island specimens. The mandibular morphometric similarity between M. musculus from Yonagunijima Island and M. caroli from Okinawajima Island is presumably related to adaptations in producing similar mechanical advantages and equivalent masticatory forces.