Geographic Variation in Skull Morphology of the Large Japanese Field Mice, Apodemus speciosus (Rodentia: Muridae) Revealed by Geometric Morphometric Analysis.

We analyzed geographic variation in skull morphology of the large Japanese field mouse (Apodemus speciosus) and determined changes in skull morphology that occurred during the evolutionary history of A. speciosus in relation to the estimated distribution range in the last glacial maximum (LGM). We analyzed 1,416 specimens from 78 localities using geometric morphometric techniques applied to the dorsal side of the cranium and mandible. While large variations within and among the populations in Honshu, Shikoku, and Kyushu were observed, geographic patterns were not observed. Hokkaido and peripheral island populations showed shared differentiation from the Honshu, Shikoku, and Kyushu populations with a larger skull and distinct mandible shape. In addition, these two groups also differed from each other in accumulated random shape variation. Common characteristics found in Hokkaido and peripheral island populations were considered to be the ancestral states, which were retained by geographic isolation from the main islands. Random variations in Hokkaido and the peripheral island populations were formed through stochastic processes in relation to their isolation. Characteristic morphologies widely found in the populations of Honshu, Shikoku, and Kyushu were considered to be derived states that expanded after separation from the peripheral islands. Complex geomorphology and a shift in distribution range related to climate change and altitudinal distribution are suggested to have formed the complex geographic variation in this species.