Skull-shape variation and modularity in two Japanese field mice, Apodemus speciosus and Apodemus argenteus (Rodentia: Muridae)

Abstract

Two congeneric species of Japanese field mice, Apodemus speciosus and Apodemus argenteus, differ considerably in their feeding strategies and habitat utilizations. Although both species are omnivorous, previous studies demonstrated that A. argenteus typically eats a wide variety of food items than A. speciosus. Moreover, A. speciosus is a ground dweller, while A. argenteus commonly shows arboreal movements. These dietary and lifestyle differences suggest potential disparity in morphological structure. In this study, we used geometric morphometrics to examine the skull shape differences are related to their feeding strategies and habitat preferences. We found that A. argenteus showed wider zygomatic arches and a broader mandibular ramus. These traits are linked to enlargement of the attachment sites for the masticatory muscles. We also found a relatively short mandibular diastema in A. argenteus, which may increase the mechanical advantage of the lower jaw by reducing the out-lever arm. These results suggest that A. argenteus has a greater relative bite force that is suitable for diversified diets as a more generalist feeder. Interspecific differences in the upper incisor, inter-orbital width, and auditory bullae might be associated with the lifestyles of these two species. Additionally, the presence of two distinct modules in the cranium (rostrum and braincase) and mandible (ascending ramus and alveolar region) may reflect similar patterns of intraspecific variation within both species. In conclusion, skull shape enables distinction between these two species and may favor adaptation to their ecology and lifestyle, despite the similarities in craniomandibular modules associated with common developmental factors.