In rodents, an adaptation for biting off hard materials, accompanied by extension of the masseter origin anterior to the orbit, might interfere with vision in the anteroventral direction. In arboreal rodents, the anatomy must be modified somewhat to allow for the clear anteroventral vision necessary to safely descend from, or move about on trees. In the present study, we tested this hypothesis by comparing the craniometric data between two Apodemus murids, semiarboreal A. argenteus and terrestrial A. speciosus. Based on the correlation between the index of relative encephalization (IRE; cube root of the brain case volume divided by the basicranial length) and the angle of the foramen magnum, the head in A. argenteus is situated so that the snout is more depressed than in A. speciosus, which has the same IRE value. The head posture of A. argenteus does not seem to be due to the relative brain size, but rather to a semiarboreal adaptation that ensures clear anteroventral vision by lowering the anterior portion of the masseter to a greater degree than the eyeball.
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