Penile morphology of the house mouse (Mus musculus) and harvest mouse (Micromys minutus): focus on the position and function of stalk and lateral bacular mounds

We examined the penile morphology of two murid species: Mus musculus Linnaeus, 1758 and Micromys minutus (Pallas, 1771) and discussed how the position of the baculum affects the diversity of penile morphology. The ratio of stalk length to total penis length was not significantly different between Mu. musculus and Mi. minutus; however, distal end of the stalk was more distal in Mu. musculus than in Mi. minutus. Lateral bacular mound of Mu. musculus is short but has a function in cross-sectional volume expansion of glans near the distal end of the internal prepuce with outward turning of lateral bacular mounds induced by blood flowing into the well-developed cavernous space. In contrast, lateral bacular mounds in Mi. minutus were located more proximally. The cavernous space of lateral bacular mounds was not developed. Considering that the pivot point of the anterior tip of lateral bacular mound and distal tip of the stalk is located proximally, and lateral bacular mounds are shorter and do not reach distal tip of the internal prepuce, lateral bacular mounds could not contribute to the increased cross-sectional areas of the distal portion of penis. Overall, we found that development of the cavernous space enabling the cross-sectional volume increase of penis via blood flow, possibly enhances success of fertilization; further, the position of the stalk tip functions as the pivot point for moving lateral bacular mounds, and affects the position of the increase in penile volume. These two factors are thought to have been changed during the evolutionary process.