Takashi O. Yato and Masaharu Motokawa (2021) Comparative Morphology of the Male Genitalia of Japanese Muroidea Species. Mammal Study 46 (4): 269-278.

　This study examined the diversity of the morphological structures of the male genitalia, especially the distal parts of the penis, of 11 rodent species in Muridae and Cricetidae. The morphological diversity of the male reproductive organ is thought to reflect adaptation to different reproductive strategies or sexual selection. Therefore, a morphological study of male genitalia improves our understanding of the mammalian reproductive system. While previous studies have rarely examined morphology other than baculum structures, this study demonstrated that the Muridae and Cricetidae rodents examined had a medial bacular mound and two lateral bacular mounds in the distal penis, forming a trident structure, and clarified how the degree of development of the trident structure differed significantly among rodent species. The authors also showed histologically, for the first time, that the movement of the lateral bacular mounds to the outside during copulation contributed to enlargement of the distal part of the penis. This was a truly unique study of the morphological diversity and functional significance of male genitalia, and should contribute to scientific developments in mammalogy, taxonomy, and morphology by applying the study findings to a broad range of taxa, not just rodents. The high quality of the authors’ illustrations should also be appreciated, ensuring their value as a longstanding future reference.