The penile baculum in monkey (Macaca radiata), rat (Rattus norvegicus), and bat (Pipistrellus mimus) and its functional significance—a mini review

Abstract Background Evolution of the penis in sexual method of reproduction had showed complex developmental processes, leading to extreme species variation. The presence and absence of the os penis vary in taxa within mammals indicate adaptive value of the mammalian penis bone, and understanding the diversity is challenging. Surprisingly, little is known about its details. Methods Comparative analysis of the penis with special reference to the os penis (penile baculum) was analyzed in monkey (Macaca radiata), rat (Rattus norvegicus), and bat (Pipistrellus mimus). Results In monkey, the baculum appears like a miniature of a long bone, a clear central medullary cavity, and surrounded by osteons. Similarly, the baculum in rat showed a medullary canal and surrounded by smaller channels of osteons. In bat, it is seated over the corpus spongiosum that shows a small diameter channel which forms the main lamellar bone and appears to be a single osteon or Haversian system. Though microanatomical examination revealed that the presence of vascular and non-vascular structures contribute for penile erection, the presence of the os penis would be helpful for the penis erection and stiffness and vaginal friction. Conclusions Anatomical relationship to other penile elements and its specific functional significance and the explanation for this general adaptive diversity across species is challenging.