Structural Organization of the Penis in the South American Plains Vizcacha (Lagostomus maximus), a Caviomorph Rodent.

The male genital morphology of the plains vizcacha (Lagostomus maximus) remains poorly characterized despite the species' distinctive reproductive biology and its phylogenetic position within Caviomorpha. In this study, we describe the gross anatomy, histology, and ultrastructure of the penis of L. maximus using light microscopy and scanning electron microscopy. The genitalia of the vizcacha shares several features with other caviomorph rodents, including the absence of a pendulous scrotum, subcutaneous inguinal testes, and a retracted inverted S-shaped penile configuration. Notably, we identify a previously undescribed narrow stylet-like distal process supported by hyaline cartilage and bearing caudally oriented surface spine-like projections, representing a novel morphological specialization among caviomorph rodents. These distal specializations may influence copulatory mechanics by enhancing mechanical stimulation of the female reproductive tract. When considered alongside recent evidence suggesting a mixed ovulatory strategy in L. maximus, including a seminal plasma-induced ovulatory process, these findings raise the possibility that penile morphology contributes synergistically to ovulation induction. Our results expand current knowledge of genital diversity in caviomorph rodents and provide a morphological framework for future functional and evolutionary investigations.