The Presence of and Morphological Variation in the Baculum in the Northern Pygmy Loris (Xanthonycticebus intermedius)

The genital bones of primates, such as the baculum, display remarkable morphological diversity and offer valuable insights into reproductive biology, systematics, and evolutionary history. However, genital skeletal anatomy remains understudied in many primate taxa, particularly within Strepsirrhini. Pygmy lorises (Xanthonycticebus spp.) are nocturnal primates native to Southeast Asia, and their skeletal anatomy is poorly understood, particularly in relation to their genital bones. We examined the morphology, structure, and development of the genitalis in the northern pygmy loris (X. intermedius). We examined the genitalia of four male pygmy lorises from the Endangered Primate Rescue Centre, Vietnam. We identified three specimens as X. intermedius using DNA barcoding. DNA in the fourth sample was degraded, preventing species identification, but observations of the individual suggested that it was also X. intermedius. We used palpation to detect the baculum, radiography to confirm the presence of the baculum and visualize its anatomical position within the penis, alpha-shape analysis to assess morphological variation in the external surface of the baculum, and micro-CT to evaluate internal vascularization patterns. The baculum was present in all samples, with lengths ranging from 10.44 to 14.78 mm and various shapes. Alpha-shape analysis confirmed intraspecific variation in shape, with one sample showing a distinct morphology of the external surface. We found a significant negative correlation between the Vacuity Index (Vi) and both baculum length and volume, suggesting an ontogenetic relationship. Apparent progressive ossification, from uniform to localized vascularization, mirrors patterns in long bones. The anatomical position of the baculum in all samples supports a positional difference between Strepsirrhini and Haplorrhini, with the baculum occupying the full length of the penis in Strepsirrhini. Our results further suggest that baculum morphology, length, volume, and Vi could serve as indirect proxies for age in detached, museum, or unchipped samples. This study contributes to our understanding of lorisid skeletal biology and lays the foundation for future research on primate genital morphology.