Micro-CT-Based 3D Morphometrics and Internal Microstructure of the Baculum in Wild Vietnamese Primates

In primates, the baculum exhibits substantial morphological variability related to developmental and reproductive functions, yet data on its occurrence, form, and internal structure are scarce for many taxa. We investigated baculum morphology and microstructure in three genera of wild Vietnamese primates (i.e., Pygathrix, Trachypithecus, and Nomascus) using high-resolution micro-computed tomography and 3D morphometric modelling. We studied 33 wild males, including 21 of known age. We measured the morphological parameters length, volume, alpha-shape complexity, and the Vacuity index (Vi) from 3D reconstructions to describe the external form and internal canal distribution. For species with sufficient specimens of known age, we fitted candidate models to explore age-related variation in baculum size and microstructure. Length and volume generally increased with age and tended to stabilize in older individuals, consistent with sigmoidal growth patterns, whereas alpha-shape complexity showed only weak age-related variation. The Vacuity index provided indirect information on internal porosity and Haversian canal distribution; however, histological validation is needed before these microstructural patterns can be interpreted as direct evidence of a specific ossification pathway. Overall, this study expands the anatomical record of the baculum in primates and supports the value of integrating micro-CT based 3D morphometrics with internal microstructural descriptors to investigate baculum development. Estimating chronological age from these variables remains exploratory and requires validation through larger samples and histological data.