Tracing the evolutionary history of the morpho-anatomy of baculum in primates.

Abstract

Animal morphology reflects both evolutionary history and present-day adaptation. Male mammal copulatory structures such as the baculum (penile bone) are ideal for studying these processes because of their complexity and high interspecific variability. In primates, however, research has focused mostly on baculum length. Here we investigate the evolution of primate baculum anatomy and morphology using the largest dataset assembled to date. We combined high-resolution 3D micro-CT reconstructions with advanced non-landmark methods to quantify key traits, including baculum position, discrete shape categories, and a continuous descriptor of overall baculum complexity derived from alpha-shapes analysis. Using stochastic character mapping on a primate phylogeny, we inferred ancestral states and evolutionary transitions for baculum position, shape type, and complexity. Reconstructions indicate that a proximally positioned baculum extending beyond the penile mid-shaft represents the ancestral condition, retained in Strepsirrhini and shifted distally in Haplorrhini. A stick-shaped baculum is inferred as ancestral, with subsequent transitions to Y-shaped and pear-shaped morphologies. Overall, this study provides the first phylogenetically explicit reconstruction of primate baculum evolution, revealing repeated shifts in anatomy and morphology complexity. These results offer a framework for testing functional and selective hypotheses on copulatory structures and highlight the value of 3D morphometrics for understanding morphological diversity across species.