Testing for Functional Adaptation in a Genital Bone: Baculum Size, Shape, and Microstructure Show Little Plastic Response to Mating Across Five Rodent Species.

Plasticity-the ability of a genotype to produce different phenotypes in response to environmental input-can provide insight into the mechanical forces experienced by biological structures. Bones are particularly plastic, remodeling in response to sustained and repetitive mechanical stress. In many mammals, males possess a bone in their penis called a baculum. Across species, its extreme morphological divergence suggests it undergoes recurrent adaptive evolution, and several studies have explicitly linked baculum size and shape to reproductive fitness. However, the precise functions of this bone remain largely unknown. Here we tested whether copulation imposes mechanical loading sufficient to induce remodeling in the baculum. We compared bacula of mated vs. unmated males across five rodent species with divergent baculum morphologies and mating ecologies, using high-resolution morphometric analyses of size and shape, and synchrotron-enabled quantification of bone microstructure. We found no consistent effect of mating on size, shape, or microstructure of the baculum. These results suggest that copulation does not impose mechanical loads sufficient to induce remodeling in the baculum. More broadly, our findings indicate that the rapid evolutionary divergence of the baculum is unlikely to be enabled by plasticity associated with copulation.