Morphology and development of the baculum in harbour seals (Phoca vitulina)

The baculum is a morphologically highly diverse genital bone found in most male eutherian mammals. Its exact function is still debated, but its role during mating and/or copulation might be linked to, among others, support during intromission or sexual selection. Less is known about how the shape of the baculum develops throughout ontogeny. Differences between juveniles and adults could reveal morphological changes related to sexual maturity and, in turn, provide evidence for a functional role in sexual reproduction. In this paper, we investigated the postnatal development of baculum shape in the harbour seal ( Phoca vitulina ) by using μCT scans of 298 specimens, ranging from juveniles to 28 years of age. We applied a 3D geometric morphometrics approach to analyse how the shape and centroid size of the baculum change throughout ontogeny. We found that in the first 3 years of life, barely any changes happen to size, but before reaching sexual maturity, both shape and size change immensely within a short period of time. After becoming fully grown around the age of seven, we did not find any further major changes in these attributes. The postnatal development of the baculum appears to be linked to changes in hormone levels, which would explain the rapid changes during puberty. The largest shape variation is characterized by an increase in bone curvature from juveniles to adults, while the baculum appears to be most robust (in terms of relative thickness) during puberty, which is difficult to explain. Still, the relatively small shape variation after reaching sexual maturity indicates that the shape of the baculum is functionally constrained in the context of sexual reproduction.