Genital bone in male harbour seals

Most but not all male eutherian mammals have an os penis or baculum. It has evolved and was lost multiple times independently. The genital bone displays a great diversity in morphology which has been used for taxonomic purposes.

Here, we investigated the ontogenetic changes in morphology of the baculum in Phoca vitulina using 3D geometric morphometrics. A total of 302 bones from the Zoological Institute at Kiel University (ZIK, curated by Böhmer Research Group, Zoology and Functional Morphology of Vertebrates, Zoological Institute, Christian-Albrechts-Universität zu Kiel, Germany) were digitized and their shape was quantified to reveal the postnatal development.

Our results reveal that the baculum tends to stop growing in the seventh year of age. This correlates with the age of sexual maturity. Bacula of some older individuals showed an continuous increase in bone thickness, but this does not appear to be an overall trend.

References:

Berghäuser T, Wölfer J, Böhmer C and Nyakatura JA (in press, 2025) Morphology and development of the baculum in harbour seals (Phoca vitulina) J Zoology