The baculum (also penis bone, penile bone or os penis) is a bone found in the penis of most mammals. It is absent in humans. It is used for copulation and varies in size and shape by species. Its characteristics are sometimes used to differentiate between similar species.
Although many mammals have a penile bone, humans do not, instead providing the rigidity of the erection entirely through blood pressure in the corpus cavernosum.
Examples of species with a penile bone include the rat, dogs and the walrus.