The baculum (also penis bone, penile bone, or os penis, or os priapi) is a bone found in the penis of many placental mammals. It is absent in the human penis, but present in the penises of other primates, such as the gorilla and chimpanzee. The bone is located above the male urethra, and it aids sexual reproduction by maintaining sufficient stiffness during sexual penetration. The homologue to the baculum in female mammals is known as the baubellum or os clitoridis – a bone in the clitoris.
The penile bone is always present in the male dog, formed by a paired ossification center of the corpora cavernosa at 35 days of age. In large dogs, it is approximately 10 cm long, 1,3 cm wide and 1 cm thick. It is divided into three parts: