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, equids, marsupials, lagomorphs, and hyenas, amongst others. It is used for copulation and varies in size and shape by species. Its characteristics are sometimes used to differentiate between similar species.
The oosik of Native Alaskan cultures is a polished and sometimes carved baculum of various large northern carnivores such as walruses. The raccoon baculum is sometimes worn as a luck or fertility charm.
The word baculum originally meant "stick" or "staff" in Latin. The homologue to the baculum in female mammals is known as the baubellum or os clitoridis. Clellan S. Ford and Frank A Beach, Patterns of Sexual Behavior, p. 30 says "Both gorillas and chimpanzees possess a penile bone. In the latter species the os penis is located in the lower part of the organ and measures approximately three-quarters of an inch in length."
In humans, which lack the baculum and baubellum, the rigidity of the erection is provided entirely through blood pressure in the corpus cavernosum. Humans occasionally are born with the penile bone. It is generally surgically removed.
2 to 3 inches long.