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"Os penis" redirects here. It is not to be confused with Os Pinos.
Fossil baculum of a bear from the Miocene.

The baculum (also penis bone, penile bone or os penis) 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.[1] The bone is located above the male urethra,[2] and it aids sexual intercourse by maintaining sufficient stiffness during sexual penetration. The female equivalent is the os clitoridis – a bone in the clitoris.[3][4]


The phallic meaning of the term is unique to English; however, similar words exist in other languages with the same meaning, such as the Slovenian word bakulum. It is derived from a Latin term which meant cudgel, scepter, staff or stick. The phrase argumentum ad baculum exemplifies the Latin meaning.


The baculum is used for copulation and varies in size and shape by species. Its characteristics are sometimes used to differentiate between similar species. A bone in the penis allows a male to mate for a long time with a female,[5] which can be a distinct advantage[clarification needed] in some mating strategies.[6]

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 or (though poor Latin) os clitoris.[7]

Presence in mammals[edit]

A raccoon baculum.


It is absent in humans, ungulates, elephants, monotremes, marsupials, lagomorphs,[citation needed] hyenas,[14] and cetaceans (whales, dolphins, and porpoises), among others.[citation needed]

Such a wide distribution among placental mammals suggests that the bone evolved early in the history of these mammals, and was subsequently lost in certain groups.

Among the primates the marmoset, weighing around 500g, has a baculum measuring around 2mm, while the tiny 63g galago has one around 13mm long. The great apes, despite their size, tend to have very small penis bones, and humans are the only ones to have lost them altogether.[6]

In some mammalian species, such as the raccoon (Procyon lotor), the baculum can be used to determine relative age. If the baculum tip is made up of uncalcified cartilage, has a porous base, masses less than 1.2g, and measures less than 90mm long, then the baculum belongs to a juvenile male.[12]

Absence in humans[edit]

Unlike other primates, humans lack an os penis or os clitoris; however, this bone is much reduced among the great apes: in many ape species it is a relatively insignificant 10–20 mm structure. There are reported cases of human penis ossification following trauma,[15] and one reported case of a congenital os penis surgically removed from a 5 year old boy, who also had other developmental abnormalities, including a cleft scrotum.[16] Clellan S. Ford and Frank A. Beach in Patterns of Sexual Behavior (1953), p. 30 say "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."[1] In humans, the rigidity of the erection is provided entirely through blood pressure in the corpora cavernosa.



Cultural significance[edit]

The existence of the baculum is unlikely to escape the notice of pastoralist and hunter-gatherer cultures (see also below).

The raccoon baculum is sometimes worn as a charm for luck or fertility.[citation needed]



Oosiks are also frequently sold as souvenirs to tourists by Alaska Natives. In 2007 a 4.5-foot (1.4 m) long fossilized penis bone from an extinct species of walrus, believed by the seller to be the largest in existence, was sold for $8,000.[19]

Walrus baculum, approximately 22 inches (56 centimetres) long.

See also[edit]



  7. ^ Best; Granai (2 December 1994). "Tamius merriami" (PDF). Mammalian Species (American Society of Mammalogists) 476: 1–9. doi:10.2307/3504203. 
  10. ^ Dyck, Markus G.; Bourgeois, Jackie M.; Miller, Edward H. (2004). "Growth and variation in the bacula of polar bears (Ursus maritimus) in the Canadian Arctic". Journal of Zoology 264 (1): 105–110. doi:10.1017/S0952836904005606. 
  15. ^ Sarma, Deba; Thomas Weilbaecher (1990). "Human os penis". Urology 35 (4): 349–350. doi:10.1016/0090-4295(90)80163-H. PMID 2108520. 
  16. ^ Champion, RH; J Wegrzyn (1964). "Congenital os penis". Journal of Urology 91: 663–4. PMID 14172255. 

General references[edit]


External links[edit]