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. 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 word baculum meant "stick" or "staff" in Latin (c.f. Argumentum ad baculum) from the original (Greek: βάκλον, Baklon) "stick" Aesop. 1.baculum (-I, n.), also baculus (-I, m), (i) walking-stick; (ii) flail (cf. Fr, and Engl ‘baton’) Isid. Etym. 20.13. i. ‘The walking-stick is said to have been an invention of Bacchus (c.f. Dionysus) the discoverer of the vine, for men to lean on when under the influence of wine’ commonly used for rolling dough, threshing wheat, as in Greek Baklava, French baguette and other common household uses.
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, which can be a distinct advantage in some mating strategies.
Presence in mammals
Mammals having a penile bone (in males) and a clitoral bone (in females) include various eutherians:
It is absent in humans, ungulates (hoofed mammals), elephants, monotremes (platypus, echidna), marsupials, lagomorphs, hyenas, sirenians, and cetaceans (whales, dolphins, and porpoises), among others.
Evidence suggests that the baculum was independently evolved 9 times and lost in 10 separate lineages. The baculum is an exclusive characteristic of placentals and closely related eutherians, being absent in other mammal clades, and it has been speculated that it is derived from the epipubic bones more widely spread across mammals, but notoriously absent in placentals.
Among the primates the marmoset, weighing around 500 g, has a baculum measuring around 2 mm, while the tiny 63 g galago has one around 13 mm 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.
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, is less than 1.2 g in mass, and measures less than 90 mm long, then the baculum belongs to a juvenile male.
Absence in humans
Unlike other primates, humans lack an os penis or os clitoris; however, this bone is present but 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, 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. 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." In humans, the rigidity of the erection is provided entirely through blood pressure in the corpora cavernosa.
It has been speculated that the loss of the bone in humans, when it is present in our nearest related species the chimpanzee, is because humans "evolved a mating system in which the male tended to accompany a particular female all the time to try to ensure paternity of her children" which allows for frequent matings of short duration. Observation suggests that primates with a baculum only infrequently encounter females, but engage in longer periods of copulation that the baculum makes possible, thereby maximizing their chances of fathering the female's offspring. Human females exhibit concealed ovulation also known as 'hidden estrus', meaning it is almost impossible to tell when the female is fertile, so frequent matings would be necessary to ensure paternity.
The evolutionary biologist Richard Dawkins speculated in 1989 that the loss of the bone in humans, when it is present in our nearest related species the chimpanzee, is a result of sexual selection by females looking for honest signals of good health in prospective mates. The reliance of the human penis solely on hydraulic means to achieve a rigid state makes it particularly vulnerable to blood pressure variation. Poor erectile function portrays not only physical states such as age, diabetes, and neurological disorders, but also mental states such as stress and depression.
A third view is that its loss in humans was a side-effect of neoteny during human evolution; it is noted that late-stage fetal chimpanzees lack a baculum.
The existence of the baculum is unlikely to have escaped the notice of pastoralist and hunter-gatherer cultures.
It has been argued that the "rib" in the story of Adam and Eve is actually a mistranslation of a Biblical Hebrew euphemism for baculum, and that its removal from Adam in the Book of Genesis is a creation story to explain this absence (as well as the presence of the perineal raphe– as a resultant "scar") in humans.
In hoodoo, the folk magic of the American South, the raccoon baculum is sometimes worn as an amulet for love or luck.
Oosik is a term used in Native Alaska cultures to describe the bacula of walruses, seals, sea lions, and polar bears. Sometimes as long as 60 cm (2 ft), fossilized bacula are often polished and used as a handle for knives and other tools. The oosik is a polished and sometimes carved baculum of these large northern carnivores.
Oosiks are also sold as tourist souvenirs. 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.