The phallic meaning of the term is unique to English. 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, which can be a distinct advantage[clarification needed] in some mating strategies.
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.
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.
In some mammalian species, such as the raccoon (Procyon lotor), the bacula can be used to determine relative age. If the bacula tip is made up of uncalcified cartilage, has a porous base, masses less than 1.2g, and measures less than 90mm long, then the bacula belongs to a juvenile male.
Absence in humans
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, 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.
Oosik is a term used in Native Alaska cultures to describe the baculum 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 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.
Walrus baculum, approximately 22 inches (56 centimetres) long.
^Baryshnikov, Gennady F., Olaf RP Bininda-Emonds, and Alexei V. Abramov. "Morphological variability and evolution of the baculum (os penis) in Mustelidae (Carnivora)." Journal of Mammalogy 84.2 (2003): 673-690.
^Dawkins, Richard (2006) . The Selfish Gene (30th anniversary ed.). Endnote to 30th anniversary edition: Oxford University Press. p. 158 endnote. ISBN0-19-929114-4. "It is not implausible that, with natural selection refining their diagnostic skills, females could glean all sorts of clues about a male’s health, and robustness of his ability to cope with stress, from the tone and bearing of his penis."