The baculum (: bacula), also known as the penis bone, penile bone, os penis, os genitale, or os priapi, is a bone in the penis of many placental mammals. It is absent from the human penis, but present in the penises of some primates, such as the gorilla and the chimpanzee. The baculum arises from primordial cells in soft tissues of the penis, and its formation is largely influenced by androgens. The bone lies above the urethra, and it aids sexual reproduction by maintaining stiffness during sexual penetration. The homologue to the baculum in female mammals is the baubellum (os clitoridis, os clitoris), a bone in the clitoris.
Etymology
The word baculum means "stick" or "staff" in Latin and originates from , baklon "stick".
Function
The baculum is used for copulation and varies in size and shape by species. Its evolution may be influenced by sexual selection, and 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. The length of the baculum may be related to the duration of copulation in some species. In carnivorans and primates, the length of the baculum appears to be influenced by postcopulatory sexual selection. In some bat species, the baculum can also protect the urethra from compression.
Presence in mammals
Mammals having a penile bone (in males) and a clitoral bone (in females) include various eutherians: Order Primates, although not in lorises, humans, spider monkeys, or woolly monkeys Order Rodentia (rodents), though not in the related order Lagomorpha (rabbits, hares, etc.) Order Eulipotyphla (insectivores, including shrews and hedgehogs) Order Carnivora (including members of many well-known families, such as ursids (bears), canids (dogs), pinnipeds (walruses, seals, sea lions), procyonids (raccoons etc.), mustelids (otters, weasels, skunks and others)). The baculum is usually longer in the Canoidea than in the Feloidea, although fossas have long bacula and giant pandas have short bacula. Order Chiroptera (bats).
It is absent in humans, ungulates (hoofed mammals), elephants, monotremes (platypus, echidna), marsupials, lagomorphs, hyenas, binturongs, 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 to be derived from the epipubic bones more widely spread across mammals, but notoriously absent in placentals.
Among the primates, marmosets, weighing around , have a baculum measuring around , while the tiny galago has one around 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 badgers and raccoons (Procyon lotor), the baculum can be used to determine relative age. If a raccoon's baculum tip is made up of uncalcified cartilage, has a porous base, is less than in mass, and measures less than long, then the baculum belongs to a juvenile.
Absence in humans
Unlike most other primates, humans lack an os penis or os clitoris, but the bone is present, although much reduced, among the great apes. In many ape species, it is a relatively insignificant structure. Cases of human penis ossification following trauma have been reported, and one case was reported 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 (1951), 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. An "artificial baculum" or penile implant is sometimes used to treat erectile dysfunction in humans.
In The Selfish Gene, Richard Dawkins proposed honest advertising as the evolutionary explanation for the loss of the baculum. The hypothesis states that if erection failure is a sensitive early warning of ill health (physical or mental), females could have gauged the health of a potential mate based on his ability to achieve erection without the support of a baculum.