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 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
- Order Primates, although not in 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), felids (cats), 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, 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, 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.
Absence in humans
Unlike other primates, humans lack an os penis or os clitoris; however, this bone is present but much reduced among the great apes. In many ape species, it is a relatively insignificant 10–20 mm 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 (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.
The loss of the bone in humans, when it is present in our nearest related species the chimpanzee, is thought to be 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.
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.
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-ft-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.
- Patterns of Sexual Behavior Clellan S. Ford and Frank A. Beach, published by Harper & Row, New York in 1951. ISBN 0-313-22355-6
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- R. F. Ewer (1973). The Carnivores. Cornell University Press. ISBN 978-0-8014-8493-3. Retrieved 16 December 2012.
- Liddell, Henry George; Scott, Robert. "βάκλον". An Intermediate Greek-English Lexicon. Tufts University. Retrieved 9 April 2017.
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- DIXSON33, Alan, N. YHOL T. Jenna, and Matt Anderson. "A positive relationship between baculum length and prolonged intromission patterns in mammals." 动物学报 50.4 (2004): 490-503.
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- Harvey, Suzanne. "How Did Man Lose His Penis Bone?". University College London, Researchers In Museums blog, 26 November 2012.
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- R. F. Ewer (1973). The Carnivores. Cornell University Press. ISBN 978-0-8014-8493-3. Retrieved 16 December 2012.
- 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.
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- Richard Estes (1991). The Behavior Guide to African Mammals: Including Hoofed Mammals, Carnivores, Primates. University of California Press. pp. 323–. ISBN 978-0-520-08085-0. Retrieved 12 December 2012.
- Schultz, Nicholas G.; Lough-Stevens, Michael; Abreu, Eric; Orr, Teri; Dean, Matthew D. (2016-06-01). "The Baculum was Gained and Lost Multiple Times during Mammalian Evolution". Integrative and Comparative Biology. 56 (4): 644–56. ISSN 1540-7063. PMID 27252214. doi:10.1093/icb/icw034.
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- "Scientists have answered one of the biggest questions people have about their penis". The Independent. 2016-12-14. Retrieved 2016-12-15.
- Ridley, Mark (2007). Richard Dawkins: How a Scientist Changed the Way We Think : Reflections by Scientists, Writers, and Philosophers. Oxford University Press. p. 228. ISBN 0-19-921466-2., of page 228
- Dawkins, Richard (2006) . The Selfish Gene (30th anniversary ed.). Endnote to 30th anniversary edition: Oxford University Press. p. 158 endnote. ISBN 0-19-929114-4.
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.
- Bednarik, R. G. (2011). "The Human Condition". ISBN 978-1-4419-9352-6. doi:10.1007/978-1-4419-9353-3. (page 134), cited by:
Achrati, Ahmed (November 2014). "Neoteny, female hominin and cognitive evolution". Rock Art Research. 31 (1): 232–238.
"In humans, neoteny is manifested in the resemblance of many physiological features of a human to a late-stage foetal chimpanzee. These foetal characteristics include hair on the head, a globular skull, ear shape, vertical plane face, absence of penal bone (baculum) in foetal male chimpanzees, the vagina pointing forward in foetal ape, the presence of hymen in neonate ape, and the structure of the foot. 'These and many other features', Bednarik says, 'define the anatomical relationship between ape and man as the latter's neoteny'"
- Gilbert, S. F.; Zevit, Z. (2001). "Congenital human baculum deficiency: The generative bone of Genesis 2:21-23". American Journal of Medical Genetics. 101 (3): 284–5. PMID 11424148. doi:10.1002/ajmg.1387.
- Joanne O'Sullivan (1 March 2010). Book of Superstitious Stuff: Weird Happenings, Wacky Rites, Frightening Fears, Mysterious Myths & Other Bizarre Beliefs. Charlesbridge Publishing. p. 87. ISBN 978-1-60734-367-7.
"In the hoodoo (folk magic) tradition of the American South, a raccoon penis bone (scientifically known as the baculum) is a lucky charm used to attract love. In some areas, it's boiled to remove any trace of the animal, and then tied to a red ribbon and worn as a necklace. In other areas, the bones were traditionally given to girls and young women by suitors, and in still other places, the charms are worn by men. Earrings made from cast raccoon penis bones became a fad in 2004, and celebrities such as Sarah Jessica Parker and Vanessa Williams[disambiguation needed] were photographed wearing them. New Orleans gamblers are said to use the bones (also called coon dogs and Texas toothpicks) for luck."
- "Walrus penis sells for $8,000 at Beverly Hills action". AP. Archived from the original on 6 November 2007. Retrieved 30 August 2007.
- Gilbert SF, Zevit Z (July 2001). "Congenital human baculum deficiency: the generative bone of Genesis 2:21–23". Am. J. Med. Genet. 101 (3): 284–5. PMID 11424148. doi:10.1002/ajmg.1387.
- Clellan S., Frank A. Beach (1951). Patterns of Sexual Behavior. New York: Harper, and Paul B. Hoeber, Inc. Medical Books. ISBN 0-313-22355-6.
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- Beresford WA, Burkart S (December 1977). "The penile bone and anterior process of the rat in scanning electron microscopy". J. Anat. 124 (3): 589–97. PMC . PMID 604330.
- The San Diego Zoo's Conservation and research for endangered species projects. 'What is the significance of the baculum in animals?'
- University of Utah Biology 3315 – Comparative Vertebrate Morphology § 16
- On the evolution of the mammalian baculum: vaginal friction, prolonged intromission or induced ovulation?
- The structure of the penis with the associated baculum in the male greater cane rat (Thryonomys swinderianus)