Baculum

The baculum (also penis bone, penile bone, or os penis, os genitale[1] or os priapi[2]) 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.[3][4] The os penis arises from primordial cells within soft tissues of the penis, and its formation is largely under the influence of androgens.[5] The bone is located above the male urethra,[6] 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 (also os clitoris), a bone in the clitoris.[7][8][9]

Baculum of a dog's penis; the arrow shows the urethral sulcus.

Etymology

The word baculum meant "stick" or "staff" in Latin and originated from Greek: βάκλον, baklon "stick".[10]

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.[11] A bone in the penis allows a male to mate for a long time with a female,[12][13] which can be a distinct advantage in some mating strategies.[14][15] The length of the baculum may be related to the duration of copulation in some species.[16][17] In carnivorans and primates, the length of the baculum appears to be influenced by postcopulatory sexual selection.[18] In some bat species, the baculum can also protect the urethra from compression.[19]

Presence in mammals
A raccoon baculum

Mammals having a penile bone (in males) and a clitoral bone (in females) include various eutherians:

It is absent in humans, ungulates (hoofed mammals),[31] elephants, monotremes (platypus, echidna),[32] marsupials,[33] lagomorphs,[23] hyenas,[34] binturongs,[24] sirenians,[6] and cetaceans (whales, dolphins, and porpoises),[6] among others.

Evidence suggests that the baculum was independently evolved 9 times and lost in 10 separate lineages.[24] 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.[33]

Among the primates, marmosets, weighing around 500 grams (18 oz), have a baculum measuring around 2 millimetres (0.079 in), while the tiny 63 g (2.2 oz) galago has one around 13 millimetres (0.51 in) 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.[15]

In some mammalian species, such as badgers[35][36] 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 1.2 g (0.042 oz) in mass, and measures less than 90 mm (3.5 in) long, then the baculum belongs to a juvenile.[26]

Absence in humans

Unlike most other primates, humans lack an os penis or os clitoris,[37][38] but the bone is present, although much reduced, among the great apes. In many ape species, it is a relatively insignificant 10–20 mm (0.39–0.79 in) structure. Cases of human penis ossification following trauma have been reported,[39] 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.[40] 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."[4] 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.[41]

The first recorded attempts to explain the lack of baculum in humans might be more than two thousand years old: the Biblical ‘rib’ that was ‘taken’ from Adam may actually be the baculum.[42] However, the evolutionary history of this anatomical loss remains enigmatic to this day.[43] Several scientific hypotheses for the loss in humans have been proposed.

In The Selfish Gene, Dawkins [44] 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 their ability to achieve erection without the support of a baculum.

The tactile stimulation hypothesis proposes that the loss of the baculum in humans is linked to the female choice for tactile stimulation: a boneless penis would be more flexible, facilitating a larger range of copulatory positions and whole body movement, giving females greater general physical stimulation.[45]

The elongation-loss hypothesis proposed that the evolution of bipedalism and sperm competition resulted in an increased evolutionary pressure for a longer penis, possibly with a larger supporting baculum. Following this, progression towards mating systems where males controlled access to females (monogamy or polygyny) may have favoured selection against the baculum, possibly as a large penis with a large baculum would have been a discomfort to males and/or vulnerable to injuries.[46]

The mating system shift hypothesis proposes that the shift towards monogamy as the dominant reproductive strategy may have reduced the intensity of copulatory and post-copulatory sexual selection, and made the baculum obsolete.[47]

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"[15] 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.[15][48][49]

Strengths and weaknesses of these hypotheses were revised in a 2021 study, which also proposed an alternative hypothesis: that conspecific aggression, in combination with the development of self-awareness, may have played a role in the loss. If the presence of a baculum exacerbated the prevalence and severity of penile injuries resulting from blunt trauma to a flaccid penis, increasing ability to foresee the consequences of their actions would also enable hominins to realise that these injuries are a useful tool in male-male competition. This behavioural innovation, planned conspecific aggression with the goal of temporary exclusion of competitors from the breeding pool, would create an environment in which a genetic mutation for a penis without a baculum (or with an unossified baculum) would strongly increase the fitness of the mutant phenotype. Along with the hominin propensity for social learning and cultural transmission, this hypothetical scenario may explain why this phenotype became fixed in all human populations.[50]

An alternative view is that its loss in humans is an example of neoteny during human evolution; late-stage fetal chimpanzees lack a baculum.[51]

Cultural significance
Walrus baculum, around 22 inches (56 cm) long

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" (Hebrew צֵלׇע ṣēlā', also translated "flank" or "side") 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.[52]

In hoodoo, the folk magic of the American South, the raccoon baculum is sometimes worn as an amulet for love or luck.[53]

Oosik

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 (24 in), 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 (1.4 m) fossilized penis bone from an extinct species of walrus, believed by the seller to be the largest in existence, was sold for $8,000.[54]

See also

References
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  43. Jakovlić, Ivan (2021) “The Missing Human Baculum: A Victim of Conspecific Aggression and Budding Self-Awareness?” Mammal Review. https://doi.org/10.1111/mam.12237
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  45. Cormier LA, Jones SR (2015) The Domesticated Penis: How Womanhood Has Shaped Manhood. University of Alabama Press, Tuscaloosa, USA.
  46. Hobday AJ (2000) Where is the human baculum? The Mankind Quarterly 41: 43–58
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  50. Jakovlić, Ivan (2021) “The Missing Human Baculum: A Victim of Conspecific Aggression and Budding Self-Awareness?” Mammal Review. https://doi.org/10.1111/mam.12237
  51. Bednarik, R. G. (2011). The Human Condition. doi:10.1007/978-1-4419-9353-3. ISBN 978-1-4419-9352-6. (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'"
  52. 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): 284–85. doi:10.1002/ajmg.1387. PMID 11424148.
  53. 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 were photographed wearing them. New Orleans gamblers are said to use the bones (also called coon dogs and Texas toothpicks) for luck.
  54. "Walrus penis sells for $8,000 at Beverly Hills action". AP. Archived from the original on 6 November 2007. Retrieved 30 August 2007.

Further reading
  • 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. doi:10.1002/ajmg.1387. PMID 11424148.
  • Clellan S., Frank A. Beach (1951). Patterns of Sexual Behavior. New York: Harper, and Paul B. Hoeber, Inc. Medical Books. ISBN 978-0-313-22355-6.
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