Os clitoridis

The os clitoridis (also called the os clitoris, clitoral bone or baubellum; pl.: baubella) is a bone inside the clitoris of many placental mammals. It is absent from the human clitoris, but present in the clitoris of some primates, such as ring-tailed lemurs and non-human great apes. However, in the latter case, the bone is greatly reduced in size.^[1][2][3] It is homologous to the baculum in male mammals.

1914 illustration of the os clitoridis in the clitoris of a greater dwarf lemur (labeled "O.c.")

The structure is more evolutionarily labile than the baculum, exhibiting both more inherent variability and more gains and losses over time,^[4] which has been interpreted as evidence for its non-functionality.^[4]

Other work posits that the variation in the os clitoridis could be driven by intersexual conflict, lock-and-key genital evolution, and cryptic female choice, especially given the high level of variation within species as well as between them.^[5]

History

The os clitoridis was described in 1666 by Claude Perrault in otters and in the lioness.^[6]

The term os clitoridis was used in 1819 by Friedrich Sigismund Leuckart regarding the capuchin monkey.^[7]

This bone was named baubellum by Guy Chester Shortridge in 1934,^[8] but it is much less common in comparison to the use of the word baculum. The Latin terms os clitoris and os clitoridis are most often found in scientific publications.

Presence in mammals

The os clitoridis has been described in species belonging to the orders Chiroptera, Primates, Rodentia and Carnivora.^[9] As with the baculum, this wide distribution suggests a primitive character that has been lost in some phylogenetic branches of the class Mammalia.

Depending on the species, the presence of this bone varies from one specimen to another. It has been observed, for example, in only 30% of American red squirrels.^[10] Its presence is even rarer in dogs: 3% (6 out of 200) presence on a radiological sample of American Cocker Spaniels and 2% (4 out of 200) for the German Shorthaired Pointer.^[11]

The shape and size vary greatly from one species to another. The size is often very small:^{[lower-alpha 1]}

• 2 x 6 mm in the mountain beaver^[12]
• 2 x 0.4 x 0.2 mm in an adult lioness^[13]
• 0.5 x 0.05 x 0.01 mm in a 3-year-old adult cat^[13]
• From 10 to 30 mm in a walrus^[14] (note that the male of this species has the longest baculum, with a size of 63 cm (24 inches))
• 9.3 mm in the American badger^[15]

Development

The os clitoridis is often present, or even prominent, during the embryonic or immature phase, and then decreases with age. For example, in a walrus, the size of the bone tends to shrink as the years pass.^[14]

Function

The exact function of the os clitoridis is not known, but a function during copulation is assumed.^[16] For some, the species distribution would be the same as the baculum.^[15] The os clitoridis would be an equivalent, without evolutionary function, of the baculum, persisting or disappearing during sexual differentiation under hormonal influence. Experiments with treatment of the spleen with testosterone, led to a persistence or increase in the size of the os clitoridis.^[17]

Notes

1. All measurements listed below are less than an inch, with the exception of the walrus, which can be up to 1 inch.

References

1. "A Long-Lost Bone". National Geographic Society. 3 September 2013. Archived from the original on March 3, 2021.
2. Martin, Robert D. (2007). "The evolution of human reproduction: A primatological perspective". American Journal of Physical Anthropology. 134: 59–84. doi:10.1002/ajpa.20734. PMID 18046752. S2CID 44416632.
3. Friderun Ankel-Simons (27 July 2010). Primate Anatomy: An Introduction. Elsevier. ISBN 978-0-08-046911-9.
4. Lough-Stevens, Michael; et al. (January 2018). "The baubellum is more developmentally and evolutionarily labile than the baculum". Ecology and Evolution. 8 (2): 1073–1083. doi:10.1002/ece3.3634. PMC 5773289. PMID 29375780.
5. Sloan, Nadia (September 2019). "The evolution of female genitalia". Journal of Evolutionary Biology. 32 (9): 882–899. doi:10.1111/jeb.13503. PMID 31267594.
6. Claude Perrault (1748). Mémoires pour servir à l'histoire naturelle des animaux (in French). Arkstee & Merkus.
7. Zoologische Bruchstücke, p. 41, at Google Books
8. Shortridge, G. C. (1934). The mammals of South West Africa. William Heinemann Ltd.
9. James N. Layne (August 1954). "The os clitoridis of some North American Sciuridae". Journal of Mammalogy. 35 (3): 357–366. doi:10.2307/1375960. JSTOR 1375960. Retrieved September 21, 2022.
10. James N. Layne (1952). "The os genitale of the red quirrel, Tamiasciurus". Journal of Mammalogy. 33 (4): 457–459. doi:10.2307/1376017. JSTOR 1376017. Retrieved 21 September 2022.
11. Kutzler M.; Keller G.G.; Smith F. (2020-01-11). Os clitoridis incidence on radiographs submitted for coxofemoral dysplasia evaluations. International Symposium on Canine and Feline Reproduction | IVIS. www.ivis.org. Retrieved 2022-09-21.
12. "Aplodontia rufa" (PDF). Mammalian Species. 431: 1–10. April 23, 1993.
13. H. Burrows (2013). Biological Actions of Sex Hormones. Cambridge University Press. p. 226. ISBN 978-1-107-62550-1.
14. Leonard Janet; Alex Cordoba-Aguilar (2010). The Evolution of Primary Sexual Characters in Animals. Cambridge University Press. ISBN 978-0-19-971703-3.
15. WH (1960). Bacula of North American Mammals. Museum of Zoology, University of Michigan. hdl:2027.42/56357.
16. Pascal Picq; Philippe Brenot (2009). Odile Jacob (ed.). Le Sexe, l'Homme et l'Évolution (in French). Odile Jacob. ISBN 9782738195661.
17. Bones and Cartilage: Developmental and Evolutionary Skeletal Biology, p. 344, at Google Books