Penis of an Asian elephant
PrecursorGenital tubercle (amniotes)
SystemReproductive system, sometimes with the genitourinary system
Anatomical terminology

In many animals, a penis (/ˈpnɪs/; pl.: penises or penes) is the main male sexual organ used to inseminate females (or hermaphrodites) during copulation.[1][2] Such organs occur in both vertebrates and invertebrates, but males do not bear a penis in every animal species.

The term penis applies to many intromittent organs, but not to all. As an example, the intromittent organ of most Cephalopoda is the hectocotylus, a specialized arm, and male spiders use their pedipalps. Even within the Vertebrata, there are morphological variants with specific terminology, such as hemipenes.


The word "penis" is taken from the Latin word for "tail". Some derive that from Indo-European *pesnis, and the Greek word πέος = "penis" from Indo-European *pesos. Prior to the adoption of the Latin word in English, the penis was referred to as a "yard". The Oxford English Dictionary cites an example of the word yard used in this sense from 1379,[3] and notes that in his Physical Dictionary of 1684, Steven Blankaart defined the word penis as "the Yard, made up of two nervous Bodies, the Channel, Nut, Skin, and Fore-skin, etc."[4] According to Wiktionary, this term meant (among other senses) "rod" or "bar".

As with nearly any aspect of the body involved in sexual or excretory functions, the penis is the subject of many slang words and euphemisms for it, a particularly common and enduring one being "cock". See WikiSaurus:penis for a list of alternative words for penis.

The Latin word "phallus" (from Greek φαλλος) is sometimes used to describe the penis, although "phallus" originally was used to describe representations, pictorial or carved, of the penis.[5]

Evolution and function

The external genital organs appeared in the Devonian, about 410 million years ago, when tetrapods began to abandon the aquatic environment.[6] In fact, it was necessary to overcome the absence of a liquid phase in which to release the gametes was achieved through the transition to internal fertilization.

Among amniotes, the development of an erectile penis occurred independently for mammals, reptiles and archosaurs (crocodiles and birds).

Over time, birds have lost this organ, with the exception of Paleognathae and Anseriformes.[7]

The penis is an intromittent organ used to transfer sperm into the female genital tract for potential fertilization and, in the case of placentals, also for the excretion of urine.[8] The penises of different animal groups are not homologous with each other, but were created several times independently of each other in the course of evolution.

An erection is the stiffening and rising of the penis, which occurs during sexual arousal, though it can also happen in non-sexual situations. Ejaculation is the ejecting of semen from the penis and is usually accompanied by orgasm. A series of muscular contractions delivers semen, containing male gametes known as sperm cells or spermatozoa, from the penis.

The last common ancestor of all living amniotes (mammals, birds and reptiles) likely possessed a penis.[9]



A mallard pseudo-penis[10]
Male ducks have a corkscrew-shaped penis while female ducks have corkscrew vaginas with many blind pockets designed for difficult penetration and to prevent becoming pregnant. This reduced the likelihood of fertilization by unwanted aggressors in favor of fitter mates.

Most male birds (e.g., roosters and turkeys) have a cloaca (also present on the female), but not a penis. Among bird species with a penis are paleognathes (tinamous and ratites)[11] and Anatidae (ducks, geese and swans).[12] A bird penis is different in structure from mammal penises, being an erectile expansion of the cloacal wall (in ducks) and being erected by lymph, not blood.[13] It is usually partially feathered and in some species features spines and brush-like filaments, and in a flaccid state, curls up inside the cloaca.


Penis of a hamadryas baboon
Penis of a horse

As with any other bodily attribute, the length and girth of the penis can be highly variable between mammals of different species.[14][15] In many mammals, the penis' flaccid size is smaller than its erect size and is usually retracted into a sheath. The penis bears distal part of the urethra in placental mammals. [8] The perineum of testicond mammals separates the anus and the penis.

A bone called the baculum is present in most placental mammals but absent in humans, cattle and horses.

In mammals, the penis is divided into three parts:[16]

The internal structures of the penis consist mainly of cavernous, erectile tissue, which is a collection of blood sinusoids separated by sheets of connective tissue (trabeculae).

Genitorinary system of a male raccoon (Procyon lotor)

Canine penises have a structure at the base called the bulbus glandis.[17][18] During copulation, the spotted hyena inserts his penis through the female's pseudo-penis instead of directly through the vagina, which is blocked by the false scrotum. The pseudo-penis closely resembles the male hyena's penis, but can be distinguished from the male's genitalia by its greater thickness and more rounded glans.[19] Domestic cats have barbed penises, with about 120–150 one millimeter long backwards-pointing spines.[20] Monotremes and marsupial moles are the only mammals in which the penis is located inside the cloaca.[21][22]

Fish and reptiles

Male turtles and crocodiles have a penis, while male specimens of the reptile order Squamata, which are snakes and lizards, have two paired organs called hemipenes. Tuataras must use their cloacae for reproduction.[23] Due to evolutionary convergence, turtle and mammal penises have a similar structure.[24]

In some fish, the gonopodium, andropodium, and claspers are intromittent organs (to introduce sperm into the female) developed from modified fins.


The spine-covered penis of Callosobruchus analis, a bean weevil

In male insects, the structure analogous to a penis is known as aedeagus. The male copulatory organ of various lower invertebrate animals is often called the cirrus.[25]

In 2010, entomologist Charles Linehard described Neotrogla, a new genus of barkflies. Species of this genus have sex-reversed genitalia. Females have penis-like organs called gynosomes that are inserted into vagina-like openings of males during mating.[26]


Main article: Pizzle

Pizzles are represented in heraldry, where the adjective pizzled (or vilené[27]) indicates that part of an animate charge's anatomy, especially if coloured differently.

See also



  1. ^ Janet Leonard; Alex Cordoba-Aguilar R (18 June 2010). The Evolution of Primary Sexual Characters in Animals. Oxford University Press. ISBN 978-0-19-971703-3. Archived from the original on 11 October 2013. Retrieved 20 July 2013.
  2. ^ Schmitt, V.; Anthes, N.; Michiels, N. K. (2007). "Mating behaviour in the sea slug Elysia timida (Opisthobranchia, Sacoglossa): hypodermic injection, sperm transfer and balanced reciprocity". Frontiers in Zoology. 4: 17. doi:10.1186/1742-9994-4-17. PMC 1934903. PMID 17610714.
  3. ^ Basu, S. C. (2011). Male Reproductive Dysfunction. JP Medical Ltd. p. 101. ISBN 9789350252208.
  4. ^ Simpson, John, ed. (1989). "penis, n.". Oxford English Dictionary (second ed.). Oxford University Press. ISBN 978-0-19-861186-8.[dead link]
  5. ^ "Online Etymology Dictionary". Archived from the original on 2011-06-06. Retrieved 2011-05-28.
  6. ^ Dunlop, Jason A.; Penney, David. Fossil Arachnids. Siri Scientific Press. p. 44. ISBN 978-0-95677-954-0.
  7. ^ Cordoba-Aguilar, Alex; Leonard, Janet (2010). The Evolution of Primary Sexual Characters in Animals. Oxford University Press. pp. 216–221. ISBN 978-0-19971-703-3.
  8. ^ a b Marvalee H. Wake (15 September 1992). Hyman's Comparative Vertebrate Anatomy. University of Chicago Press. p. 583. ISBN 978-0-226-87013-7. Archived from the original on 31 December 2013. Retrieved 23 July 2013.
  9. ^ Sanger TJ, Gredler ML, Cohn MJ (October 2015). "Resurrecting embryos of the tuatara, Sphenodon punctatus, to resolve vertebrate phallus evolution". Biology Letters. 11 (10): 20150694. doi:10.1098/rsbl.2015.0694. PMC 4650183. PMID 26510679.
  10. ^ Brennan, Patricia L. R.; Clark, Christopher J.; Prum, Richard O. (2010-05-07). "Explosive eversion and functional morphology of the duck penis supports sexual conflict in waterfowl genitalia". Proceedings of the Royal Society B: Biological Sciences. 277 (1686): 1309–1314. doi:10.1098/rspb.2009.2139. ISSN 0962-8452. PMC 2871948. PMID 20031991.
  11. ^ Julian Lombardi (1998). Comparative Vertebrate Reproduction. Springer. ISBN 978-0-7923-8336-9. Archived from the original on 26 March 2014. Retrieved 5 December 2012.
  12. ^ MobileReference (15 December 2009). The Illustrated Encyclopedia of European Birds: An Essential Guide to Birds of Europe. MobileReference. ISBN 978-1-60501-557-6. Archived from the original on 26 March 2014. Retrieved 5 December 2012.
  13. ^ Frank B. Gill (6 October 2006). Ornithology. Macmillan. pp. 414–. ISBN 978-0-7167-4983-7. Archived from the original on 7 January 2014. Retrieved 5 December 2012.
  14. ^ Tim Birkhead (2000). Promiscuity: An Evolutionary History of Sperm Competition. Harvard University Press. p. 102. ISBN 978-0-674-00666-9. Archived from the original on 26 March 2014. Retrieved 23 July 2013.
  15. ^ Virginia Douglass Hayssen; Ari Van Tienhoven (1993). Asdell's Patterns of Mammalian Reproduction: A Compendium of Species-Specific Data. Cornell University Press. ISBN 978-0-8014-1753-5. Retrieved 23 July 2013.
  16. ^ William O. Reece (2009-03-04). Functional Anatomy and Physiology of Domestic Animals. John Wiley & Sons. ISBN 9780813814513. Archived from the original on 2018-03-20.
  17. ^ Susan Long (2006). Veterinary Genetics and Reproductive Physiology. Churchill Livingstone Elsevier. ISBN 978-0-7506-8877-2. Archived from the original on 2014-03-26. Retrieved 2013-11-08.
  18. ^ R. F. Ewer (1998). The Carnivores. Cornell University Press. p. 116. ISBN 978-0-8014-8493-3. Archived from the original on 26 March 2014. Retrieved 23 July 2013.
  19. ^ Glickman, SE; Cunha, GR; Drea, CM; Conley, AJ; Place, NJ (2006). "Mammalian sexual differentiation: lessons from the spotted hyena" (PDF). Trends Endocrinol Metab. 17 (9): 349–356. doi:10.1016/j.tem.2006.09.005. PMID 17010637. S2CID 18227659. Archived (PDF) from the original on 2013-02-22.
  20. ^ Aronson, L. R.; Cooper, M. L. (1967). "Penile spines of the domestic cat: their endocrine-behavior relations" (PDF). Anat. Rec. 157 (1): 71–8. doi:10.1002/ar.1091570111. PMID 6030760. S2CID 13070242. Archived from the original (PDF) on 2017-06-20.
  21. ^ Gadow, H. On the systematic position of Notoryctes typhlops. Proc. Zool. Soc. London 1892, 361–370 (1892).
  22. ^ Riedelsheimer, B., Unterberger, P., Künzle, H. and U. Welsch. 2007. Histological study of the cloacal region and associated structures in the hedgehog tenrec Echinops telfairi. Mammalian Biology 72(6): 330-341.
  23. ^ Lutz, Dick (2005), Tuatara: A Living Fossil, Salem, Oregon: DIMI PRESS, ISBN 0-931625-43-2
  24. ^ Kelly, D. A. (2004). "Turtle and mammal penis designs are anatomically convergent". Proceedings. Biological Sciences. 271 (Suppl 5): S293–S295. doi:10.1098/rsbl.2004.0161. PMC 1810052. PMID 15503998.
  25. ^ "Penis | Description, Anatomy, & Physiology | Britannica". Encyclopædia Britannica. January 2024.
  26. ^ Lienhard, Charles; Oliveira do Carmo, Thais; Lopes Ferreira, Rodrigo (2010). "A new genus of Sensitibillini from Brazilian caves (Psocodea: 'Psocoptera': Prionoglarididae)". Revue Suisse de Zoologie. 117 (4): 611–635. doi:10.5962/bhl.part.117600. ISSN 0035-418X. Archived from the original on 2014-11-03.
  27. ^ Rietstap, J. B. (1884). "Armorial général; précédé d'un Dictionnaire des termes du blason". G. B. van Goor zonen: XXXI. Vilené: se dit un animal qui a la marque du sexe d'un autre émail que le corps ((cite journal)): Cite journal requires |journal= (help)

General and cited references



Other animals