Intersex (biology)

Sex characteristics between male and female

This article is about intersex across species from a biological perspective. For humans specifically, see Intersex.

Not to be confused with Hermaphrodite.

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Sex
Biological terms

* Sexual dimorphism
* Sexual differentiation
* Feminization
* Virilization
* Sex-determination system
* XY
* XO
* ZW
* ZO
* Temperature-dependent
* Haplodiploidy
* Heterogametic sex
* Homogametic sex
* Sex chromosome
* X chromosome
* Y chromosome
* Sex chromosome anomalies
* Testis-determining factor
* Hermaphrodite
* Sequential hermaphroditism
* Simultaneous hermaphroditism
* Intersex (biology)
* Mating type

Sexual reproduction

* Evolution of sexual reproduction
* Anisogamy
* Isogamy
* Germ cell
* Meiosis
* Gametogenesis
* Spermatogenesis
* Oogenesis
* Gamete
* spermatozoon
* ovum
* Fertilization
* External fertilization
* Internal fertilization
* Sexual selection
* Plant reproduction
* Fungal reproduction
* Sexual reproduction in animals
* Sexual intercourse
* Penile–vaginal intercourse
* Copulation
* Human reproduction
* Lordosis behavior
* Pelvic thrust

Sexuality

* Plant sexuality
* Animal sexuality
* Human sexuality
* Mechanics
* Differentiation
* Activity

Sexual system

* Dioecy
* Gonochorism
* Hermaphrodite
* Intersex

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Intersex is a general term for an organism that has sex characteristics that are between male and female.[1] It typically applies to a minority of members of gonochoric animal species such as mammals (as opposed to hermaphroditic species in which the majority of members can have both male and female sex characteristics).[2] Such organisms are usually sterile.[3]

Female spotted hyena

Intersexuality can occur due to both genetic and environmental factors[4] and has been reported in mammals, fish, nematodes, and crustaceans.

## Mammals

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Intersex can occur in mammals such as pigs, with it being estimated that 0.1% to 1.4% of pigs are intersex.[5] In Vanuatu, Narave pigs are sacred intersex pigs that are found on Malo Island. An analysis of Narave pig mitochondrial DNA by Lum et al. (2006) found that they are descended from Southeast Asian pigs.[6][7][8] Female spotted hyenas have a pseudo-penis which led to a myth that they are hermaphroditic.[9]

In cattle, a freemartin is the infertile female twin of a bull. Freemartins have both XX and XY chromosomes, non-functional ovaries, and exhibit masculine behavior.

At least six different mole species have an adaption where by the female mole has an ovotestis, "a hybrid organ made up of both ovarian and testicular tissue. The evolved purpose of this adoption is to give them an extra dose of testosterone to make them just as muscular and aggressive as male moles". Only the ovarian part of the ovotestis is reproductively functional.[10][11]

Intersexuality in humans is relatively rare. Depending on the definition, the prevalence of intersex among humans has been reported to range around a figure of 0.018%.[12][13]

## Nematodes

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Adult Caenorhabditis elegans

Intersex is known to occur in all main groups of nematodes. Most of them are functionally female. Male intersexes with female characteristics have been reported but are less common.[14]

## Fishes

[edit]

Gonadal intersex occurs in fishes, where the individual has both ovarian and testicular tissue. Although it is a rare anomaly among gonochoric fishes, it is a transitional state in fishes that are protandric or protogynous.[15] Intersexuality has been reported in 23 fish families.[16]

## Crustaceans

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The oldest evidence for intersexuality in crustaceans comes from fossils dating back 70 million years ago.[4] Intersex has been reported in gonochoric crustaceans as early as 1729. A large amount of literature exists on intersexuality for isopoda and amphipoda, with there being reports of both intersex males and intersex females.[17]

## Birds

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According to National Geographic, scientists have discovered that some chickens possess a mixture of genetically male and female cells.[18] Hens can take on male characteristics if their functioning ovary is damaged or diseased.[19]

## See also

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* Gynandromorphism
* Hermaphrodite
* Sexual differentiation

## References

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1. ^ "intersex | Definition & Facts". Encyclopædia Britannica. Retrieved 2021-07-10. "Intersex, in biology, an organism having physical characteristics intermediate between a true male and a true female of its species."
2. ^ Farrell A (2011-06-01). Encyclopedia of Fish Physiology: From Genome to Environment. Academic Press. ISBN 978-0-08-092323-9. "Thus, strictly speaking, all hermaphrodites are intersex at one time point, but not all intersexes are hermaphrodites. This definition is usually applied to gonochoristic species to describe those individuals that are not normal for the species."
3. ^ "Malformation – Sexual anomalies". Encyclopedia Britannica. Encyclopædia Britannica, Inc. Retrieved 19 July 2021.
4. ^ a b Ford, Alex T. (February 2012). "Intersexuality in Crustacea: an environmental issue?". Aquatic Toxicology (Amsterdam, Netherlands). 108: 125–129\. Bibcode:2012AqTox.108..125F. doi:10.1016/j.aquatox.2011.08.016. ISSN 1879-1514. PMID 22265612.
5. ^ Hunter, R. H. F.; Hunter, Ronald Henry Fraser (1995-03-09). Sex Determination, Differentiation and Intersexuality in Placental Mammals. Cambridge University Press. p. 157\. ISBN 978-0-521-46218-1.
6. ^ Lum, J. Koji; McIntyre, James K.; Greger, Douglas L.; Huffman, Kirk W.; Vilar, Miguel G. (November 14, 2006). "Recent Southeast Asian domestication and Lapita dispersal of sacred male pseudohermaphroditic "tuskers" and hairless pigs of Vanuatu". Proceedings of the National Academy of Sciences. 103 (46): 17190–17195\. Bibcode:2006PNAS..10317190L. doi:10.1073/pnas.0608220103. PMC 1859908. PMID 17088556.
7. ^ "Intersex Pigs". Southwest Pacific Research Project. Retrieved 2023-03-07.
8. ^ "Pigs in Paradise". Penn State University. Retrieved 2023-03-07.
9. ^ "Hyena Myths Busted: Are They Really Hermaphrodites?". Animals. 2025-05-01. Retrieved 2025-05-01.
10. ^ https://www.cbc.ca/radio/quirks/oct-17-coronavirus-and-pain-sampling-an-asteroid-intersex-moles-and-more-1.5763905/female-moles-are-intersex-they-have-testicle-like-tissue-that-helps-them-grow-big-and-tough-1.5763925,CBC Radio, accessed 2023-04-13
11. ^ M. Real, Francisca; Haas, Stefan A.; Franchini, Paolo; Xiong, Peiwen; Simakov, Oleg; Kuhl, Heiner; Schöpflin, Robert; Heller, David; Moeinzadeh, M-Hossein; Heinrich, Verena; Krannich, Thomas; Bressin, Annkatrin; Hartmann, Michaela F.; Wudy, Stefan A.; Dechmann, Dina K. N. (2020). "The mole genome reveals regulatory rearrangements associated with adaptive intersexuality". Science. 370 (6513): 208–214\. Bibcode:2020Sci...370..208M. doi:10.1126/science.aaz2582. PMC 8243244. PMID 33033216.
12. ^ Leonard Sax (1 August 2002). "How common is intersex? a response to Anne Fausto-Sterling". Journal of Sex Research. 39 (3): 174–178\. doi:10.1080/00224490209552139. ISSN 0022-4499. PMID 12476264. Wikidata Q34163911.
13. ^ Selma Feldman Witchel (2018). "Disorders of Sex Development". Best Practice & Research. Clinical Obstetrics & Gynaecology. 48: 90–102\. doi:10.1016/j.bpobgyn.2017.11.005. ISSN 1521-6934. PMC 5866176. PMID 29503125. "The estimated frequency of genital ambiguity is reported to be in the range of 1:2000-1:4500"
14. ^ El-Bawab, Fatma (2020-01-18). Invertebrate Embryology and Reproduction. Academic Press. p. 431\. ISBN 978-0-12-814115-1.
15. ^ Norris, David O.; Lopez, Kristin H. (2010-11-25). Hormones and Reproduction of Vertebrates. Academic Press. pp. 249–251\. ISBN 978-0-08-095809-5.
16. ^ Pandian, T. J. (2010-09-15). Sexuality in Fishes. CRC Press. p. 38\. ISBN 978-1-4398-4669-8.
17. ^ Cothran, Rickey; Thiel, Martin (2020-04-24). The Natural History of the Crustacea: Reproductive Biology: Volume VI. Oxford University Press. pp. 395–396\. ISBN 978-0-19-068855-4.
18. ^ "Half-Male Chicken Mystery Solved". Culture. 2025-05-01. Retrieved 2025-05-01.
19. ^ Co, The Chama Valley Meat. "Hermaphrodite Chickens?". The Chama Valley Meat Co. Retrieved 2025-05-01.

* v
* t
* e

Sex

Biological
terms|

* Sexual dimorphism
* Male
* Female
* Sexual differentiation
* Feminization
* Virilization
* Sex-determination system
* XY
* ZW
* XO
* ZO
* Temperature-dependent
* Haplodiploidy
* Sex chromosome
* Testis-determining factor
* Hermaphrodite
* Sequential hermaphroditism
* Simultaneous hermaphroditism
* Intersex
* parasexuality
* Sex as a biological variable

Sexual
reproduction|

* Evolution of sexual reproduction
* Anisogamy
* Isogamy
* Germ cell
* Reproductive system
* Sex organ
* Mating
* Meiosis
* Gametogenesis
* Spermatogenesis
* Oogenesis
* Gamete
* spermatozoon
* ovum
* Fertilization
* External
* Internal
* Sexual selection
* Plant reproduction
* Fungal reproduction
* Sexual reproduction in animals
* Sexual intercourse
* Penile–vaginal intercourse
* Copulation
* Hormonal motivation
* Human reproduction
* Lordosis behavior
* Pelvic thrust

Sexuality|

* Plant sexuality
* Animal sexuality
* Human sexuality
* Mechanics
* Differentiation
* Activity

Sexual system|

* Dioecy
* Gonochorism
* Hermaphrodite
* Intersex

* Category

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