Many taxonomic groups of animals (mostly invertebrates) do not have separate sexes. In these groups, hermaphroditism is a normal condition, enabling a form of sexual reproduction in which both partners can act as the "female" or "male". For example, the great majority of pulmonate snails, opisthobranch snails and slugs are hermaphrodites. Hermaphroditism is also found in some fish species and to a lesser degree in other vertebrates. Most plants are also hermaphrodites.
Historically, the term hermaphrodite has also been used to describe ambiguous genitalia and gonadal mosaicism in individuals of gonochoristic species, especially human beings. The word hermaphrodite entered the English lexicon in the late 14th century, derived from Greek Ερμαφρόδιτος Hermaphroditos, the son of the gods Hermes and Aphrodite in Greek mythology. The word intersex has come into preferred usage for humans, since the word hermaphrodite is considered to be misleading and stigmatizing, as well as "scientifically specious and clinically problematic".
Sequential hermaphrodites 
Sequential hermaphrodites (dichogamy) occur in species in which the individual is born as one sex, but can later change into the opposite sex. This contrasts simultaneous hermaphrodites, in which an individual may possess fully functional male and female gonads. Sequential hermaphroditism is common in fish (particularly teleost fish) and some jellyfish, many gastropods (such as the common slipper shell), and some flowering plants. While some sequential hermaphrodites can change sex multiple times, most can only change sex once. Sequential hermaphrodism can best be understood in terms of behavioral ecology and evolutionary life history theory, as described in the size-advantage mode first proposed by Michael T. Ghiselin which states that if an individual of a certain sex could significantly increase its reproductive success after reaching a certain size, it would be to their advantage to switch to that sex.
Sequential hermaphrodites can be divided into three broad categories:
- Protandry: Where an organism is born as a male, and then changes sex to a female.
- Example: The clownfish (genus Amphiprion) are colorful reef fish found living in symbiosis with sea anemones. Generally one anemone contains a 'harem ', consisting of a large female, a smaller reproductive male, and even smaller non-reproductive males. If the female is removed, the reproductive male will change sex and the largest of the non-reproductive males will mature and become reproductive. It has been shown that fishing pressure can change when the switch from male to female occurs, since fishermen usually prefer to catch the larger fish. The populations are generally changing sex at a smaller size, due to natural selection.
- Protogyny: Where the organism is born as a female, and then changes sex to a male.
- Example: wrasses (Family Labridae) are a group of reef fish in which protogyny is common. Wrasses also have an uncommon life history strategy, which is termed diandry (literally, two males). In these species, two male morphs exists: an initial phase male and a terminal phase male. Initial phase males do not look like males and spawn in groups with other females. They are not territorial. They are, perhaps, female mimics (which is why they are found swimming in group with other females). Terminal phase males are territorial and have a distinctively bright coloration. Individuals are born as males or females, but if they are born males, they are not born as terminal phase males. Females and initial phase males can become terminal phase males. Usually, the most dominant female or initial phase male replaces any terminal phase male when those males die or abandon the group.
- Bidirectional Sex Changers: where an organism has both female and male gonads, but act as either female or male during different stages in life.
- Example: Lythrypnus dalli (Family Lythrypnus) are a group of coral reef fish in which bidirectional sex change occurs. Once a social hierarchy is established a fish changes sex according to its social status, regardless of the initial sex, based on a simple principle: if the fish expresses subordinate behavior then it changes its sex to male, and if the fish expresses dominant or not subordinate behavior then the fish changes it sex to female.
Dichogamy can have both conservation-related implications for humans, as mentioned above, as well as economic implications. For instance, groupers are favoured fish for eating in many Asian countries and are often aquacultured. Since the adults take several years to change from female to male, the broodstock are extremely valuable individuals.
Simultaneous hermaphrodites 
- Reproductive system of gastropods: Pulmonate land snails and land slugs are perhaps the best-known kind of simultaneous hermaphrodite, and are the most widespread of terrestrial animals possessing this sexual polymorphism. Sexual material is exchanged between both animals via spermatophore, which can then be stored in the spermatheca. After exchange of spermatozoa, both animals will lay fertilized eggs after a period of gestation; then the eggs will proceed to hatch after a development period. Snails typically reproduce in early spring and late autumn.
- Banana slugs are one example of a hermaphroditic gastropod. Mating with a partner is more desirable biologically, as the genetic material of the resultant offspring is varied, but if mating with a partner is not possible, self-fertilization is practiced. The male sexual organ of an adult banana slug is quite large in proportion to its size, as well as compared to the female organ. It is possible for banana slugs, while mating, to become stuck together. If a substantial amount of wiggling fails to separate them, the male organ will be bitten off (using the slug's radula), see apophallation. If a banana slug has lost its male sexual organ, it can still mate as a female, making its hermaphroditic quality a valuable adaptation.
- Hamlets, unlike other fish, seem quite at ease mating in front of divers, allowing observations in the wild to occur readily. They do not practice self-fertilization, but when they find a mate, the pair takes turns between which one acts as the male and which acts as the female through multiple matings, usually over the course of several nights.
- Earthworms are another example of a simultaneous hermaphrodite. Although they possess ovaries and testes, they have a protective mechanism against self-fertilization. Sexual reproduction occurs when two worms meet and exchange gametes, copulating on damp nights during warm seasons. Fertilized eggs are protected by a cocoon, which is buried on or near the surface of the ground.
When spotted hyenas were first discovered by explorers, they were thought to be hermaphrodites. Early observations of spotted hyenas in the wild led researchers to believe that all spotted hyenas, male and female, were born with what appeared to be a penis. The apparent penis in females is in fact an enlarged clitoris, which contains an external birth canal. It can be difficult to determine the sex of wild spotted hyenas until sexual maturity, when they may become pregnant. When a female spotted hyena gives birth, they pass the cub through the cervix internally, but then pass it out through the elongated clitoris.
Aside from having an ambiguous-looking external genitalia, true hermaphroditism in humans differs from pseudohermaphroditism in which the person's karyotype has both XX and XY chromosome pairs (47XXY, 46XX/46XY, 46XX/47XXY or 45X/XY mosaic) and having both testicular and ovarian tissue. One possible pathophysiologic explanation of this rare phenomenon is a parthenogenetic division of a haploid ovum into two haploid ova. Upon fertilization of the two ova by two sperm cells (one carrying an X and the other carrying a Y chromosome), the two fertilized ova are then fused together resulting in a person having dual genitalial, gonadal (ovotestes) and genetic sex.
Another common cause of hermaphroditism is the crossing over of the SRY from the Y chromosome to the X chromosome during meiosis. The SRY is then activated in only certain areas, causing development of testes in some areas by beginning a series of events starting with the upregulation of SOX9, and in other areas not being active (causing the growth of ovarian tissues). Thus, testicular and ovarian tissues will both be present in the same individual.
Hermaphrodite is used in botany to describe a flower that has both staminate (male, pollen-producing) and carpellate (female, ovule-producing) parts. This condition is seen in many common garden plants. A closer analogy to hermaphroditism in botany is the presence of separate male and female flowers on the same individual—such plants are called monoecious. Monoecy is especially common in conifers, but occurs in only about 7% of angiosperm species.
Other uses of the term 
Hermaphrodite was used to describe any person incompatible with the biological gender binary, but has recently been replaced by intersex in medicine. Humans with typical reproductive organs but atypical clitoris/penis are called pseudohermaphrodites in medical literature. Pseudohermaphroditism also refers to a human possessing both the clitoris and testicles.
People with intersex conditions sometimes choose to live exclusively as one sex or the other, using clothing, social cues, genital surgery, and hormone replacement therapy to blend into the sex they identify with more closely. Some people who are intersex, such as some of those with androgen insensitivity syndrome, outwardly appear completely female or male already, without realizing they are intersex. Other kinds of intersex conditions are identified immediately at birth because those with the condition have a sexual organ larger than a clitoris and smaller than a penis. Intersex is thought by some to be caused by unusual sex hormones; the unusual hormones may be caused by an atypical set of sex chromosomes.
Sigmund Freud (based on work by his associate Wilhelm Fliess) held fetal hermaphroditism to be a fact of the physiological development of humans. He based much of his theory of innate sexuality on that assumption. Similarly, in contemporary times, fetuses before sexual differentiation are sometimes described as female by doctors explaining the process. Neither concept is technically true. Before this stage, humans are simply undifferentiated and possess a Müllerian duct, a Wolffian duct, and a genital tubercle.
The term "hermaphrodite" derives from Hermaphroditus, the son of Hermes and Aphrodite in Greek mythology. According to Ovid, he fused with a nymph, Salmacis, resulting in one individual possessing physical traits of both sexes. Although, according to Diodorus, who mentions at an earlier period, he was born with a physical body combining both sexes.
See also 
- Konstantinidis, G. (2005). Elsevier's Dictionary of Medicine and Biology. Elsevier. p. 667. ISBN 0080460127.
- Merriam-Webster Dictionary Retrieved 28 June 2011
- "hermaphroditism". Encyclopædia Britannica Online. Retrieved 9 April 2013.
- Online Etymology Dictionary Retrieved 3 June 2012
- "Is a person who is intersex a hermaphrodite?". Intersex Society of North America. Retrieved 2 October 2011.
- Herndon, April. "Getting Rid of "Hermaphroditism" Once and For All". Intersex Society of North America. Retrieved 2 October 2011.
- Barrows, Edward M. (2001). Animal behavior desk reference: a dictionary of animal behavior, ecology, and evolution (2nd ed.). Boca Raton, Fla: CRC Press. p. 317. ISBN 0-8493-2005-4. OCLC 299866547.
- Warner, Robert R (June 1988). "Sex change and the size-advantage model". Trends in Ecology and Evolution 3 (6): 133–136. doi:10.1016/0169-5347(88)90176-0. PMID 21227182.
- Ghiselin, Micahel T. (1969). "The evolution of hermaphroditism among animals". Quarterly Review of Biology 44 (2): 189–208. doi:10.1086/406066. PMID 4901396.
- Rodgers, E.W.; Early, R.L., abd Grober, M.S. (2007). "Social status determines sexual phenotype in the bi-directional sex changing bluebanded goby Lythrypnus dalli". J Fish Biol 70: 1660–1668.
- Schultheiss, Herrmann & Jonas 2006, p. 358.
- The Painful Realities of Hyena Sex
- Graphic depiction of female hyena's reproductive system
- Hermaphrodite Hyenas? – Animal Life by Mary Ellen Schoeman
- Margarit, E.; Coll, M. D.; Oliva, R.; g�Mez, D.; Soler, A.; Ballesta, F. (2000). "SRY gene transferred to the long arm of the X chromosome in a Y-positive XX true hermaphrodite". American Journal of Medical Genetics 90 (1): 25–28. doi:10.1002/(SICI)1096-8628(20000103)90:1<25::AID-AJMG5>3.0.CO;2-5. PMID 10602113.
- Molnar, Sebastian (17 February 2004). "Plant Reproductive Systems". Evolution and the Origins of Life. Geocities.com. Archived from the original on 2009-10-22. Retrieved 12 September 2009.
- Voss, Heinz-Juergen: Sex In The Making – A Biological Account. Online: http://DasEndeDesSex.blogsport.de/images/voss_2011_sex_in_the_making.pdf
- Leyner, Mark; Goldberg M.D., Billy (2005). Why Do Men Have Nipples?: Hundreds of Questions You'd Only Ask a Doctor After Your Third Martini. New York: Three Rivers Press. ISBN 1-4000-8231-5. OCLC 57722472.
- Ovid, Metamorphoses, Book IV: The story of Hermaphroditus and Salmacis.
- Diodorus Siculus –- Book IV Chapters 1–7
Further reading 
- "Bony Fishes: Reproduction". SeaWorld/Busch Gardens Animal Infobooks. Busch Entertainment Corporation. 2009. Retrieved 12 September 2009.
- Discovery Health Channel, (2007) "I Am My Own Twin"
- Kim M.D., Kyu-Rae; Youngmee Kwon M.D., Jae Young Joung M.D., Kun Suk Kim M.D., Alberto G Ayala M.D. and Jae Y Ro M.D. (2002). "True Hermaphroditism and Mixed Gonadal Dysgenesis in Young Children: A Clinicopathologic Study of 10 Cases". Modern Pathology (Nature Publishing Group) 15 (10): 1013–1019. doi:10.1097/01.MP.0000027623.23885.0D. ISSN 0893-3952. OCLC 357415945. PMID 12379746. Retrieved 12 September 2009.
- Randall, John E. (2005). Reef and Shore Fishes of the South Pacific: New Caledonia to Tahiti and the Pitcairn Islands. Honolulu, Hawaii: University of Hawaii Press. pp. 346, 387. ISBN 0-8248-2698-1. OCLC 52152732.
- Chase, Cheryl (1998). "Affronting Reason". In Atkins, Dawn. Looking Queer: Body Image and Identity in Lesbian, Bisexual, Gay, and Transgender Communities. New York: Haworth Press. pp. 205–219. ISBN 978-1-56023-931-4. OCLC 38519315.
- Fausto-Sterling, Anne (12 March 1993). "How Many Sexes Are There?". The New York Times (New York). p. Op–Ed., reprinted in: Harwood, Sterling, ed. (1996). Business As Ethical and Business As Usual: Text, Readings, and Cases. Belmont, CA: Wadsworth Pub. pp. 168–170. ISBN 0-534-54251-4. OCLC 141382073.
- Grumbach, Melvin M.; Conte, F. A. (1998). "Disorders of sex differentiation". In Williams, Robert Hardin; Wilson, Jean D. Williams Textbook of Endocrinology. Philadelphia: Saunders. pp. 1303–1425. ISBN 0-7216-6152-1. OCLC 35364729.
- Schultheiss, Dirk; Herrmann, Thomas; Jonas, Udo (March 2006). "Early Photo-Illustration of a Hermaphrodite by the French Photographer and Artist Nadar in 1860". The Journal of Sexual Medicine 3 (2): 355–360. doi:10.1111/j.1743-6109.2005.00157.x. PMID 16490032. (subscription required)
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- Britannica Online Encyclopedia: hermaphroditism (biology)
- Current Biology – Gender trading in a hermaphrodite
- The Evolution of Self-Fertile Hermaphroditism: The Fog Is Clearing
- "Born True Hermaphrodite – Pictorial Profile", about Lynn Edward Harris