Many taxonomic groups of animals (mostly invertebrates) do not have separate sexes.[4] In these groups, hermaphroditism is a normal condition, enabling a form of sexual reproduction in which either partner can act as the female or male. For example, the great majority of tunicates, pulmonate molluscs, opisthobranch, earthworms, 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. Animal species having different sexes, male and female, are called gonochoric, which is the opposite of hermaphrodite.[5][6][7]
There are also species where hermaphrodites exist alongside males (called androdioecy) or alongside females (called gynodioecy), or all three exist in the same species (called trioecy); these three systems are sometimes called mixed breeding systems. In plants there are cases where male flowers and hermaphrodite flowers occur on the same plant (andromonoecy) or female flowers and hermaphrodite flowers on the same plant (gynomonoecy).[8]
The term hermaphrodite is commonly used for abnormal cases of dioecious animal species but according to geneticist Michael Majerus this definition should be distinguished from the scientific definition.[9]
In recent years the term hermaphrodite applied to humans has fallen out of favor since there have been no identified cases of a human reproducing as both male and female,[10] with some biologists saying hermaphroditism cannot occur in humans.[11][12] Intersex activists have preferred the word intersex, since the word hermaphrodite is considered to be stigmatizing,[13][14][15][16] as well as "scientifically specious and clinically problematic."[17]
There are no hermaphroditic species among mammals[18][19]or birds.[20][21] According to David B. Rivers there is controversy around hermaphroditism in insects with some experts believing it does not occur.[22] Hermaphroditism is said to occur in one or two insect species.[23]
A rough estimate of the number of hermaphroditic animal species is 65,000.[24] The percentage of animal species that are hermaphroditic is about 5% in all animal species, or 33% excluding insects. (Although the current estimated total number of animal species is about 7.7 million, the study, which estimated the number, 65,000, used an estimated total number of animal species, 1,211,577 from "Classification phylogénétique du vivant (Vol. 2)" - Lecointre and Le Guyader (2001)). Most hermaphroditic species exhibit some degree of self-fertilization. The distribution of self-fertilization rates among animals is similar to that of plants, suggesting that similar pressures are operating to direct the evolution of selfing in animals and plants.[24]
Etymology
The term derives from the Latin: hermaphroditus, from Ancient Greek: ἑρμαφρόδιτος, romanized: hermaphroditos,[25] which derives from Hermaphroditus (Ἑρμαφρόδιτος), the son of Hermes and Aphrodite in Greek mythology. According to Ovid, he fused with the nymphSalmacis resulting in one individual possessing physical traits of male and female sexes;[26] according to the earlier Diodorus Siculus, he was born with a physical body combining male and female sexes.[27] The word hermaphrodite entered the English lexicon as early as the late fourteenth century.[28]Alexander ab Alexandro stated, using the term hermaphrodite, that the people who bore the sexes of both man and woman were regarded by the Athenians and the Romans as monsters, and thrown into the sea at Athens and into the Tiber at Rome.[29]
Clownfish are initially male; the largest fish in a group becomes a female.
Most species of parrotfish start life as females and later change into males.
Sequential hermaphrodites (dichogamy) occur in species in which the individual is born as one sex, but can later change into the opposite sex.[30] This contrasts simultaneous hermaphrodites, in which an individual may possess fully functional male and female genitalia. Sequential hermaphroditism is common in fish (particularly teleost fish) and many gastropods (such as the common slipper shell), and some flowering plants. Sequential hermaphrodites can only change sex once.[31] Sequential hermaphroditism can best be understood in terms of behavioral ecology and evolutionary life history theory, as described in the size-advantage mode[32] first proposed by Michael T. Ghiselin[33][better source needed] 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.[30]
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.[30]
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 female and male reproductive organs, but act as either female or male during different stages in life.[30]
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 female, and if the fish expresses dominant or not subordinate behavior then the fish changes its sex to male.[34]
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
Turbellarians mating by penis fencing. Each has two penises on the undersides of their heads which they use to inject sperm.
Earthworms are simultaneous hermaphrodites, having both male and female reproductive organs.
A simultaneous (or synchronous) hermaphrodite (or homogamous) is an adult organism that has both male and female sexual organs at the same time.[30]Simultaneous hermaphrodites can be regarded as both sexes present in the same individual.[2]Self-fertilization often occurs.
Reproductive system of gastropods: Pulmonateland 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 from early spring through late autumn.[35]
Banana slugs are another 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.[36]
The species of colourful sea slugsGoniobranchus reticulatus is hermaphroditic, with both male and female organs active at the same time during copulation. After mating, the external portion of the penis detaches, but is able to regrow within 24 hours.[37][38]
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.[39][failed verification]
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.[citation needed]
The free-living hermaphroditic nematode Caenorhabditis elegans reproduces primarily by self-fertilization, but infrequent out-crossing events occur at a rate of approximately 1%.[40]
The mangrove killifish (Kryptolebias marmoratus) is a species of fish that lives along the east coast of North, Central and South America. These fish are simultaneous hermaphrodites. K. marmoratus produces eggs and sperm by meiosis and routinely reproduces by self-fertilization. Each individual hermaphrodite normally fertilizes itself when an egg and sperm produced by an internal organ unite inside the fish's body.[41] This species is also regarded as the only known vertebrate species that can reproduce by self fertilization.[42]
When spotted hyenas were first scientifically observed 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 female spotted hyenas is in fact an enlarged clitoris, which contains an external birth canal.[43][44] 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.[45]
Hermaphrodite is used in botany to describe, for example, a flower that has both staminate (male, pollen-producing) and carpellate (female, ovule-producing) parts.[46]
Monoecy
Flowering plant species with separate male and female flowers on the same individual are called monoecious. Monoecious plants are often referred to as hermaphroditic because they produce both male and female gametes. However, the individual flowers are not hermaphroditic because they are only one sex.[47] Monoecy only occurs in about 7% of angiosperm species.[48]Conifers are almost all monoecious, but 65% of gymnosperm species are dioecious.[49] Some plants can change their sex throughout their lifetime. This process is called Sequential hermaphroditism.[citation needed]
Andromonecy
In andromonecious species, the plants produce perfect (hermaphrodite) flowers and separate staminate flowers that function as male but are sterile as female.[50] Andromonecy occurs in about 4000 species of flowering plants (2% of flowering plants).[51]
1860 photograph by Nadar of an intersex person displaying genitalia, one of a nine-part series. The series may be the earliest medical photographic documentation of an intersex person.[53]
Historically, the term hermaphrodite was used in law to refer to people whose sex was in doubt. The 12th-century Decretum Gratiani states that "Whether an hermaphrodite may witness a testament, depends on which sex prevails" ("Hermafroditus an ad testamentum adhiberi possit, qualitas sexus incalescentis ostendit.").[54][55] Similarly, the 17th-century English jurist and judge Edward Coke (Lord Coke), wrote in his Institutes of the Lawes of England on laws of succession stating, "Every heire is either a male, a female, or an hermaphrodite, that is both male and female. And an hermaphrodite (which is also called Androgynus) shall be heire, either as male or female, according to that kind of sexe which doth prevaile."[56][57]
During the Victorian era, medical authors attempted to ascertain whether or not humans could be hermaphrodites, adopting a precise biological definition to the term.[58] From that period until the early 21st century, intersex individuals were termed true hermaphrodites if their gonadal tissue contained both testicular and ovarian tissue, or pseudohermaphrodites if their external appearance (phenotype) differed from sex expected from internal gonads. This language has fallen out of favor due to misconceptions and pejorative connotations associated with the terms,[59][better source needed] and also a shift to nomenclature based on genetics.
The term intersex describes a wide variety of combinations of what are considered male and female biological characteristics. Intersex biology may include, for example, ambiguous-looking external genitalia, karyotypes that include mixed XX and XY chromosome pairs (46XX/46XY, 46XX/47XXY or 45X/XY mosaic). Clinically, medicine currently describes intersex people as having disorders of sex development, a term vigorously contested.[60][61] This is particularly because of a relationship between medical terminology and medical intervention.[62]
In some cases, intersex traits are caused by unusual levels of sex hormones, which may be the result of an atypical set of sex chromosomes. One possible pathophysiological explanation of intersex in humans 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 chromosome 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 being intersex is the crossing over of the testis-determining factor (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 the transcription factor (SOX9), and in other areas not being active (causing the growth of ovariantissues). Thus, testicular and ovarian tissues will both be present in the same individual.[65]
The evolution of anisogamy may have contributed to the evolution of simultaneous hermaphroditism and sequential hermaphroditism[67] but, as of 2016 it remains unclear if the evolution of anisogamy first led to hermaphroditism or gonochorism.[68]: 213 It is possible that hermaphroditism evolved from gonochorism, or vice versa. Most studies on its evolution focus on plants, and its evolution in animals is unclear as of December 2017[update].[69]
In animals
According to Nelson Çabej simultaneous hermaphroditism in animals most likely evolved due to a limited number of mating partners.[70]
In nematode species, self-fertilizing hermaphrodites evolved from gonochoric ancestors.[71]
It is widely accepted that the first vascular plants were outcrossing hermaphrodites.[72] In flowering plants, hermaphroditism is ancestral to dioecy.[73] Flowers in angiosperms evolved to attract insects as vectors for pollination.[74]
Hermaphroditism in plants may promote self fertilization in pioneer populations.[75] However, plants have evolved multiple different mechanisms to avoid self-fertilization in hermaphrodites, including sequential hermaphroditism, molecular recognition systems and mechanical or morphological mechanisms such as heterostyly.[76]: 73, 74
^Kerr AM, Baird AH, Hughes TP (January 2011). "Correlated evolution of sex and reproductive mode in corals (Anthozoa: Scleractinia)". Proceedings. Biological Sciences. 278 (1702): 75–81. doi:10.1098/rspb.2010.1196. PMC2992726. PMID20659935. For example, the cnidarian class Anthozoa (corals and anemones) is mainly comprised of gonochoric (separate sex) brooders or spawners, while one order, Scleractinia (skeleton-forming corals), appears to be mostly hermaphroditic spawners. Here, using the most complete phylogeny of scleractinians, we reconstruct how evolutionary transitions between sexual systems (gonochorism versus hermaphrodism) and reproductive modes (brooding versus spawning) have generated large-scale taxonomic patterns in these characters.
^Rosenfield KA (2018), "Hermaphrodite", in Vonk J, Shackelford T (eds.), Encyclopedia of Animal Cognition and Behavior, Cham: Springer International Publishing, pp. 1–2, doi:10.1007/978-3-319-47829-6_329-1, ISBN978-3-319-47829-6
^""I Want to Be Like Nature Made Me"". Human Rights Watch. 2017-07-25. Retrieved 28 May 2021. A term once commonly used to refer to individuals with intersex traits. It is now considered pejorative and outdated, although a small number of intersex people have reclaimed the term.
^Hinchcliffe N. "Protect Transgender and Intersex Patients". www.aafp.org. Retrieved 28 May 2021. The historical terms "hermaphrodite" and "pseudo-hermaphrodite" are outdated, pejorative, stigmatizing and no longer used in medical literature or research.
^Schärer L (February 2017). "The varied ways of being male and female". Molecular Reproduction and Development. 84 (2): 94–104. doi:10.1002/mrd.22775. PMID28032683. Of note, the otherwise well-studied insects, birds, and mammals are strikingly absent here—with not a single species among these groups showing hermaphroditism (for details on a supposedly hermaphroditic scale insect, however, see Gardner and Ross, 2011).
^ abJarne P, Auld JR (September 2006). "Animals mix it up too: the distribution of self-fertilization among hermaphroditic animals". Evolution; International Journal of Organic Evolution. 60 (9): 1816–24. doi:10.1554/06-246.1. PMID17089966. S2CID23849389.
^Oxford English Dictionary, 1st edn, s.v. hermaphrodite, n. and adj.; "Online Etymology Dictionary". Archived from the original on 5 November 2013. Retrieved 3 June 2012.
^"Hermaphrodite". Encyclopedia of Diderot & d'Alembert - Collaborative Translation Project. October 2003. Archived from the original on 2015-04-02. Retrieved 2015-04-01.
^Ghiselin MT (June 1969). "The evolution of hermaphroditism among animals". The Quarterly Review of Biology. 44 (2): 189–208. doi:10.1086/406066. PMID4901396. S2CID38139187.
^Rodgers EW, Early RL, Grober MA (2007). "Social status determines sexual phenotype in the bi-directional sex changing bluebanded goby Lythrypnus dalli". J Fish Biol. 70 (6): 1660–1668. doi:10.1111/j.1095-8649.2007.01427.x.
^Sakakura, Y., Soyano, K., Noakes, D.L.G. & Hagiwara, A. (2006). Gonadal morphology in the self-fertilizing mangrove killifish, Kryptolebias marmoratus. Ichthyological Research, Vol. 53, pp. 427-430
^Kanamori A, Yamamura A, Koshiba S, Lee JS, Orlando EF, Hori H (October 2006). "Methyltestosterone efficiently induces male development in the self-fertilizing hermaphrodite fish, Kryptolebias marmoratus". Genesis. 44 (10): 495–503. doi:10.1002/dvg.20240. PMID17029221. S2CID13639701.
^Molnar S (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.
^Vallejo-Marín M, Rausher MD (February 2007). "The role of male flowers in andromonoecious species: energetic costs and siring success in Solanum carolinense L". Evolution; International Journal of Organic Evolution. 61 (2): 404–12. doi:10.1111/j.1558-5646.2007.00031.x. PMID17348949. S2CID781464.
^E Coke, The First Part of the Institutes of the Laws of England, Institutes 8.a. (1st Am. Ed. 1812).
^Greenberg J (1999). "Defining Male and Female: Intersexuality and the Collision Between Law and Biology". Arizona Law Review. 41: 277–278. SSRN896307.
^Reis E (2009). Bodies in Doubt: an American History of Intersex. Baltimore: Johns Hopkins University Press. pp. 55–81. ISBN978-0-8018-9155-7.
^Judd, Walter S.; Campbell, Christopher S.; Kellogg, Elizabeth A.; Stevens, Peter F.; Donoghue, Michael J. (2002). Plant systematics, a phylogenetic approach (2 ed.). Sunderland MA, USA: Sinauer Associates Inc. ISBN0-87893-403-0.
Further reading
"Bony Fishes: Reproduction". SeaWorld/Busch Gardens Animal Infobooks. Busch Entertainment Corporation. 2009. Archived from the original on 25 July 2011. Retrieved 12 September 2009.
Discovery Health Channel, (2007) "I Am My Own Twin"
Randall JE (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. ISBN978-0-8248-2698-7. OCLC52152732.
Chase C (1998). "Affronting Reason". In Atkins D (ed.). Looking Queer: Body Image and Identity in Lesbian, Bisexual, Gay, and Transgender Communities. New York: Haworth Press. pp. 205–219. ISBN978-1-56023-931-4. OCLC38519315.
Fausto-Sterling A (12 March 1993). "How Many Sexes Are There?". The New York Times. New York. p. Op–Ed., reprinted in: Harwood S, ed. (1996). Business As Ethical and Business As Usual: Text, Readings, and Cases. Belmont, CA: Wadsworth Pub. pp. 168–170. ISBN978-0-534-54251-1. OCLC141382073.
Schultheiss D, Herrmann TR, Jonas U (March 2006). "Early photo-illustration of a hermaphrodite by the French photographer and artist Nadar in 1860". The Journal of Sexual Medicine. 3 (2): 355–60. doi:10.1111/j.1743-6109.2005.00157.x. PMID16490032. (subscription required)
![The Obando Fertility Rites in the Philippines, before becoming a Catholic festival, was initially an Anitist ritual dedicated to the hermaphrodite deity, Lakapati, who presided over fertility.[52]](http://upload.wikimedia.org/wikipedia/commons/thumb/0/05/Fertilityobandojf.JPG/440px-Fertilityobandojf.JPG)
![1860 photograph by Nadar of an intersex person displaying genitalia, one of a nine-part series. The series may be the earliest medical photographic documentation of an intersex person.[53]](http://upload.wikimedia.org/wikipedia/commons/thumb/2/2d/Nadar_-_%22Hermaphrodite%22_%28Seventh_Gallica_image%29.jpg/340px-Nadar_-_%22Hermaphrodite%22_%28Seventh_Gallica_image%29.jpg)
