The individuals of manytaxonomic groups ofanimals, primarily invertebrates, are hermaphrodites, capable of producing viable gametes of bothsexes. In the great majority oftunicates,mollusks, andearthworms, hermaphroditism is a normal condition, enabling a form of sexual reproduction in which either partner can act as the female or male. Hermaphroditism isalso found in some fish species, but is rare in othervertebrate groups. 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 ofselfing in animals and plants.[3]
A rough estimate of the number of hermaphroditic animal species is 65,000, about 5% of all animal species, or 33% excluding insects. Insects are almost exclusively gonochoric, and no definitive cases of hermaphroditism have been demonstrated in this group.[4] There are no known hermaphroditic species amongmammals[5] orbirds.[6]
About 94% offlowering plant species are either hermaphroditic (all flowers produce both male and female gametes) ormonoecious, where both male and female flowers occur on the same plant. There are alsomixed breeding systems, in both plants and animals, where hermaphrodite individuals coexist with males (calledandrodioecy) or with females (calledgynodioecy), or all three exist in the same species (calledtrioecy). Sometimes, both male and hermaphrodite flowers occur on the same plant (andromonoecy) or both female and hermaphrodite flowers occur on the same plant (gynomonoecy).
Hermaphrodism is not to be confused withovotesticular syndrome in mammals, which is a separate and unrelated phenomenon. While people with the condition were previously called "true hermaphrodites" in medical literature, this usage is now considered to be outdated as of 2006 and misleading,[7][8] as people with ovotesticular syndrome do not have functional sets of both male and female organs.[9][10]
The term hermaphrodite derives from theLatin:hermaphroditus, fromAncient Greek:ἑρμαφρόδιτος,romanized: hermaphroditos,[11] which derives fromHermaphroditus (Ἑρμαφρόδιτος), the son ofHermes andAphrodite inGreek mythology. According toOvid, he fused with thenymphSalmacis resulting in one individual possessing physical traits of male and female sexes.[12] According to the earlierDiodorus Siculus, he was born with a physical body combining male and female sexes.[13] The wordhermaphrodite entered theEnglish lexicon as early as the late fourteenth century.[14]
Shells ofCrepidula fornicata (common slipper shell)Clownfish are initially male; the largest fish in a group becomes a female.Most species ofparrotfish start life as females and later change into males.
Sequential hermaphrodites (dichogamy) occur inspecies in which the individual first develops as one sex, but can later change into the opposite sex.[15] (Definitions differ on whether sequential hermaphroditism encompasses serial hermaphroditism; for authors who exclude serial hermaphroditism, a sequential hermaphrodite is also stipulated to only change sex once.[16]) This contrasts with simultaneous hermaphrodites, in which an individual possesses fully functional male and female genitalia. Sequential hermaphroditism is common in fish (particularlyteleost fish) and manygastropods (such as thecommon slipper shell). Sequential hermaphroditism can best be understood in terms ofbehavioral ecology and evolutionarylife history theory, as described in the size-advantage mode[17] first proposed byMichael T. Ghiselin[18] 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 develops as a male, and then changes sex to a female.[15]
Example: Theclownfish (genusAmphiprion) are colorful reef fish found living insymbiosis withsea 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 tonatural selection.
Protogyny: Where the organism develops as a female, and then changes sex to a male.[15]
Example:Wrasses (FamilyLabridae) 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 females. They are not territorial. They are, perhaps, female mimics (which is why they are found swimming in group with females). Terminal phase males are territorial and have a distinctively bright coloration.[19] 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 may act either as a female or as a male during different stages in life.[15]
Example:Lythrypnus dalli (FamilyLythrypnus) are a group ofcoral 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 non-dominant superior behavior then it changes its sex to male.[20]
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 oftenaquacultured. Since the adults take several years to change from female to male, thebroodstock are extremely valuable individuals.
Turbellarians mating bypenis 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.
Pulmonateland snails and landslugs are perhaps the best-known kinds of simultaneous hermaphrodites, and are the most widespread of terrestrial animals possessing this sexual polymorphism. Sexual material is exchanged between both animals viaspermatophores, and is then stored in thespermatheca. After exchange ofspermatozoa, both animals will lay fertilized eggs after a period of gestation. The eggs will proceed to hatch after a development period. Snails typically reproduce from early spring through late autumn.[21]
Banana slugs are an example of a hermaphroditic gastropod. Mating with a partner is more desirable biologically than self-fertilization, 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'sradula), seeapophallation. If a banana slug has lost its male sexual organ, it can still mate as a female, making hermaphroditism a valuable adaptation.[22]
The species of colourfulsea 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.[23][24]
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 exchangegametes, copulating on damp nights during warm seasons.
The free-living hermaphroditic nematodeCaenorhabditis elegans reproduces primarily by self-fertilization, but infrequent out-crossing events occur at a rate of approximately 1%.[25]
Hamlets do not practice self-fertilization, but a pair will mate multiple times over several nights, taking turns between which one acts as the male and which acts as the female.[26][failed verification]
The mangrove killifish (Kryptolebias marmoratus) are simultaneous hermaphrodites, producing both eggs and sperm and routinely reproducing by self-fertilization. Each individual normally fertilizes itself when an egg and sperm produced by an internal organ unite inside the fish's body.[27] This species is also regarded as the only known vertebrate species that can reproduce byself fertilization.[28]
Whenspotted hyenas were first scientifically observed by explorers, they were thought to be hermaphrodites. Early observations of wild spotted hyenas led researchers to believe that all spotted hyenas, male or female, were born with what looked to be a penis. Afemale spotted hyena's apparent penis is in fact an enlarged clitoris, which contains an external birth canal.[29][30] It can be difficult to determine the sex of spotted hyenas untilsexual maturity, when they may become pregnant. When a female spotted hyena gives birth, she passes the cub through the cervix internally, but then passes it out through the elongated clitoris.[31]
The termhermaphrodite is used inbotany to describe, for example, aperfectflower that has bothstaminate (male, pollen-producing) andcarpellate (female, ovule-producing) parts. The overwhelming majority of flowering plant species are hermaphroditic.[32]
Flowering plant species with separate, imperfect, male and female flowers on the same individual are calledmonoecious. Monoecy only occurs in about 7% of flowering plant species.[33] Monoecious plants are often referred to as hermaphroditic because they produce both male and female gametes. However, the individual flowers are not hermaphroditic if they only produce gametes of one sex.[34] 65% ofgymnosperm species are dioecious, butconifers are almost all monoecious.[35] Some plants can change their sex throughout their lifetime, a phenomenon calledsequential hermaphroditism.[citation needed]
Inandromonoecious species, the plants produceperfect (hermaphrodite) flowers and separate fertile male flowers that are sterile as female.[36][37] Andromonoecy occurs in about 4000 species of flowering plants (2% of flowering plants).[38]
Ingynomonoecious species, the plants produce hermaphrodite flowers and separate male-sterile pistillate flowers.[36] One example is the meadow saxifrage,Saxifraga granulata.[39] Charles Darwin gave several other examples in his 1877 book "The Different Forms of Flowers on Plants of the Same Species".[40]
About 57% of moss species and 68% of liverworts areunisexual, meaning that their gametophytes produce either male or female gametes, but not both.[41]: 377
Hermaphroditus, the "son" of the Greek godHermes and the goddessAphrodite, origin of the word "hermaphrodite"TheObando Fertility Rites in thePhilippines, before becoming a Catholic festival, was initially anAnitist ritual dedicated to the hermaphrodite deity, Lakapati, who presided over fertility.[42]1860 photograph byNadar of a person displaying ambiguous genitalia, one of anine-part series. The series may be the earliest medical photographic documentation of an intersex person.[43]: 358
Historically, the term hermaphrodite was used in law to refer to people whose sex was in doubt. The 12th-centuryDecretum 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.").[44][45]
Alexander ab Alexandro (1461–1523) stated, using the termhermaphrodite, 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.[46] Similarly, the 17th-century English jurist and judgeEdward Coke (Lord Coke), wrote in hisInstitutes 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 calledAndrogynus) shall be heire, either as male or female, according to that kind of sexe which doth prevaile."[47][48]
During theVictorian era, medical authors attempted to ascertain whether or not humans could be hermaphrodites, adopting a precise biological definition to the term.[49] From that period until the early 21st century, individuals withovotesticular syndrome were termedtrue hermaphrodites if theirgonadal tissue contained both testicular and ovarian tissue, andpseudohermaphrodites if their external appearance (phenotype) differed from sex expected from internal gonads. This language has fallen out of favor due to misconceptions and stigma associated with the terms,[50][51][8][10] and also a shift to nomenclature based on genetics.
The term "intersex" described a wide variety of combinations of what are ambiguous 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/XYmosaic). Clinically, medicine currently uses the terminology "disorders of sex development"[52] (also known asvariations in sex characteristics.)[53] This is particularly significant because of the relationship between medical terminology and medical intervention.[54]
In some cases, variations in sex characteristics are caused by unusual levels of sex hormones, which may be the result of an atypical set of sex chromosomes.[medical citation needed] One common cause of variations in sex characteristics traits is the crossing over of thetestis-determining factor (SRY) from the Y chromosome to the X chromosome duringmeiosis. The SRY is then activated in only certain areas, causing development oftestes in some areas by beginning a series of events starting with the upregulation of thetranscription factor (SOX9), and in other areas not being active (causing the growth ofovariantissues). Thus,testicular and ovarian tissues will both be present in the same individual.[57] Of all total recorded cases of ovotesticular DSD, in only 8% percent of all cases was SRY present, leaving the rest of cases that could be explained to other or less common causes, with the vast majority simply being currently unexplainable.
The evolution ofanisogamy may have contributed to the evolution of simultaneous hermaphroditism and sequential hermaphroditism,[61] it remains unclear if the evolution of anisogamy first led to hermaphroditism orgonochorism.[62]: 213
A 2023 study argued that hermaphroditism can evolve directly frommating types under certain circumstances, such as if the fertilization is well organized and the average size of groups is small.[63] Simultaneous hermaphroditism that exclusively reproduces through self-fertilization has evolved many times in plants and animals, but it might not last long evolutionarily.[64]: 14
Joan Roughgarden and Priya Iyer argued that the lastcommon ancestor for animals was hermaphroditic and that transitions from hermaphroditism to gonochorism were more numerous than the reverse. Other scientists have criticized this argument; saying it’s based onparaphyleticSpiralia, assignments of sexual modes for thephylum level than the species level, and methods exclusively based onmaximum parsimony.[65]
It is widely accepted that the firstvascular plants were outcrossing hermaphrodites.[68] In flowering plants, hermaphroditism is ancestral to dioecy.[69]
Hermaphroditism in plants may promote self fertilization in pioneer populations.[70] However, plants have evolved multiple different mechanisms to avoid self-fertilization in hermaphrodites, includingsequential hermaphroditism, molecular recognition systems and mechanical or morphological mechanisms such asheterostyly.[71]: 73, 74
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^Royer M (1975). "Hermaphroditism in Insects. Studies on Icerya purchasi". In Reinboth R (ed.).Intersexuality in the Animal Kingdom. Berlin, Heidelberg: Springer. pp. 135–145.doi:10.1007/978-3-642-66069-6_14.ISBN978-3-642-66071-9.
^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).
^ab"Is a person who is intersex a hermaphrodite?".Intersex Society of North America. Retrieved2024-01-20.The mythological term "hermaphrodite" implies that a person is both fully male and fully female. This is a physiologic impossibility. The words "hermaphrodite" and "pseudo-hermaphrodite" are stigmatizing and misleading words.
^abMills A (2018-01-01).Biology of Sex. University of Toronto Press. p. 309.ISBN978-1-4875-9337-7.In the past, the termhermaphrodite was widely applied in such cases, but humans are not hermaphroditic. In a truly hermaphroditic species, individuals have functional sets of male and female organs.
^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 fromthe original on 2009-10-22. Retrieved12 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.
^Stevens D, Richards A (1985). "Gynodioecy inSaxifraga granulata L.".Plant Systematics and Evolution.151:43–54.doi:10.1007/BF02418018.S2CID21632274.
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^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)
^Vilain E, Achermann JC, Eugster EA, Harley VR, Morel Y, Wilson JD, et al. (February 2007)."We used to call them hermaphrodites".Genetics in Medicine.9 (2):65–66.doi:10.1097/GIM.0b013e31802cffcf.PMID17304046.use of the words "hermaphrodite," "pseudohermaphrodite," and "intersex" should be abandoned, as they either are confusing or have a negative social connotation that may be perceived as harmful by some patients and parents.
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