The individuals of many taxonomic groups of animals, primarily invertebrates, are hermaphrodites, capable of producing viable gametes of both sexes. In the great majority of tunicates, mollusks, and earthworms, hermaphroditism is a normal condition, enabling a form of sexual reproduction in which either partner can act as the female or male. Hermaphroditism is also found in some fish species, but is rare or absent in other vertebrate 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 of selfing 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 hermaphroditic species among mammals[5] or birds.[6]
About 94% of flowering plant species are either hermaphroditic (all flowers produce both male and female gametes) or monoecious, where both male and female flowers occur on the same plant. There are also mixed breeding systems, in both plants and animals, where hermaphrodite individuals coexist with males (called androdioecy) or with females (called gynodioecy), or all three exist in the same species (called trioecy). 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 with intersexuality, which is a separate and unrelated phenomenon. The usage of the term hermaphrodite to describe intersex people is considered to be offensive,[7] and it is also scientifically incorrect as hermaphrodism does not exist in humans.[8][9]
Etymology
The term hermaphrodite derives from the Latin: hermaphroditus, from Ancient Greek: ἑρμαφρόδιτος, romanized: hermaphroditos,[10] 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.[11] According to the earlier Diodorus Siculus, he was born with a physical body combining male and female sexes.[12] The word hermaphrodite entered the English lexicon as early as the late fourteenth century.[13]
Shells of Crepidula fornicata (common slipper shell)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 first develops as one sex, but can later change into the opposite sex.[14] This contrasts with simultaneous hermaphrodites, in which an individual possesses fully functional male and female genitalia. Sequential hermaphroditism is common in fish (particularly teleost fish) and many gastropods (such as the common slipper shell). Sequential hermaphrodites can only change sex once.[15] Sequential hermaphroditism can best be understood in terms of behavioral ecology and evolutionary life history theory, as described in the size-advantage mode[16] first proposed by Michael T. Ghiselin[17] 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.[14]
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 develops as a female, and then changes sex to a male.[14]
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 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.[18] 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.[14]
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 non-dominant superior behavior then it changes its sex to male.[19]
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.
Pulmonateland snails and land slugs 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 via spermatophores, and is then stored in the spermatheca. After exchange of spermatozoa, 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.[20]
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's radula), see apophallation. If a banana slug has lost its male sexual organ, it can still mate as a female, making hermaphroditism a valuable adaptation.[21]
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.[22][23]
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.
The free-living hermaphroditic nematode Caenorhabditis elegans reproduces primarily by self-fertilization, but infrequent out-crossing events occur at a rate of approximately 1%.[24]
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.[25][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.[26] This species is also regarded as the only known vertebrate species that can reproduce by self fertilization.[27]
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.[28][29] 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.[30]
The term hermaphrodite is used in botany to describe, for example, a perfectflower that has both staminate (male, pollen-producing) and carpellate (female, ovule-producing) parts. The overwhelming majority of flowering plant species are hermaphroditic.[31]
Monoecy
Flowering plant species with separate, imperfect, male and female flowers on the same individual are called monoecious. Monoecy only occurs in about 7% of flowering plant species.[32] 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.[33] 65% of gymnosperm species are dioecious, but conifers are almost all monoecious.[34] Some plants can change their sex throughout their lifetime, a phenomenon called sequential hermaphroditism.[citation needed]
Andromonecy
In andromonecious species, the plants produce perfect (hermaphrodite) flowers and separate fertile male flowers that are sterile as female.[35][36] Andromonecy occurs in about 4000 species of flowering plants (2% of flowering plants).[37]
Gynomonoecy
In gynomonoecious species, the plants produce hermaphrodite flowers and separate male-sterile pistillate flowers.[35] One example is the meadow saxifrage, Saxifraga granulata.[38] Charles Darwin gave several other examples in his 1877 book "The Different Forms of Flowers on Plants of the Same Species".[39]
About 57% of moss species and 68% of liverworts are unisexual, meaning that their gametophytes produce either male or female gametes, but not both.[40]: 377
Hermaphroditus, the "son" of the Greek god Hermes and the goddess Aphrodite, origin of the word "hermaphrodite"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.[41]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.[42]
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.").[43][44].
Alexander ab Alexandro (1461–1523) 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.[45]
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."[46][47]
During the Victorian era, medical authors attempted to ascertain whether or not humans could be hermaphrodites, adopting a precise biological definition to the term.[48] 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,[49][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,[50] a term that has been vigorously challenged.[51] This is particularly significant because of the relationship between medical terminology and medical intervention.[52]
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.[medical citation needed] One common cause of intersex traits 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.[55]
The evolution of anisogamy may have contributed to the evolution of simultaneous hermaphroditism and sequential hermaphroditism,[6] but it remains unclear if the evolution of anisogamy first led to hermaphroditism or gonochorism.[57]: 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].[58]
Simultaneous hermaphroditism that exclusively reproduces through self-fertilization has evolved many times in plants and animals, but it might not last long evolutionarily.[59]: 14
In animals
Joan Roughgarden and Priya Iyer argued that the last common ancestor for animals was hermaphroditic and that transitions from hermaphroditism to gonochorism were more numerous than the reverse. However, their argument was based on paraphyleticSpiralia, assignments of sexual modes for the phylum level than the species level, and methods exclusively based on maximum parsimony.[58]
It is widely accepted that the first vascular plants were outcrossing hermaphrodites.[62] In flowering plants, hermaphroditism is ancestral to dioecy.[63]
Hermaphroditism in plants may promote self fertilization in pioneer populations.[64] 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.[65]: 73, 74
^Jarne 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.
^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.
^ abSchärer 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).
^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
^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.
^Bagemihl, Bruce (1999). Biological exuberance : animal homosexuality and natural diversity (2nd ed.). New York: St. Martin's Press. ISBN978-0312253776.
^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, DLG; Hagiwara, A (2006). "Gonadal morphology in the self-fertilizing mangrove killifish, Kryptolebias marmoratus". Ichthyological Research. 53 (4): 427–430. doi:10.1007/s10228-006-0362-2. hdl:10069/35713. S2CID9474211.
^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.
^Stevens, D.P.; Richards, A.J. (1985). "Gynodioecy in Saxifraga granulata L.". Plant Systematics and Evolution. 151: 43–54. doi:10.1007/BF02418018. S2CID21632274.
^Darwin, Charles (1877). The different forms of flowers on plants of the same species. London: John Murray.
^Shaw, A.J. (2000). "12". In Shaw, A.J.; Goffinet, B. (eds.). Population ecology, population genetics, and microevolution. Cambridge, UK: Cambridge University Press. pp. 369–402. ISBN978-0-521-66794-4. ((cite book)): |work= ignored (help)
^chevalier de Jaucourt, Louis (1765). "Hermaphrodite". Encyclopedia of Diderot & d'Alembert. 8: 165–167. Retrieved 13 April 2023.
^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, US: 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)
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