Intersex people are individuals born with any of several variations in sex characteristics including chromosomesgonadssex hormonesor genitals that, according to the UN Office of the High Commissioner for Human Rights"do not fit the typical definitions for male or female bodies". Intersex people were previously referred to as hermaphrodites or "congenital eunuchs".
It was the first attempt sex creating a taxonomic classification system of intersex conditions. Intersex people were categorized as either having true hermaphroditismfemale pseudohermaphroditismor male pseudohermaphroditism.
Intersex people face stigmatization and discrimination from birth, or from discovery of an zex trait, such as from puberty. This may include infanticide, hremofrodit and the stigmatization of families.
However, this is considered controversial, with no firm evidence of favorable outcomes. Adults, including elite female athletes, have also been subjects of such treatment. Some intersex persons may be assigned and raised as a girl or boy but then identify with another gender later in life, while most continue to identify with their assigned sex. Intersex people are born with sex characteristics including genitals, gonads and chromosome patterns that do not fit typical binary notions of male or female bodies.
Intersex is an umbrella term hrmofrodit to describe a wide range of natural bodily variations. In some cases, intersex traits are visible at birth while in others, they are not apparent until puberty. Some chromosomal hermofrodit variations may not be physically apparent at all. In biological terms, sex may be determined by a number of factors present at birth, including: .
People whose characteristics are not either all typically male or all typically female at birth are intersex. Some intersex traits are not always visible at birth; some babies may be born with ambiguous genitals, while others may have ambiguous internal organs testes and ovaries. Others will not become aware that they are intersex unless they receive genetic testing, because it does not manifest in their phenotype.
Whether or not they were socially tolerated or accepted by any particular culture, the existence of intersex people was known to many ancient and pre-modern cultures. The Greek historian Diodorus Siculus wrote hermofrorit the mythological Hermaphroditus in the first century BCE, who was "born with a physical body which is a combination of that of a man and that of hermofrodit woman", and reputedly possessed supernatural properties.
In European societies, Roman lawpost-classical canon lawand later common lawreferred to a person's sex as male, female or hermaphrodite, with legal rights as male or female depending on the characteristics that appeared most dominant. Some of these cultures, for instance the South-Asian Hijra communities,  may include intersex people in a hermodrodit gender category.
Although—according to Morgan Holmes —early Western anthropologists categorized such cultures "primitive," Holmes has argued that analyses of these cultures have been simplistic or romanticized and fail to take account of the ways that subjects of all categories are treated.
During the Victorian eramedical authors introduced the terms " true hermaphrodite " for an individual who has both ovarian and testicular tissue, "male pseudo-hermaphrodite" for a person with testicular tissue, but either female or ambiguous sexual anatomy, and "female pseudo-hermaphrodite" for a person with ovarian tissue, but sex male or ambiguous sexual anatomy.
Some later shifts in terminology have reflected advances in genetics, while other shifts are suggested to be due to pejorative nermofrodit. The term intersexuality was coined by Richard Goldschmidt in Since the rise of modern medical science, some intersex people with ambiguous external genitalia have had their genitalia surgically jermofrodit to resemble either female or male genitals.
Surgeons pinpointed intersex babies as a "social emergency" when born. Dialogue between what were once antagonistic groups of activists and sex has led to only slight changes in hermofrodit policies and how intersex patients and their families are treated in some locations.
Human rights institutions are placing increasing scrutiny on harmful practices and issues of discrimination against intersex people. These issues have been addressed by a rapidly increasing number of international institutions including, inthe Council of Europe, sxe United Nations Office of the United Nations High Commissioner for Human Rights and the World Health Organization.
These developments have been accompanied by International Intersex Forums and increased cooperation amongst civil society organizations. However, the implementation, codification, and enforcement of intersex human rights in national legal systems remains slow. Stigmatization and discrimination from birth may include infanticide, abandonment, and the stigmatization of families. As noted in the "Intersex human rights" page, the birth of an intersex child was often viewed as a curse or a sign of a hermofrodih mother, especially in parts of Africa.
Infants, children and adolescents also experience "normalising" interventions on intersex persons that are medically unnecessary and the pathologisation of variations in sex characteristics. In countries where the human rights of intersex people have been studied, medical interventions to modify the sex characteristics of intersex people have still taken place without the consent of the intersex person.
Such interventions have been criticized by the World Health Organization, other UN bodies such as the Office of the High Commissioner for Human Rights, and an increasing number of regional and national institutions due to their sex consequences, including trauma, impact on sexual function and sensation, and hermoffrodit of rights to physical and mental integrity.
People born with intersex bodies are seen as different, intersex infants, hermofridit, adolescents and adults "are often stigmatized and subjected to multiple human rights violations", including discrimination in education, healthcare, employment, sport, and public services. Access to informationmedical records, peer and other counselling and support.
With the rise of modern medical science in Western societies, a secrecy-based model was also adopted, in the belief that this was necessary to ensure "normal" physical and psychosocial development. The Asia Pacific Forum of National Human Rights Institutions states that legal recognition is firstly "about intersex people who have been issued a male or a female birth certificate being able to enjoy the same legal rights as other men and women.
A Kenyan court case in established the right of an intersex boy, "Baby A", yermofrodit a birth certificate.
Like all individuals, some intersex individuals may be raised as a certain sex male or female but then identify with another later in life, while most do not.
Research in the late 20th century led to a growing medical consensus that diverse intersex bodies are normal, but relatively rare, forms of human biology.
Foremost, we advocate use of the terms "typical", "usual", or "most frequent" where it is more common to use the term "normal. Emphasize that all of these conditions are biologically understandable while they are statistically uncommon. Some people with intersex traits self-identify as intersex, and some do not. Some intersex organizations reference "intersex people" and "intersex variations sex traits"  while others use more medicalized language such as "people with intersex conditions",  or people "with intersex conditions or DSDs differences of sex development " and "children born with variations of sex anatomy".
A hermofrodit is an organism that has both male and female reproductive organs. Until the sex century, "hermaphrodite" was used synonymously with "intersex".
Currently, hermaphroditism is not to be confused with intersex, as the former refers only to a specific phenotypical presentation of sex organs and the latter to a more complex combination of phenotypical and genotypical presentation.
Using hermaphrodite to refer to intersex individuals is considered to be stigmatizing and misleading. Members of the Lawson Wilkins Pediatric Ses Society and sex European Society for Paediatric Endocrinology adopted this term in their "Consensus statement on management hermofrodit intersex disorders".
Alternatives to categorizing intersex conditions as "disorders" have been suggested, including "variations of sex development". Intersex can be contrasted with homosexuality or same-sex attraction. Intersex can therefore be contrasted with transgender which is the condition in which one's gender identity does not match one's assigned sex. The relationship of intersex to lesbian, gay, bisexual and trans, and queer communities is complex,  but intersex people are often added to LGBT to create an LGBTI community.
Emi Koyama describes how inclusion of intersex in LGBTI can fail to address intersex-specific human rights issues, including creating false impressions "that intersex people's rights are protected" by laws protecting LGBT people, and failing to acknowledge that many intersex people are not LGBT.
Television works about intersex and films about intersex are scarce. Intersex peer support and advocacy organizations have existed since at leasthermofroddit the establishment of the Androgen Insensitivity Syndrome Support Group Australia in Intersex Awareness Day is an internationally observed civil awareness day designed to highlight the challenges faced by intersex people, occurring annually on 26 October.
It marks the first public demonstration by intersex people, which took place in Boston on 26 Octoberoutside a venue where the American Academy of Pediatrics was holding its annual conference. Intersex Day of Remembrancealso known as Intersex Solidarity Day, is an internationally observed civil awareness day designed to highlight issues faced by intersex people, occurring annually on 8 November.
In HinduismSangam literature uses the word pedi to refer to people born with an intersex condition; it also refers to antharlinga hijras and various other hijras.
In Islamscholars of Islamic jurisprudence have detailed discussions on the status and rights of intersex based on what mainly exhibits in their external sexual organs. Yet, modern Islamic jurisprudence scholars turn to medical screening to hermofrodiy the dominance of their sex.
The intersex rights include rights of inheritance, rights to marriage, rights to live like any other male or female. The rights are generally based on whether they are true hermaphrodites or sex. Scholars of Islamic jurisprudence generally consider their rights based on the majority of sex appears from their external sexual organs. In Judaismthe Talmud contains hermofrodit discussion concerning the status of two intersex types in Jewish law; namely the androgynouswhich exhibits both male and female external sexual organs, and the tumtum which exhibits neither.
In the s and s, the treatment of intersex babies started to be discussed in Orthodox Jewish medical halacha by prominent rabbinic leaders, for example Eliezer Waldenberg and Moshe Feinstein. In Anitismthe wife of Bathalathe supreme hermofrodot of the Tagalog peoplewas the hermaphrodite deity Lakapati, who served as queen of hermofrodit celestial abode and court called Kaluwalhatian.
She was also the ancient hermofrodit of fertility and is highly regarded as the Tagalog pantheon's most important feminine figure. Her relationship with the sed god, Bathala, was symbolic for the ancient Tagalogs as it referred to marriage as a mutual bond between two parties regardless of gender, which was a common hermofrodlt at the time.
The chant and prayer portrayed Lakapati as an all-powerful deity who had control of one's life. Prominent among deities who received full-blown sacrifices, Lakapati is fittingly represented by a her,ofrodit image with both male and female parts and was worshiped in the fields at planting time.
Her bodily expression is notably feminine. The ancient Tagalogs believed that the hermaphrodite image of Lakapati depicted the "balance of everything". During early Spanish rule, Lakapati was depicted as the Holy Spirit, as the people continued to revere her despite Spanish threats. Modern interpretations have stated that Lakapati was transgender, although in a historical context, Lakapati was known as a hermaphrodite or intersex and not a transgender person.
The South African middle-distance runner Caster Semenya won gold at the World Championships in the women's metres and won silver in the Summer Olympics. The results were not released. Semenya was ruled eligible to compete. Katrina KarkazisRebecca Jordan-YoungGeorgiann Davis and Silvia Hermocrodit have claimed that IAAF policies on "hyperandrogenism" in female athletes, are "significantly flawed", arguing that the policy will not protect against breaches of hermifrodit, will require athletes to undergo unnecessary treatment in order to compete, and will intensify "gender policing".
They recommend that athletes be able to compete in accordance with their legally recognised gender. In Aprilthe BMJ reported that four elite women athletes with 5-ARD an intersex medical condition were subjected to sterilization hermofrodit "partial clitoridectomies" in order to compete in sport. The authors noted that partial clitoridectomy was "not medically indicated" and "does not relate to real or perceived athletic 'advantage'.
There are few firm estimates of the number of intersex people. The now-defunct Intersex Society of North America stated that:. If herrmofrodit ask experts at medical centers how often a child is born so noticeably atypical in terms of genitalia that a specialist in sex differentiation is called in, the number comes out to about 1 in to 1 in births [0. But a lot more people than that are born with subtler forms of sex anatomy variations, some of which won't show up hermofrodit later in life.
Blackless, Fausto-Sterling et al. The figure of 1. Individuals with diagnoses of disorders of sex development DSD may or may not experience stigma and discrimination due to their sex characteristics, including sex "normalizing" interventions. Human rights institutions have called for the de-medicalization of intersex traits, as far as possible. The following summarizes some prevalence figures of intersex traits a fuller sex of conditions' is provided below, at the end of 'Medical classifications' :.
Population figures can vary due to genetic hermlfrodit. In the Dominican Republic5-alpha-reductase deficiency is not uncommon in the town of Las Salinasresulting in social acceptance of the intersex trait. The overall incidence for the town was 1 in every 90 males were carriers, with other males either non-carriers or non-affected carriers.
Janet L. Over the last years, research has established that a sexual selection exists and is widespread in the plant and animal kingdoms; b it does not necessarily entail sexual dimorphism; even hermaphrodites have it; c it does not require intelligence or a sophisticated sense of esthetics; even tapeworms and plants choose mates; and d it does not require brawn or even mobility for competition; plants may compete for pollinators, and broadcast spawning invertebrates may also compete for matings.
Although discussions of sexual selection often focus on sexual dimorphism, several phenomena that are commonly associated with sexual selection are widespread and highly developed in hermaphrodites.
These phenomena include a bizarre and expensive courtship and copulatory behavior, b multiple mating and sperm competition, c rapid evolution of genitalia, d special structures associated with courtship, and e sexual polymorphism. The skewed breeding sex ratios associated with sequential hermaphroditism have long been recognized as contributory to sexual selection. In many simultaneous hermaphrodites, although the sex ratio at mating may be one to one, the actual reproductive sex ratio may also be skewed, creating a high potential for sexual selection.
Reproductive biology in hermaphroditic taxa also involves a lot of complexity unknown in dioecious taxa, such as sex change, facultative sex allocation and conditional reciprocity that offers opportunities to enrich our understanding of sexual selection and to test the assumptions and predictions of theory.
Sexual selection has come to be seen as a keystone of Charles Darwin's theory of evolution by natural selection, being the exception that proves the rule that evolution proceeds through differential reproduction see Ghiselin b for discussion.
Famously, Darwin developed the theory of sexual selection to account for certain traits such as the weapons used in male-male competition for example, a stag's antlers or the ornaments used to attract members of the opposite sex for example, a peacock's tail which seemed to be very important in obtaining mates but unimportant otherwise.
In his view, where females and males share the same habitat, food sources, predators, and so on, sexually dimorphic characters must have evolved as a hermofrodit of differential mating success. The first questions to address, then, are the nature of sexual selection and how it might apply to hermaphrodites. Sexual selection is a term that has meant different things to different people.
In a recent review, Tim Clutton-Brock listed 9 different definitions sex sexual selection and the list is not exhaustive see Table 1. Of those definitions, sex involve sexual dimorphism sex differences and 2 refer only to males.
However, the essence of sexual selection as Darwin defined it is selection through competition for mates. This depends not on a struggle for existence, but on a struggle between the males for the possession of the females; the result is not death to the unsuccessful competitor, but few or no offspring. As Andersson has pointed out, this definition can be applied to all organisms, including plants, since as in ecology, the effect of competition will be the same whether it occurs as interference competition male-male combat, for examplescramble competition sperm competition; pollen competitionor indirectly such as competition to be chosen by females or by pollinators see Levitan ; Skogsmyr and Lankinen ; Delph and Ashman ; Thomson Andersson's general definition of sexual selection applies to all forms of hermaphrodites as well as taxa, such as protists, that lack anisogamy, making it appropriate to test hypotheses about the origins of sexual selection contrary to Grant Therefore, consideration of hermaphrodites shows us the weaknesses of definitions of sexual selection that are specific to sexually dimorphic traits or to males alone.
The next questions concern how to identify and measure sexual selection and what its sources may be in hermaphrodites. The hermofrodit explanation for the rule of male-male competition and female choice was seen by Darwin as anisogamy.
In surprisingly modern arguments he traced the source of male eagerness and female coyness to 1 the motility of sperm vs. Therefore, as is so often the case, in his description and definition of sexual selection, Darwin identified most of the issues that occupy us today. Bateman hypothesized that males compete for mates and females do not because reproductive success hermofrodit females is limited by the resources available for egg production, that is, the female gain curve plateaus, whereas the reproductive success of males is limited only by access to females, and the male gain curve is proportional to the number of mates or eggs available.
This hypothesis has been very influential, but problems with it have begun to be identified see Hubbell and Johnson ; Arnold and Duvall ; Gowaty ; Leonard ; see Tang-Martinez and Ryder for discussion. Another factor that has been used to explain the pattern of male-male competition and female choice in dieocious animals Alexander and Borgia and the preferred sexual role in simultaneous hermaphrodites Leonard and Lukowiak is the control of fertilization.
Alexander and Borgia argued that a fundamental difference between males and females is that in general, females retain a greater degree of control over the fate of their gametes than do males. The arguments of Bill Eberhard and Patty Gowaty suggest that female control of fertilization may represent a form of mate choice.
At present, a popular hypothesis is that multiple factors may be at work to set the stage for sexual selection see Shuster and Wade Table 2 lists factors that have been considered to be important in determining the strength and direction of sexual selection by producing differences between the sexes or sexual roles in variance in reproductive success and consequently skewed breeding sex ratio BSR.
The great difficulty is to determine which differences between the sexes are causes of sexual selection and which are effects. In hermaphrodites each individual acts as hermofrodit male and female and can act as its own control. Identification of the preferred sexual role in hermaphrodites is one way of sex alternative hypotheses as to the source of sexual conflict see following sections; review in Leonard Variance in reproductive success has long been of interest as a measure of sexual selection Bateman ; Payne ; Fincke ; Raffetto and others ; etc.
The theoretical upper limit to the intensity of selection on a population is given by Crow's Index; the ratio of the variance in sex number to the square of the mean number of progeny Crow ; Shuster and Wade The use of Crow's Index as a basis for measuring the intensity of sexual selection Wade ; Shuster and Wade is an important development in sexual selection theory but the current usage, I mates Shuster and Wadeis inappropriate for hermaphroditic taxa because it focuses on male mating success as a fraction of the total.
With separate sexes, in each generation sexual selection acting on males will be separate from that acting on females. Sex either sequential or simultaneous hermaphrodites however, sexual selection on an individual comes from the total of its success in both sexual roles.
An appropriate measure of sexual selection for hermaphrodites would therefore have to include variance in total reproductive success. Clearly, the intensity of sexual selection cannot be larger than total selection so that Crow's Index represents a theoretical upper limit to the strength of sexual selection. In reality, sexual selection will seldom or never reach this upper limit since other forces of selection are expected to be in operation, although it might be possible to construct experimental conditions that would bring a population close.
Also, it should be emphasized that the magnitude of the variance is a measure of the potential for selection and sex a measure of the intensity of selection see discussion Wade ; Grafen ; random factors could also produce variance in reproductive success see Sutherland However, differences in variance in reproductive success among populations with similar life histories and biology, would offer a first approximation to the potential for sexual selection.
Defining sexual selection as the product of competition with conspecifics for reproductive opportunities is clear in principle, however identifying and measuring it in practice is far from simple, as Darwin predicted.
Despite the increasingly sophisticated approaches of quantitative genetics that are beginning to be employed Holland and Rice ; Shuster and Wade ; Mead and Arnoldmost of sexual selection research still relies on the same sorts of evidence that attracted Darwin's attention; the morphology, sexual behavior, and social structure of animals.
Although the biology of many hermaphroditic taxa, particularly invertebrate animals, remains poorly known, review of the literature on hermaphrodite mating systems shows evidence for a variety of phenomena that are associated with sexual selection in dieocious organisms Table 3. Male-male competition hermofrodit been the major focus of sexual selection research in dioecious species see Table 1 and among hermaphrodites has been long studied in plants in the form of pollen competition see reviews in Willson and Burley ; Delph and Havens ; Skogsmyr and Lankinen ; Delph and Ashman ; Thomson More indirect competition in the form of competition for pollinators has been an important evolutionary force in angiosperms and a very active field of research see reviews in Skogsmyr and Lankinen ; Thomson Less work has been done in animals, but there is strong circumstantial and some direct evidence to suggest that sperm competition is common and important in many internally fertilizing simultaneously hermaphroditic taxa, especially gastropods and flatworms see sex in Baur ; Michiels The sperm-cast mating system of sessile hermaphroditic animals that brood eggs may lead to significant levels of sperm competition see review in Bishop and Pemberton Multiple mating and hermofrodit storage are common in hermaphroditic animals, including many androdieocious taxa see review in Weeks and others Direct evidence for sperm competition is available from a flatworm Pongratz and Michielsa leech Tan and othersand the gastropods, H.
Some serranines also have harem polygamy see following section and Petersen and an increase in male mating opportunities seems to be associated with social dominance in this group Leonard Fighting between males has been reported in a sequentially hermaphroditic polychaete see discussion in Berglund; Sella and Ramella It seems likely that classic interference competition for access to mates occurs in other hermaphrodites, although documented examples are rare.
This may reflect either a lack of attention to the phenomenon or a difference in the biology of hermaphrodites. For example, in many androdioecious animals, males are rare relative to self-fertilizing hermaphrodites see review in Weeks and others so that competition among males for access to hermaphrodites may seldom occur. The prevalence of conditional reciprocity in mating systems in simultaneous hermaphrodites see discussion that follows and in Leonard may inhibit the success of interference competition.
If this is true, species with hypodermic insemination, unilateral copulation without reciprocity, or chain copulation may be fruitful sources of evidence for direct interference competition. Competition among hermaphrodites for access to a partner's sperm or for matings in the female role has not been studied but is theoretically possible see discussion in Leonard Female choice of mates is a key aspect of classical sexual selection.
In an elegant series of studies, Webster and colleagues see review in Webster and Gower have demonstrated that Biomphalaria glabrata snails with resistant genotypes discriminate against mates on the basis of parasitic infection. There is also evidence that simultaneous hermaphrodites may discriminate among sperm on the basis of whether spermatophores were exchanged reciprocally, in an opisthobranch Karlsson and Haase and in the stylommatophoran Cantareus Helix aspersaon the basis of whether a dart was received review in Koene although, in the latter case, it is not clear whether cryptic female choice or sperm competition is involved.
In a planarian, individuals gave more sperm to previously isolated individuals, suggesting a preference for lower sperm competition Michiels and Bakovski In some simultaneously hermaphroditic serranine fish, individuals with eggs to spawn mate preferentially with larger harem-holding hermaphrodites or pure males see reviews in Leonard ; Petersen Data on the number of offspring sired by first vs.
Selective abortion in plants may represent a form of cryptic female choice see reviews in Willson and Burley ; Skogsmyr and Lankinenalthough in plants female choice may be difficult to distinguish from parent-offspring conflict Skogsmyr and Lankinen Morphological sexual dimorphism is common in sequential hermaphrodites such as the well-studied examples of fish with social control of sex change sex example, Warner and others ; Shapiro Monoecious plants, that is, those with separate male and female flowers on the hermofrodit individual, may have sexually dimorphic flowers Barrett Ectoprocts with morphologically distinct male and female zooids in a hermaphroditic colony also may be considered to have sexual dimorphism see Bishop and Pemberton There is one report McLauchlan of stylommatophoran land snails with a shell dimorphism that allegedly reflects differences in sexual behavior associated with age and size.
What is more common in simultaneously hermaphroditic taxa than morphological sexual dimorphism is behavioral sexual dimorphism. That is, it is sometimes the case that individuals will behave quite differently when mating in one role rather than the other. For example, the opisthobranch sea slug Navanax inermis copulates unilaterally with one individual acting as male and the other female and this is associated with distinct male and female courtship behaviors Leonard and Lukowiak This is also the case in another opisthobranch, Aplysia californica Leonard and Lukowiak and in the basommatophorans Lymnaea stagnalis review in KoenePhysa acuta Ohbayashi-Hodoki and others and Biomphalaria glabrata Webster and Gower Simultaneously hermaphroditic serranine fish also show sexually dimorphic courtship behavior Fischer ; Petersen and in both Navanax and the serranines there is a preference for mating in one of the sexual roles with Navanax preferring the female role Leonard and Lukowiak ; Michiels and others and the serranines preferring the male role Leonard ; Petersen Such behavioral dimorphism will probably sex to be more widespread when more hermaphroditic taxa have been studied in detail.
In contrast, a species of stylommatophoran banana slug with unilateral intromission Ariolimax californicus has symmetrical courtship behavior Leonard and others The factors that account for asymmetrical vs. Perhaps the most basic criterion for suspecting sexual selection in a species is the observation of sexual behavior or other reproductive characters that seem bizarre, or sex to be costly in terms of either energy, time or risk of harm to the possessor.
Perhaps even more spectacular is the aerial mating of the land slug Limax maximus in which a pair of simultaneous hermaphrodites climb a tree or wall to an overhang, drop down on 10—25 cm of mucus strands and, hanging, exchange spermatophores between entertwined penes, without intromission Gerhardt ; Langlois The energetic cost of locomotion and mucus production in stylommatophorans is high Denny and mucus production is a major source of water loss in these terrestrial mollusks Luchtel and Deyrup-Olsen The mucus threads are often eaten after copulation, suggesting that the material content of the threads represents an important resource.
Limax is one of the many hermaphrodites that will repay investigation from the standpoint of sexual selection. Another bizarre behavior is the apophallation found in two species of Ariolimax slugs, whereby copulation is occasionally terminated by amputation of the penis hermofrodit one or both individuals; Heath ; Mead ; Leonard and others A similar behavior has been reported for the genus Deroceras Rymzhanov with the difference that in this case, individuals are reported to amputate their own penis after a unilateral copulation and present it to the partner.
Michiels and Koene argue, on theoretical grounds, that sexual behavior involving damage to the individual acting as a female is particularly likely to evolve in hermaphrodites.
One of the most important differences between sexual selection in dioecious vs. That is, in each generation males compete with males for their contribution to the next generation, and females compete with females. Sexual conflict is an epiphenomenon. In hermaphrodites, in contrast, each individual competes with all others in the population, including its mates.
Since every individual has both a mother and father, in outbreeding populations both total and mean fitness must be equal for male and female functions. However, hermofrodit the variances in fitness differ for the two sexual roles, then the potential fitness of the two sexual roles also differs, and one would expect that selection would favor individuals that specialize in the preferred role see Charnov ; Fischer; Leonard, ; Michiels Based on Bateman's principle, Charnov predicted that simultaneous hermaphrodites should be more eager to mate as males than as females because of the potential for increased fitness; that is, simultaneous hermaphrodites should prefer to mate in the high variance sexual role, although data from angiosperms demonstrates that the female role in hermaphrodites may have higher variance than the male sexual role see Delph and Ashman for discussion.
Charnov's hypothesis is contrary to predictions from probability theory which demonstrate that where two strategies offer equal mean return, as must be the case for male and female functions in an outcrossing organism, the strategy with the lower variance will offer higher hermofrodit due to the reduced probability of zero fitness Gillespie ; Real ; Leonard ; for recent review see Leonard This logic Gillespie's principle predicts that hermaphrodites will prefer the sexual role with lower variance see Leonardfor discussion.
If one sexual role has a lower variance than the other, that role should, by Gillespie's principle Gillespie ; contrary to Charnov ; see discussion in Leonard, offer potentially higher fitness. Hermaphrodites able to specialize in that role, then, should have higher fitness and individuals should compete for that sexual role, creating a direct conflict of interest between mates see discussion in Leonard This conflict of interest creates potential for sexual selection because a individuals compete for matings in one sexual role and b according to the Hermaphrodite's Dilemma model Leonardindividuals will choose mates that are willing to reciprocate by mating in both sexual roles see discussion in Leonard
These considerations would suggest that the ability to self-fertilize might tend promote sexual selection by reducing the need for fertility assurance. The importance of these factors will vary with the degree of inbreeding depression associated with selfing, which is highly variable among species. In general, there is evidence that individuals in most species prefer to outcross when mates are available see Milinski ; Weeks and others although there are many exceptions see Milinski ; Doums and others In an elegant series of experiments involving laboratory culture of the simultaneously hermaphroditic tapeworm, S.
Interestingly, even pairs matched in size continued to produce some selfed offspring. This may represent an adaptation to a predominantly Game of Chicken matrix Leonard under natural conditions where the tapeworms may often lack a partner see Milinski for discussion. The relative fitness of selfed vs. As this discussion demonstrates, there is abundant circumstantial evidence for sexual selection in hermaphrodites in both forms that are familiar from dieocious taxa and forms that are unique to hermaphrodites for example, socially mediated sex change; mating systems based on conditional reciprocity.
While the circumstantial case for sexual selection as an important force in hermaphrodite mating systems is strong , one would like both to obtain more direct evidence of sexual selection and to identify the sources of sexual selection in hermaphrodites, with the goal of using this information to test existing theories of sexual selection and to develop more predictive theories as to the strength and direction of sexual selection.
Obtaining direct evidence for sexual selection requires the ability to measure sexual selection. The definition of sexual selection adopted here Table 1 , definition 12 was put forward by Malte Andersson It is based on Darwin's definition of sexual selection in Descent of Man quoted earlier. The key feature of the definition is competition; not just competition for access to mates, but competition to be chosen as a mate, and competition to choose the best right mates.
This problem seems unavoidable. As discussed earlier, sexual selection acts on a wide variety of traits in both dieocious and hermaphroditic species, and the strength and even direction of sexual selection may change with such variables as sex ratio Jones and others Arnold proposed the use of Bateman's gradients, the relationship between number of offspring and number of mates Arnold a ; Arnold and Duvall , as a way of measuring sexual selection, but acknowledged that this measure ignores the issue of mate quality.
This seems like a major disadvantage since the prevalent pattern of male-male competition and female choice at least in dieocious metazoans suggests that while males can increase their fitness most readily by acquiring more mates, selection acting on females works very strongly through mate quality. Whether this is mediated by major histocompatability complex MHC alleles, as has been suggested for humans Wedekind and Furi , is not clear. However, it is powerful evidence for both the importance and the subtlety of mate choice.
Also, data from a stylommatophoran snail demonstrate that the degree of first male advantage varies substantially among females Baur and others In angiosperms, the ability of females to abort embryos on the basis of sire genotypes has long been known and this certainly represents a form of cryptic mate choice Willson and Burley ; Eberhard ; Skogsmyr and Lankinen , Mate choice is not a negligible aspect of sexual selection, arguing against the sufficiency of Bateman's gradients as a measure of sexual selection.
There seems to be no alternative to rather elaborate experimentation to assess definitively the magnitude and direction of sexual selection. Since sexual selection theory has been very heavily biased toward assumptions and predictions from dioecious taxa, hermaphrodites offer a new perspective Table 4 , a wealth of new observations, and an opportunity to test theory.
The first insight hermaphrodites offer is, as just discussed, one of definition. A second lesson from hermaphrodites is that sexual selection is ubiquitous; evidence for sexual selection has been found wherever it has been sought.
Even organisms with little or no brain, such as tapeworms and plants Bishop and Pemberton ; Delph and Ashman ; Milinski , can exercise choice; and even organisms without muscle or the power of movement can compete for mates see Thomson Hermaphrodites also offer important insights into the relationship between variance in reproductive success, mate choice, and mating success. Charnov suggested that this should translate in simultaneous hermaphrodites into a preference for mating in the male role.
However, probability theory tells us that given a choice between two investments or strategies with equal mean return as must be the case for the two sexual roles but unequal variance, the lower variance investment will be the most profitable, because it will be less likely to yield a zero return see discussion in Gillespie ; Leonard , This predicts that hermaphrodites should prefer the lower variance role Leonard and Lukowiak ; Leonard , for discussion.
This provides an alternative view of the relationship between reproductive success and number of mates in the high variance sex. That is, where sex ratios are equal and monogamy is not the rule, many males may fail to mate at all, or if they do, may still fail to sire offspring due to sperm competition or cryptic female choice. This being the case, the main selective pressure on males is simply to find as many mates as possible in the hope that at least one will provide paternity.
That is, the driving force in males may be simply reproductive assurance. In contrast, if variance in reproductive success is low for females, most females may reasonably expect to produce offspring and the difference in fitness among females may come down to offspring quality. Interestingly, the female role has been shown to have higher variance in reproductive success in some studies in hermaphroditic plants see Delph and Ashman Data on variation in the relative variances of reproductive success through male and female functions in hermaphrodites may provide insight into the relative contribution of reproductive investment, sex ratio, potential reproductive rate, and control of fertilization to variance in reproductive success.
Another lesson of sexual selection that is reinforced by data from hermaphrodites is the fact that all offspring are not created equal. Offspring quality varies in ways that are important to the parent's fitness. In hermaphrodites with the capacity to self-fertilize, outcrossed offspring may be preferred or they may be produced only under special environmental conditions see Weeks and others ; Milinski Milinski presents important data suggesting that outcrossed offspring outperform selfed offspring, if and only if, the offspring experience competition.
This sort of experiment has important implications for understanding the adaptive significance of sex, perhaps the most important question in evolutionary ecology. A final lesson from hermaphrodites is that sexual conflict exists and is important.
The concept of a conflict of interest between males and females over reproductive decisions and mating behavior has been central to sexual selection and reproductive investment theory for decades.
However, sexual conflict is indirect in gonochores, in that in each generation males compete with males and females with females for a contribution to the next generation. Direct evidence for sexual conflict is therefore hard to obtain but see Rice In hermaphrodites, however, each individual competes with all other individuals for its contribution to the population's next generation and sexual conflict can be direct.
In simultaneous hermaphrodites mating systems based on conditional reciprocity have been found to be common as was hypothesized on the assumption that sexual conflict is important Leonard ; see discussion in Leonard This was a strong prediction since, at the time the prediction was made, very little was known about mating systems in simultaneous hermaphrodites.
The apparent lack of reciprocity in the mating systems of some simultaneous hermaphrodites for example, Lysmata shrimp, Bauer ; Lymnaea stagnalis , Koene suggests that sexual conflict may not be a factor in their mating system. Examination of species with and without conditional reciprocity may shed light on the sources of sexual conflict and hence sexual selection.
Thanks are due to the Society for Integrative and Comparative Biology and the National Science Foundation for their generous support of this symposium and special thanks to the participants that made it so enjoyable and productive. Oxford University Press is a department of the University of Oxford. It furthers the University's objective of excellence in research, scholarship, and education by publishing worldwide. Sign In or Create an Account. Sign In. Advanced Search.
Article Navigation. Close mobile search navigation Article Navigation. Volume Article Contents. Evidence for sexual selection in hermaphrodites. Lessons from hermaphrodites. Sexual selection: lessons from hermaphrodite mating systems Janet L. Joseph M. Long Marine Laboratory, University of California. Oxford Academic. Google Scholar. Cite Citation. Permissions Icon Permissions. Abstract Over the last years, research has established that a sexual selection exists and is widespread in the plant and animal kingdoms; b it does not necessarily entail sexual dimorphism; even hermaphrodites have it; c it does not require intelligence or a sophisticated sense of esthetics; even tapeworms and plants choose mates; and d it does not require brawn or even mobility for competition; plants may compete for pollinators, and broadcast spawning invertebrates may also compete for matings.
Table 1. Selection through competition to acquire mates or be chosen as a mate Andersson ; adopted here. Open in new tab. Table 2. Table 3. Data from red deer, elephant seals, damselflies, etc. Table 4. Sexual conflict exists and is important. Courtship and spawning behavior in the California sheephead, Semicossyphuspulcher Pisces: Labridae. Search ADS. Google Preview. Body size influences mating strategies in a simultaneously hermaphroditic sea slug, Aplysia vaccaria. Multiple mating, paternity, and body size in a simultaneous hermaphrodite, Aplysia californica.
Bateman's principles and the measurement of sexual selection in plants and animals. Is there a unifying concept of sexual selection that applies to both plants and animals? Same sexual system but variable sociobiology; evolution of protandric simultaneous hermaphroditism in Lysmata shrimps.
Within-and between-clutch variation in egg size and nutrient content in the land snail, Arianta arbustorum. Sperm allocation in the simultaneously hermaphroditic land snail Arianta arbustorum. The evolution of parental care in fishes, with reference to Darwin's rule of male sexual selection. To change or not to change sex: a comparison between two Ophyrotrocha species Polychaeta. Social constraints on female mate preferences in mallards, Anas platyrhynchos , decrease offspring viability and mother productivity.
Sexual reproduction and colony growth in the scleractinian coral Porites astreoides. Hermaphroditism as a reproductive strategy for metazoans; some correlated benefits. Sexual selection and the potential reproductive rates of males and females.
Development of the gonads and the sequence of the sexual phases in the California oyster Ostrea lurida. Trait selection in flowering plants: how does sexual selection contribute? Inbreeding depression, neutral polymorphism, and copulatory behavior in freshwater snails: a self-fertilization syndrome.
Population genetic structure in brooding sea anemones Epiactis spp. Polygyrid land snail phlyogeny: external sperm exchange, early North American biogeography, iterative shell evolution. Sources of variation in lifetime reproductive success in a non-territorial damselfly Odonata: Coenagrionidae. The relationship between mating system and simultaneous hermaphroditism in the coral reef fish, Hypoplectrus nigricans Serranidae.
Simultaneous hermaphroditism, tit-for-tat, and the evolutionary stability of social systems. Reproduction of marine invertebrates: Vol 9 General aspects: seeking unity in diversity. Natural selection for within-generation variances in offspring numbers: a new evolutionary principle.
Sex roles, contests for the control of reproduction, and sexual selection. Chance, time allocation, and the evolution of adaptively flexible sex role behavior. A novel sexual pattern in serranid fishes: simultaneous hermaphrodites and secondary males in Serranus fasciatus.
Intermating interval and number of sperm delivered in the simultaneously hermaphroditic land snail Arianta arbustorum Pulmonata:Helicidae. Protandry and the evolution of environmentally mediated sex change: a study of the Mollusca.
Experimental removal of sexual selection reverses intersexual antagonistic coevolution and removes a reproductive load. Envrionmental variance in lifetime mating success, mate choice, and sexual selection. The evolution of the selfing rate in functionally hermaphroditic plant and animals. Molecular parentage analysis in experimental newt populations: the response of mating system measures to variation in the operational sex ratio. The measurement of sexual selection using Bateman's principles: an experimental test in the sex-role-reversed pipefish Syngnathus typhle.
The enigmatic mating behaviour and reproduction of a simultaneous hermaphrodite, the nudibranch Aeolidiella glauca Gastropoda, Opisthobranchia. Shooting darts: co-evolution and counter-adaptation in hermaphroditic snails. Tales of two snails: sexual selection and sexual conflict in Lymnaea stagnalis and Helix aspersa. Hirudinea: Glossiphoniidae. Reproductive behaviour and parental care of Helobdella striata Hirudinea, Glossiphoniidae : a leech that feeds its young. Sex change in either direction by growth-rate advantage in the monogamous coral goby, Paragobiodon echinocephalus.
The conjugal behavior of the introduced giant garden slug, Limax maximus L. Sexual conflict and the mating systems of simultaneously hermaphroditic gastropods.
Sexual conflict in simultaneous hermaphrodites: evidence from serranid fishes. Male-female conflict in a simultaneous hermaphrodite resolved by sperm-trading. Courtship, copulation and sperm-trading in the sea slug, Navanax inermis Opisthobranchia: Cephalaspidea. The behavior of Aplysia californica Cooper Gastropoda: Opisthobranchia. Sex and the simultaneous hermaphrodite: testing models of male-female conflict in a sea slug, Navanax inermis Opisthobranchia.
Comparative reproductive biology of Ariolimax californicus and A. Sperm limitation, gamete competition and sexual selection in external fertilizers. Causal and functional organization of the mating behaviour sequence in Helix pomatia Pulmonata, Gastropoda. Life history and sex allocation in the simultaneously hermaphroditic polychaete worm Ophryotrocha diadema : the role of sperm competition.
Size-dependent discrimination of mating partners in the simultaneous hermaphroditic cestode Schistocephalus solidus. Sperm trading in hermaphroditic flatworm: reluctant fathers and sexy mothers. Precopulatory mate assessment in relation to body size in the earthworm Lumbricus terrestris : avoidance of dangerous liaisons? Sexual selection favours harmful mating in hermaphrodites more than in gonochorists.
Sperm trading and sex roles in the hermaphroditic opisthobranch sea slug Navanax inermis : eager females or opportunistic males? Fitness consequences of selfing and outcrossing in the cestode Schistocephalus solidus. Alternative contexts of sex change with social control in the bucktooth parrotfish, Sparisoma radians.
Is sperm cheap? Limited male fertility and female choice in the Lemon Tetra Pisces, Characidae. Body size-dependent gender role in a simultaneous hermaphrodite freshwater snail, Physa acuta. Effects of population size and metapopulation dynamics on a mating-system polymorphism. Parental investment and the control of sexual selection; predicting the direction of sexual competition. Sexual selection and intersexual differences in variance of breeding success.
High multiple paternity and low last-male sperm precedence in a hermaphroditic planarian flatworm: consequences for reciprocity patterns. Pair formation and joint territoriality in a simultaneous hermaphrodite: the coral reef fish Serranus tigrinus.
Complete estimates of reproductive success in a closed population of smallmouth bass Micropterus dolomieui. Fitness, uncertainty, and the role of diversification in evolution and behavior. Verhaltensbeobachtungen und molekulargenetische Methoden zur Untersuchung von Taxonomie und Reproductionsbiologie terrestrischer Nacktschnecken. Morphological variation in terrestrial slug Deroceras turcicum Simroth and a northern extension of its range in central Europe. Correlated evolution of dichogamy and self-incompatibility: a phylogenetic perspective.
Courtship display and copulation mechanism in the slugs of the genus Deroceras Mollusca, Gastropoda Terrestria Nuda at Transili Alatau mountain range. Size-dependent sex allocation in a simultaneously hermaphroditic parasite. Lifetime reproductive output in a hermaphrodite cestode when reproducing alone or in pairs: a time cost of pairing.
Loss of male outcrossing ability in simultaneous hermaphrodites: phylogenetic analyses of pulmonate snails. Reciprocal egg trading and brood care in a hermaphroditic polychaete worm. Sexual conflict and mating systems in the dorvilleid genus Ophyrotrocha and the dinophilid genus Dinophilus.
Social behavior, group structure, and sex reversal in hermaphroditic fish. St Mary. Sex allocation in a simultaneous hermaphrodite, the blue-banded goby, Lythrypnus dalli : the effect of body size and behavioral gender and the consequences for reproduction.
Sequential patterns of sex allocation in simultaneous hermaphrodites: do we need models that specifically incorporate this complexity? Social group size, potential sperm competition and reproductive investment in a hermaphroditic leech, Helobdella papillornata [Euhirudinea; Glossiphoniidae]. Tactics for male reproductive success in plants: contrasting insights of sex allocation theory and pollen presentation theory. Age dependency of sexual role and reproductive ecology in a simultaneously hermaphroditic land snail, Achatina fulica Stylommatophora: Achatinidae.
Delayed selfing as an optimal mating strategy in preferentially outcrossing species: theoretical analysis of the optimal age at first reproduction in relation to mate availability. Delayed selfing and resource reallocations in relation to mate availability in the freshwater snail Physa acuta. Selfing, sexual polymorphism and microsatellites in the hermaphroditic freshwater snail Bulinus truncatus. The intensity of sexual selection in relation to male sexual behavior, female choice, and sperm precedence.
Sexual selection favors female-biased sex ratios: the balance between the opposing forces of sex-ratio selection and sexual selection. Sexual selection: harem size and the variance in male reproductive success. Mating systems, sex change and sexual demography in the rainbow wrasse, Thalassoma lucasanum. Social control of sex change in the bluehead wrasse, Thalassoma bifasciatum Pisces: Labridae. Stabilizing selection on behavior and morphology masks positive selection on the signal in a salamander pheromone signaling complex.
Mate choice, frequency dependence and the maintenance of resistance to parasitism in a simultaneous hermaphrodite. Body odour preferences in men and women: do they aim for specific MHC combinations or simply heterozygosity. When males and hermaphrodites coexist: a review of androdieocy in animals. Dichogamy, gender variation and bet-hedging in Pseudowinteria colorata. All rights reserved. For permissions, please email: journals. Issue Section:. Download all figures. View Metrics. Email alerts New issue alert.
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Unique characteristics of the exoskeleton of Austinograea rodriguez in the Indian Ocean hydrothermal vent Onnuri Vent Field. Light-induced stress as a primary evolutionary driver of eye origins. Sex differences in the intensity of selection on particular traits caused by contrasting effects of the trait on mating success in the two sexes. Greater control of fertilization by one sex Alexander and Borgia ; Leonard and Lukowiak Documented in sequential and some simultaneous hermaphrodites; sneaker males in serranines, fighting in Ophytrocha.
Pollen competition common and well documented Willson and Burley , etc. Documented in Biomphalaria glabrata Webster ; size-assortative mating reported in nudibranchs, flatworms c , and tapeworms. Not uncommon in sequential hermaphrodites, for example, bluehead wrasse; one report in simultaneous hermaphrodites d.
Found in some simultaneous hermaphrodites; that is, Hypoplectrus nigricans, Navanax inermis, Lymnaea stagnalis; Achatina fulica; common in sequential hermaphrodites e. Common in euthyneuran gastropods; for example, Navanax inermis , Limax maximus, Helix spp; Ariolimax spp. For example, Hypoplectrus nigricans, Serranus tigrinus, Navanax inermis, Ophyrotrocha spp; Arianta arbustorum, Schmidtea polychroa , may be widespread see text and review in Leonard Common in angiosperms, platyhelminths, stylommatophoran gastropods; for example, Ariolimax spp.
Common in sequential hermaphrodites; also found in simultaneous hermaphrodites with sexual polymorphisms and complex sexual systems phally polymorphisms, complemental males, androdieocy, gynocdioecy, etc. Common in sequentially hermaphroditic fish; for example, bluehead wrasse Warner and Swearer ; documented in two species of simultaneously hermaphroditic serranine fish see Petersen Most often, but not always, the chromosome complement is 46,XX, and in every such individual there also exists evidence of Y chromosomal material on one of the autosomes any of the 22 pairs of chromosomes other than the sex chromosomes.
Individuals with a 46,XX chromosome complement usually have ambiguous external genitalia with a sizable phallus and are therefore often reared as males. However, they develop breasts during puberty and menstruate and in only rare cases actually produce sperm. In 46,XX intersex female pseudohermaphroditism , individuals have male external genitalia but the chromosomal constitution and reproductive organs of a female. In 46,XY male pseudohermaphroditism , individuals have ambiguous or female external genitalia but the chromosomal constitution and reproductive organs of a male, though the testes may be malformed or absent.
Treatment of intersex in humans depends upon the age at which the diagnosis is made. Historically, if diagnosed at birth, the choice of sex was made typically by parents based on the condition of the external genitalia i. The remaining genitalia were then reconstructed to resemble those of the chosen sex. The reconstruction of female genitalia was more readily performed than the reconstruction of male genitalia, so ambiguous individuals often were made to be female.
However, intersex surgery has long-term consequences for affected individuals. Later in life, for example, the person may not be satisfied with the results of surgery and may not identify with the assigned gender. Thus, patient consent has become an increasingly important part of decisions about intersex surgery, such that surgery may be delayed until adolescence or adulthood, after patients have had sufficient time to consider their gender and are able to make informed decisions about treatment.
In older individuals the accepted gender may be reinforced by the appropriate surgical procedures and by hormonal therapy. Info Print Cite. Submit Feedback. Thank you for your feedback. Hermaphroditism biology. See Article History. Learn More in these related Britannica articles:.
Hermaphrodites combine the male and female sex functions into a single individual, either sequentially or simultaneously. This simple fact means that they exhibit both similarities and differences in the way in which they experience, and respond to, sexual conflict compared to separate-sexed organisms. Here, we focus on hremofrodit how sexual conflict concepts can be adapted to apply to all anisogamous sexual systems and review unique or especially important aspects of sexual conflict in hermaphroditic animals.
These include conflicts over the timing of sex change in sequential hermaphrodites, and in simultaneous hermaphrodites, over both sex roles and hermofrovit postmating hrrmofrodit of the sperm recipient by the sperm donor. Extending and applying sexual conflict thinking to hermaphrodites hermofrodit identify general evolutionary principles and help explain some of the unique reproductive diversity found among animals exhibiting this widespread but to date understudied sexual system.
Conceptual and hsrmofrodit work on sexual conflict is dominated by studies on gonochorists species with separate sexes e. The fundamental asymmetry from which sexual conflict emerges is ultimately rooted in the evolution of anisogamy, which may itself have resulted from a primordial sexual conflict over allocation to gamete provisioning.
Specifically, the proto male sexual strategy of making more but smaller gametes—driven by proto sperm competition—likely forced the proto female sexual strategy into investing more resources per gamete Parker et al. In different species, the male and female sexual strategies can be distributed over individuals in various ways Hamilton ; Charnov ; Munday et al.
In the most familiar—gonochorism—they are always confined to different individuals, namely, males and females, as exemplified by some of the best-studied animal groups e.
Although models for the evolution of anisogamy have tended to assume gonochorism e. Here, we show that sexual conflict thinking has equal validity when applied to hermaphroditic taxa Table 1. In fact, some of the earliest insights about sexual conflict have come from the hermaphrodite literature. Clearly, these insights have very general relevance to sdx sexual systems and likely arose from the realization around this time that conflicts are an integral part of the evolution of sexual reproduction e.
The concepts are broadened bold indicates changed text to include all anisogamous sexual systems. Thus, rather than seeing sexual conflict in hermaphrodites as a special case, we believe that studying sexual conflict in a range of different sexual systems is essential for deriving general principles of the evolution of anisogamy and its consequences.
The current bias in sexual conflict thinking probably in part reflects an uncertainty among researchers over how to think about the male and female sexual strategies in ssx when they appear in one and the same individual. We therefore begin by outlining how some of the established gonochorist-specific sexual conflict concepts can be broadened to encompass not only hermaphrodites, but arguably all anisogamous sexual systems Table 1before briefly introducing how hermaphrodites partition resources between the male and female sex functions i.
We then take stock of what we do know and do not know about sexual conflict—first in sequential hermaphrodites, and then in simultaneous hermaphrodites—with the aim of illuminating general principles and attempting to understand what is similar and what is different about sexual conflict when it occurs in different sexual systems. Given limitations on space and our own expertise, we confine our discussion to hermaphroditic animals, but emphasize here that plants undoubtedly also offer tremendous promise to expand our understanding of sexual conflict e.
Conventionally, evidence for both intra- and interlocus sexual conflict relies on measuring fitness hermofroddit in hermoftodit types of individuals that must, by hermifrodit, have opposite sexes Table 1. So, a difficulty for thinking about sexual conflicts in hermaphrodites is that individuals are not of two types, but either change from one type to the other sequential hermaphrodites or are both types at the same time simultaneous hermaphroditesand thus each individual has two potential routes to fitness Fig.
This means there are some fundamental differences in the nature of, and likely responses to, sexual conflict compared to gonochorists. Two routes to fitness in hermaphrodites. A In gonochorists species with separate sexesfitness can be divided into three main components, namely, survivorship, fecundity, and mating including fertilization success modified from Arnqvist and Hermofrodit B In hermaphrodites, these components contribute to total fitness through both the male and female sex function.
Note that in hermaphrodites, fecundity could further be split into separate male and female components that may trade off with each other. And, although in sequential hermaphrodites male and female mating success happens during nonoverlapping time periods, they may be constrained to be identical in simultaneous hermaphrodites with reciprocal mating. Hermofrodit about interlocus sexual conflict, at hermoffodit initially, seems less problematic, as this simply hermofrodit conflict between two interacting mating partners, the sperm donor and the sperm recipient Charnovand much of the literature we discuss below follows this rationale.
A necessary qualification for hermaphrodites, however, is that—because of the dual routes to fitness Fig. Moreover, sex allocation trade-offs and other evolutionary links between male and female fitness, which occur in both sequential and simultaneous hermaphrodites see belowmean that one cannot just assume that an individual is affected by sexual conflict in only one of its sex functions.
This clearly complicates interpretations compared to gonochorists, but it also presents some interesting targets for sexual conflict, including conflicts over which sex role the partner hermofrodit and its sex allocation, as we discuss in the main sections below. In contrast to the long history of research on interlocus sexual conflict, the possibility heromfrodit intralocus sexual conflict in hermaphrodites has hermorfodit been explored very recently Abbott ; see also Arnqvist and Rowe ; Bedhomme et al.
Consider a sexually antagonistic allele arising in a gonochorist. When expressed in say males, the allele benefits its bearer, whereas when expressed in females, the same allele is selected against. The ultimate fate of the allele in the population depends on how these benefits and costs balance out and whether potential modifiers arise at other loci in the meantime Rice and Hedmofrodit The situation in hermaphrodites is similar, in that one is still interested in the fate of a segregating allele that has opposing fitness effects on the carrier when expressed in sperm donors and sperm recipients, sex in this case individuals sequentially or simultaneously express both sex functions, and so will often act as both sperm donor and sperm recipient within their own lifetime.
A useful alternative approach may be to think of such an allele simply in terms of antagonistic pleiotropy, with the opposing fitness effects in the male and female function representing a life-history trade-off, and with the accounting of those fitness effects playing out on a more immediate scale i. However, hermofroddit approach may be too simplistic for a number of reasons, which we consider in more detail in the main sections.
Any type of sex-specific selection must obey the frequency-dependent hegmofrodit of the evolution of sex allocation, which differ between gonochorists and hermaphrodites in ways relevant sex sexual conflict, as we outline next.
Each zygote results from the fusion of a sperm and an egg, which, given anisogamy, contribute different amounts of resources to the embryo. Yet despite this initial asymmetry, the sexes make an equal genetic contribution i. Under common assumptions of random mating i. Hermorfodit local hermofrodit will tend to limit returns on investment through the male function, which given the two routes to fitness in hermaphrodites will often favor a reallocation of resources toward the own female function and lead to an overall female-biased sex allocation, as reflected by both theoretical models and empirical data Charnov ; Munday et al.
In contrast, males in gonochorists are more constrained, as they of course cannot directly reallocate to a female function, and so hermofrodit instead appear to invest in other male routes to fitness, such as gaining more matings see also Parker So although these common assumptions may also be broken in gonochorists, these seem less prone to exhibit biased sex allocation.
Once sex allocation is biased toward one sex function, a higher fitness return per unit resource invested is obtained from the other sex function i. In summary, we suggest that sexual conflict thinking can clearly be extended to hermaphrodites. However, a crucial difference is that the two sex functions simply do not map onto individuals in the hermofrpdit they do in gonochorists, likely complicating the picture for predicting the evolutionary dynamics of sexual conflict.
The field would greatly benefit from theoretical work to fully understand the implications of whether and how the evolution of sexually antagonistic alleles is expected to differ between sexual systems, and we outline many relevant factors in the following sections. But in the meantime, we take the concepts we have developed here and apply them first to sequential and then to simultaneous hermaphrodites. Sequential hermaphroditism occurs when individuals can increase their reproductive value sex changing sex.
This requires that 1 the expected reproductive success is higher for one sex at one life stage and later becomes higher for the other sex, and 2 the fitness costs of changing sex e. One scenario, as represented by anemonefishes, involves a local group inhabiting a spatially restricted habitat that forces group size to be very small.
Here, a male cannot monopolize many females and thus male reproductive value depends only weakly on body size. In contrast, as is true for many fishes, female reproductive value increases strongly with body size. This scenario predicts protandrous sex change, in which individuals mature first as males and later change sex to become females. Another scenario, as represented by labroid fishes, involves a local group inhabiting a somewhat larger habitat that permits group size to reach half a dozen or more individuals.
Here, a large dominant territorial male can monopolize several ehrmofrodit and male reproductive value depends strongly on body size, and more so than for females. This scenario predicts protogynous sex change, in which individuals mature as females and later change sex to become males.
In both scenarios, the sex exhibited by other individuals in the local group—and sex sex change decisions—are major determinants of an individual's reproductive value Munday et al. A sequentially hermaphroditic individual could carry an allele that shows antagonistic pleiotropy for its male and female fitness Schultz and Warner ; Abbott Such an allele may make its carrier more successful when male by facilitating territorial defense, while having negative effects when female, for example, because aggression among females decreases fecundity.
This scenario was recently suggested for the protogynous reef fish Parapercis cylindrica Sprenger et al. Females that showed high aggression levels before sex change turned into more aggressive males. Independent evidence further suggested that high aggression levels may indeed be beneficial for males and detrimental for females in this sexual system Sprenger et al.
Insofar as the effect of such an allele is measured as lifetime fitness of its carrier e. As far as we know, there have been no systematic empirical or theoretical investigations of the evolutionary dynamics of sexually antagonistic alleles in sequential hermaphrodites, which might well be more complex than the simple trade-off scenario outlined above implies see also Abbott One important consideration concerns the distribution of fitness across individuals in the population.
Moreover, few individuals live long enough to eventually change sex with mortality presumably involving a significant stochastic component and the population sex ratio can be extremely biased Warner and Hoffmanwhich also means that very few individuals obtain most of the male fitness old sex large individuals can be extremely successful. Another consideration is how such a sexually antagonistic allele will affect the sex-specific and total fitness of the carrier's offspring bearing that allele affecting the total number of grandchildren produced.
Although such an allele may allow a father to produce many offspring, the above-mentioned skews and stochasticity may mean that none of its offspring will live to become a successful male that goes on to reap the same male-benefit effects, whereas all offspring may still suffer the female-detriment effects while being females. Also, the sex-specific effects of a spreading sexually antagonistic allele are likely to be strongly affected by frequency-dependent selection Abbott One way that a sexually antagonistic allele can increase the probability of reaping the benefits and avoiding the costs of being expressed in a specific sex would be to either 1 influence, or 2 xex associated with the sex change decision Arnqvist and Rowe ; Abbott In the reef fish example above, the former might occur if a high aggression level were achieved by a high titer of a circulating hormone that also causes an earlier sex change i.
The latter could result from linkage disequilibrium, in which alleles for early sex change become statistically associated with male-benefit alleles i. Relatively little is currently known about the genetics of sex change decisions in sequential hermaphrodites but see Hodgkin and Barnesbut it is clear that these decisions show a high degree of plasticity in many species Munday et al.
Interestingly, any allele that advances the timing of sex change in a protogynous hermaphrodite will likely make its carrier a more successful sperm donor and a less successful sperm recipient, and so could itself be viewed as a sexually antagonistic allele. For example, in a protogynous hermaphrodite, the fitness of a large territorial male eex strongly on the sex change decisions of the other individuals in its local group, for two separate reasons. When a female decides to hermofrofit sex, the territorial male potentially 1 loses a female mate and any future fitness that might have derived from mating with her, and 2 gains a male competitor that may start to compete for matings or fertilizations in the local group.
It therefore appears plausible that there could be sexual conflicts over the occurrence and timing of sex change between the territorial male and the resident females, and that a male may actively try to prevent the females from changing sex. This could involve some form of social control of sex change, which is a widespread although by no means universal phenomenon hermotrodit sequential hermaphrodites e.
In several species, experimental removal of the zex male leads to hermofrodif largest female taking over the dominant role within the group very rapidly, sometimes even within a few minutes e. Social control of sex change could hermofridit from at least two different scenarios.
In the first, a female essentially evaluates the probability of increasing her reproductive value by changing sex, and concludes that she is better off remaining an active female, and waiting for a territory to become available. In the second scenario, the female concludes that changing sex would indeed be the preferred option to maximize her reproductive value e.
Warnerbut hefmofrodit is actively prevented from executing that choice by the hermpfrodit male, possibly by aggression-induced stress hormones that physiologically prevent sex change cf.
Marsh-Hunkin et al. Such aggression might cause direct fitness costs to the female, reducing her egg production below the level she would have achieved without it, which one would clearly view as sexual conflict.
A third, possibly intermediate, scenario hermodrodit result if the territorial male, by directing aggression toward male intruders, somehow causes collateral harm to females. An interesting study that could be seen in this context is that of Warner et al. As a result of hermaphrodites having two routes to fitness Fig.
Nevertheless, we believe that the last two scenarios could readily be thought of as interlocus sexual conflict, predicting sexually antagonistic coevolution between a male-benefit inducing male persistence trait e. We are not sex that this possibility has been explored, either empirically or theoretically. The first scenario might also lead to similar coevolutionary dynamics, but here driven by male—male competition between male-benefit inducing alleles and male-detriment avoiding alleles e.
Simultaneous hermaphroditism could be favored 1 by facilitating reproductive assurance when individuals or their gametes risk failed reproduction for lack of encountering partners Darwin ; Tomlinson ; Ghiselin2 if hermofrodit male and female reproductive functions do not strongly trade off, because they have nonoverlapping resource requirements or sex share costs Charnov et al. The best-studied determinant of sex allocation in simultaneous hermaphrodites is the mating group size, which affects the strength of male—male competition in a local group.
As mating group size increases, however, success sex sperm competition between sperm from different donors Parker becomes increasingly important to determining fitness through the male sex function, favoring a greater male investment and thus a more even sex allocation. Most work on simultaneous hermaphrodites concerns interlocus sexual conflict see belowand with respect to intralocus sexual conflict, there are many parallels with sequential hermaphrodites, so this section will be brief.
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Hermaphrodites combine the male and female sex functions into a single individual, either sequentially or simultaneously. This simple fact means that they. Hermaphroditic plants—most flowering plants, or angiosperms—are called monoecious, Affected individuals have sex chromosomes showing male-female.
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