Sexual selection

File:Paradesia decora Keulemans.jpg

Sexual selection creates colourful differences between sexes in Goldie's bird-of-paradise. Male above; female below. Painting by John Gerrard Keulemans.

Sexual selection is a mechanism of evolution in which members of one sex choose mates of the other sex to mate with (intersexual selection), and compete with members of the same sex for access to members of the opposite sex (intrasexual selection). These two forms of selection mean that some individuals have greater reproductive success than others within a population, for example because they are more attractive or prefer more attractive partners to produce offspring. Successful males benefit from frequent mating and monopolizing access to one or more fertile females. Females can maximise the return on the energy they invest in reproduction by selecting and mating with the best males.

The concept was first articulated by Charles Darwin who wrote of a "second agency" other than natural selection, in which competition between mate candidates could lead to speciation. The theory was given a mathematical basis by Ronald Fisher in the early 20th century. Sexual selection can lead males to extreme efforts to demonstrate their fitness to be chosen by females, producing sexual dimorphism in secondary sexual characteristics, such as the ornate plumage of birds-of-paradise and peafowl, or the antlers of deer. Depending on the species, these rules can be reversed. This is caused by a positive feedback mechanism known as a Fisherian runaway, where the passing-on of the desire for a trait in one sex is as important as having the trait in the other sex in producing the runaway effect. Although the sexy son hypothesis indicates that females would prefer male offspring, Fisher's principle explains why the sex ratio is most often 1:1. Sexual selection is widely distributed in the animal kingdom, and is also found in plants and fungi.

History

Darwin

File:Darwin sexual caricature.gif

Victorian cartoonists mocked Darwin's ideas about display in sexual selection. Here he is fascinated by the apparent steatopygia in the latest fashion.

Sexual selection was first proposed by Charles Darwin in On the Origin of Species (1859) and developed in The Descent of Man, and Selection in Relation to Sex (1871), as he felt that natural selection alone was unable to account for certain types of non-survival adaptations. He once wrote to a colleague that "The sight of a feather in a peacock's tail, whenever I gaze at it, makes me sick!" His work divided sexual selection into male–male competition and female choice.<ref>Template:Cite journal</ref><ref name="Mendelson Safran 2021">Template:Cite journal</ref>

Template:ErrorTemplate:Main other{{#if:|{{#if:|}}

— {{#if:|, in }}Template:Comma separated entries

}}

{{#invoke:Check for unknown parameters|check|unknown=Template:Main other|preview=Page using Template:Blockquote with unknown parameter "_VALUE_"|ignoreblank=y| 1 | 2 | 3 | 4 | 5 | author | by | char | character | cite | class | content | multiline | personquoted | publication | quote | quotesource | quotetext | sign | source | style | text | title | ts }}

Template:ErrorTemplate:Main other{{#if:|{{#if:|}}

— {{#if:|, in }}Template:Comma separated entries

}}

{{#invoke:Check for unknown parameters|check|unknown=Template:Main other|preview=Page using Template:Blockquote with unknown parameter "_VALUE_"|ignoreblank=y| 1 | 2 | 3 | 4 | 5 | author | by | char | character | cite | class | content | multiline | personquoted | publication | quote | quotesource | quotetext | sign | source | style | text | title | ts }}

These views were to some extent opposed by Alfred Russel Wallace, mostly after Darwin's death. He accepted that sexual selection could occur, but argued that it was a relatively weak form of selection. He argued that male–male competitions were forms of natural selection, but that the "drab" peahen's coloration is itself adaptive as camouflage. In his opinion, ascribing mate choice to females was attributing the ability to judge standards of beauty to animals (such as beetles) far too cognitively undeveloped to be capable of aesthetic feeling.<ref name="Wallace">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

File:Tribolium castaneum87-300.jpg

Sexual selection protected flour beetles from extinction in a ten-year experiment.<ref name="popben"/>

Darwin's ideas on sexual selection were met with scepticism by his contemporaries and not considered of great importance, until in the 1930s biologists decided to include sexual selection as a mode of natural selection.<ref>Template:Cite book</ref> Only in the 21st century have they become more important in biology; the theory is now seen as generally applicable and analogous to natural selection.<ref name="Hosken2011">Template:Cite journal</ref> A ten-year study, experimentally varying sexual selection on flour beetles with other factors held constant, showed that sexual selection protected even an inbred population against extinction.<ref name="popben">Population benefits of sexual selection explain the existence of males phys.org May 18, 2015 Report on a study by the University of East Anglia Template:Webarchive</ref>

Fisherian runaway

{{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}}

Ronald Fisher, the English statistician and evolutionary biologist, developed his ideas about sexual selection in his 1930 book The Genetical Theory of Natural Selection. These include the sexy son hypothesis, which might suggest a preference for male offspring, and Fisher's principle, which explains why the sex ratio is usually close to 1:1. The Fisherian runaway describes how sexual selection accelerates the preference for a specific ornament, causing the preferred trait and female preference for it to increase together in a positive feedback runaway cycle.<ref name="Fisher 1930">Fisher, R. A. (1930) The Genetical Theory of Natural Selection. Oxford University Press, Template:ISBN, Template:Usurped.</ref> He remarked that:<ref name=Blind/>

Template:ErrorTemplate:Main other{{#if:|{{#if:|}}

— {{#if:|, in }}Template:Comma separated entries

}}

{{#invoke:Check for unknown parameters|check|unknown=Template:Main other|preview=Page using Template:Blockquote with unknown parameter "_VALUE_"|ignoreblank=y| 1 | 2 | 3 | 4 | 5 | author | by | char | character | cite | class | content | multiline | personquoted | publication | quote | quotesource | quotetext | sign | source | style | text | title | ts }}

File:Euplectes progne male South Africa cropped.jpg

Male long-tailed widowbird

This causes a dramatic increase in both the male's conspicuous feature and in female preference for it, resulting in marked sexual dimorphism, until practical physical constraints halt further exaggeration. A positive feedback loop is created, producing extravagant physical structures in the non-limiting sex. A classic example of female choice and potential runaway selection is the long-tailed widowbird. While males have long tails that are selected for by female choice, female tastes in tail length are still more extreme with females being attracted to tails longer than those that naturally occur.<ref name="Andersson 1994">Template:Cite book</ref> Fisher understood that female preference for long tails may be passed on genetically, in conjunction with genes for the long tail itself. Long-tailed widowbird offspring of both sexes inherit both sets of genes, with females expressing their genetic preference for long tails, and males showing off the coveted long tail itself.<ref name=Blind/>

Richard Dawkins presents a non-mathematical explanation of the runaway sexual selection process in his book The Blind Watchmaker.<ref name=Blind>Template:Cite book</ref> Females that prefer long tailed males tend to have mothers that chose long-tailed fathers. As a result, they carry both sets of genes in their bodies. That is, genes for long tails and for preferring long tails become linked. The taste for long tails and tail length itself may therefore become correlated, tending to increase together. The more tails lengthen, the more long tails are desired. Any slight initial imbalance between taste and tails may set off an explosion in tail lengths. Fisher wrote that:

The exponential element, which is the kernel of the thing, arises from the rate of change in hen taste being proportional to the absolute average degree of taste. —Ronald Fisher, 1932<ref>Ronald Fisher in a letter to Charles Galton Darwin, 22 November 1932, cited in Fisher, R. A., Bennett, J. H. 1999. The genetical theory of natural selection: A complete variorum edition, Oxford University Press, Oxford, p. 308</ref>{{#if:|{{#if:|}}

— {{#if:|, in }}Template:Comma separated entries

}}

{{#invoke:Check for unknown parameters|check|unknown=Template:Main other|preview=Page using Template:Blockquote with unknown parameter "_VALUE_"|ignoreblank=y| 1 | 2 | 3 | 4 | 5 | author | by | char | character | cite | class | content | multiline | personquoted | publication | quote | quotesource | quotetext | sign | source | style | text | title | ts }}

File:Peacock Flying.jpg

The peacock tail in flight, the proposed classic example of a Fisherian runaway

The female widowbird chooses to mate with the most attractive long-tailed male so that her progeny, if male, will themselves be attractive to females of the next generation—thereby fathering many offspring that carry the female's genes. Since the rate of change in preference is proportional to the average taste amongst females, and as females desire to secure the services of the most sexually attractive males, an additive effect is created that, if unchecked, can yield exponential increases in a given taste and in the corresponding desired sexual attribute.<ref name=Blind/>

Template:ErrorTemplate:Main other{{#if:|{{#if:|}}

— {{#if:|, in }}Template:Comma separated entries

}}

{{#invoke:Check for unknown parameters|check|unknown=Template:Main other|preview=Page using Template:Blockquote with unknown parameter "_VALUE_"|ignoreblank=y| 1 | 2 | 3 | 4 | 5 | author | by | char | character | cite | class | content | multiline | personquoted | publication | quote | quotesource | quotetext | sign | source | style | text | title | ts }}

Since Fisher's initial conceptual model of the 'runaway' process, Russell Lande and Peter O'Donald have provided detailed mathematical proofs that define the circumstances under which runaway sexual selection can take place.<ref>Template:Cite journal</ref><ref>Template:Cite book</ref> Alongside this, biologists have extended Darwin's formulation; Malte Andersson's widely accepted<ref name="Kokko Jennions 2014">Template:Cite journal</ref> 1994 definition is that "sexual selection is the differences in reproduction that arise from variation among individuals in traits that affect success in competition over mates and fertilizations".<ref name="Andersson 1994"/><ref name="Kokko Jennions 2014"/> Despite some practical challenges for biologists, the concept of sexual selection is "straightforward".<ref name="Kokko Jennions 2014"/>

Modern theory

Reproductive success

File:Irish Elk front.jpg

The enormous sexually selected antlers of the Irish elk might have helped it on its way to extinction.<ref name="Gould 1974"/>

The reproductive success of an organism is measured by the number of offspring left behind, and by their quality or probable fitness.<ref>Template:Cite journal</ref><ref>Template:Cite book</ref><ref name="PHYS-20140129">Template:Cite news</ref> Sexual preference creates a tendency towards assortative mating or homogamy. The general conditions of sexual discrimination appear to be (1) the acceptance of one mate precludes the effective acceptance of alternative mates, and (2) the rejection of an offer is followed by other offers, either certainly or at such high chance that the risk of non-occurrence is smaller than the chance advantage to be gained by selecting a mate. Bateman's principle states that the sex which invests the most in producing offspring becomes a limiting resource for which the other sex competes, illustrated by the greater nutritional investment of an egg in a zygote, and the limited capacity of females to reproduce; for example, in humans, a woman can only give birth every ten months, whereas a male can become a father numerous times in the same period.<ref>Template:Cite journal</ref> More recently, researchers have doubted whether Bateman was correct.<ref name="Newcomer">Template:Cite journal</ref>

Honest signalling

The handicap principle of Amotz Zahavi, Russell Lande and W. D. Hamilton, holds that the male's survival until and through the age of reproduction with seemingly maladaptive traits is taken by the female as a signal of his overall fitness. Such handicaps might prove he is either free of or resistant to disease, or that he possesses more speed or a greater physical strength that is used to combat the troubles brought on by the exaggerated trait.<ref name="Zahavi 1975 pp. 205–214">Template:Cite journal</ref><ref name="Zahavi 1977 pp. 603–605">Template:Cite journal</ref><ref name="Zahavi Zahavi 1997">Template:Cite book</ref> Zahavi's work spurred a re-examination of the field and several new theories. In 1984, Hamilton and Marlene Zuk introduced the "Bright Male" hypothesis, suggesting that male elaborations might serve as a marker of health, by exaggerating the effects of disease and deficiency.<ref name="HamiltonZuk1982">Template:Cite journal</ref>

Male intrasexual competition

File:Susa group, mountain gorilla.jpg

Male mountain gorilla, a species with very large males<ref>Template:Cite book</ref>

{{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}}

Male–male competition occurs when two males of the same species compete for the opportunity to mate with a female. Sexually dimorphic traits, size, sex ratio,<ref name="Weir 2012">Template:Cite journal</ref> and the social situation<ref name="Proctor-2012">Template:Cite journal</ref> may all play a role in the effects male–male competition has on the reproductive success of a male and the mate choice of a female. Larger males tend to win male–male conflicts.<ref>Template:Cite journal</ref> Males take many risks in such conflicts, so the value of the resource must be large enough to justify those risks.<ref name="Nelson-Flower 2015">Template:Cite journal</ref><ref name="Luo 2016">Template:Cite journal</ref> Winner and loser effects further influence male behaviour.<ref name="Zeng 2018">Template:Cite journal</ref> Male–male competition may also affect a female's ability to select the best mates, and therefore decrease the likelihood of successful reproduction.<ref name="Cayuela 2016">Template:Cite journal</ref>

Multiple models

More recently, the field has grown to include other areas of study, not all of which fit Darwin's definition of sexual selection. A "bewildering"<ref name="Kokko Jennions 2006"/> range of models variously attempt to relate sexual selection not only to the fundamental<ref name="Kokko Jennions 2006"/> questions of anisogamy and parental roles, but also to mechanisms such as sex ratios – governed by Fisher's principle,<ref name="Hamilton 1967">Template:Cite journal</ref> parental care, investing in sexy sons, sexual conflict, and the "most-debated effect",<ref name="Kokko Jennions 2006"/> namely mate choice.<ref name="Kokko Jennions 2006">Template:Cite journal</ref> Elaborated characteristics that might seem costly, like the tail of the Montezuma swordfish (Xiphophorus montezumae), do not always have an energetics, performance or even survival cost; this may be because "compensatory traits" have evolved in concert with the sexually selected traits.<ref name="Oufiero 2015">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

Toolkit of natural selection

File:Protarchaeopteryx 4713.JPG

Protarchaeopteryx was flightless, but had feathers, perhaps used in courtship, that pre-adapted it for flight.

Sexual selection may explain how characteristics such as feathers had survival value at an early stage in their evolution. The earliest proto-birds such as Protarchaeopteryx had well-developed feathers but could not fly. The feathers may have served as insulation, helping females incubate their eggs, but if proto-bird courtship combined displays of forelimb feathers with energetic jumps, then the transition to flight could have been relatively smooth.<ref name="Clarke 2013">Template:Cite journal</ref>

Sexual selection may sometimes generate features that help cause a species' extinction, as has historically been suggested for the giant antlers of the Irish elk (Megaloceros giganteus) that became extinct in Holocene<ref>Template:Cite journal</ref> Eurasia<ref name="Gould 1974">Template:Cite journal</ref> (although climate-induced habitat deterioration and anthropogenic pressure are now considered more likely causes).<ref>Template:Cite journal</ref> It may, however, also do the opposite, driving species divergence—sometimes through elaborate changes in genitalia<ref>Template:Cite journal</ref>—such that new species emerge.<ref>Hosken, David J.; Stockley, Paula. "Sexual selection and genital evolution Template:Webarchive." Trends in Ecology & Evolution 19.2 (2004): 87–93.</ref><ref>Arnqvist, Göran. "Comparative evidence for the evolution of genitalia by sexual selection Template:Webarchive." Nature 393.6687 (1998): 784.</ref> Sexual selection often interacts with natural selection to drive speciation.<ref>Maan, M.E.; Seehausen, O. "Ecology, sexual selection and speciation". Ecology Letters, 2011 Jun; 14(6). pp. 591-602. doi: 10.1111/j.1461-0248.2011.01606.x. PMID 21375683.</ref>

Sex role reversal

Sex role reversal (SRR) was first referred to as the phenomenon of females within a given species competing for mate access, rather than males fulfilling that role.<ref name="Fritzsche et al"/> Later, SRR was redefined to include cases where males in a species have higher parental investment. The concept's most notable mention was in Darwin’s The Descent of Man and Selection in Relation to Sex (1871). He brought attention to females having to undergo selection for access to male mating partners instead of the assumed natural order of males undergoing intra/intersexual competition of the time.<ref name="Eens et al."/>

From this point, several key studies were conducted on populations where females sought males, following Darwin’s description of the bird Barred buttonquail(Turnix suscitator).<ref name="Eens et al."/> This included studies of species that consistently subverted expected sex hierarchy norms, such as studies of the Pipefish family(Syngnathinae) or Seahorse family(Hippocampus).<ref name="Vincent et al"/> Females of these species are generally larger, more colorful, and more aggressive than males.<ref name="Eens et al."/> Most studies that succeeded after Darwin’s notes focused on this supposed reversal of animal behavior and attempted to understand what caused this female dominance.<ref name="Gwynne"/> Most studies during the first half of the 20th century believed it to be a result of unbalanced sex ratios, i.e, many more females than males of a population, and for this hierarchy to be unchanging.<ref name="Fritzsche et al"/> Keynote studies on reproduction of fruit flies(Drosophila) in the 1960s, however, illustrated sexual diversity based on environmental factors such as food availability as well as sex ratios.<ref name="Merrell"/> What was also distinct about the studies was that they demonstrated how quickly the sexes' passive and dominant roles could change given the ecological conditions.<ref name="Merrell"/>

The redefinition and wide use of SRR as a tool in animal behavior came from the 1970s. Darwin’s mention of sex role reversal resulting from a much larger ratio of females to males was later picked up by researchers Stephen T. Emlen and Lewis W. Oring, who reworked the initial concept to a concrete definition. As opposed to females, SRR became redefined by males taking on the bulk of parental investment of offspring.<ref name="Fritzsche et al"/> Researchers were attempting to exclude human-biased projections onto animal behavior. SRR showing parental investment as opposed to “masculine” sexual behaviors performed by females was an attempt to exclude human bias from animal observation. The highest consistency of males taking on higher parental investment than females was most noted in fish, bird, or amphibian species.<ref name="Fritzsche et al"/> Biologists’ new understanding of sexual selection came from observing mate selection based upon resource availability. Resource and mating trade-offs exist for any sexually reproducing organism. For example, male Sandpipers(Actitis macularia) are generally responsible for caring for the nest and protecting eggs, a task seen in other bird species to be shared with or exclusive to female members.<ref name="Gwynne"/> Thus, other mating opportunities for male sandpipers are a tradeoff for offspring care, making them unavailable for breeding. Generally, the sex that has to produce eggs and care for offspring in the zygote stage will divest resources from itself in the post-zygotic stage. This means that during this period, they would be unable to engage in other mating opportunities, providing a barrier to mating rates. Instances of the zygote carrying sex (i.e, the female) devoting less time to post-zygote parenting do present many questions to population behaviour.<ref name="Gwynne"/>

After Emlen and Oring's publications,<ref name="Emlen et al"/> questions remained on what environmental conditions led to SRR. A smaller ratio of males to females is the most widely accepted reason for SRR. Populations that consistently have a higher number of females to males have been noted for distinct dimorphism, as well as males undertaking higher parental investment in particular species. Animals exhibiting monogamous mating behaviour very rarely have traits of SRR. The most common mating type to take on SSR is polygamous species, in which dominant individuals have access to many mating partners, and conspecifics are driven away. Examples of polygamy coinciding with SRR are consistent with the Gulf pipefish(Syngnathus scovelli).<ref name="Vincent et al"/> Pipefish have been studied for decades due to observations of males being the choosier sex of mating partners, and females selected based on markers of higher quality, such as sex and the presence of secondary traits. Dominant females had numerous male mates, while other females were driven away. The study found that the same line of strong sexual dimorphism in polygamous species was carried to SRR examples, with females being the ones facing sexual selection as opposed to males.<ref name="Gwynne"/>

Other factors, like resource availability, may affect populations switching towards or away from SRR. Recently, however, biologists studying numerous different insect species have found that nest availability could be an enormous factor. The relative availability of nests for females to lay eggs was a determining factor in whether or not parental investment by males and female sexual dominance became present in the Broad-winged katydid(Microcentrum rhombifolium). When nests are less frequent in an environment, females undergo intrasexual competition to gain access to males, and males generally take on more parenting responsibilities. Co-parenting becomes much more common for populations of katydids in a habitat that allows for more nests, and female aggression decreases.<ref name="Gwynne"/>

Researchers now understand that the dynamics between male and female counterparts are more complex than “female dominant” and “male dominant.” The field is studying the adaptation of different sexual roles as a result of fluid factors such as sex ratio or resource investment.

In different taxa

Sexual selection is widely distributed among the eukaryotes, occurring in plants, fungi, and animals. Since Darwin's pioneering observations on humans, it has been studied intensively among the insects, spiders, amphibians, scaled reptiles, birds, and mammals, revealing many distinctive behaviours and physical adaptations.<ref name="Nieuwenhuis 2012"/>

In mammals

{{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}}

Darwin conjectured that heritable traits such as beards, hairlessness, and steatopygia in different human populations are results of sexual selection in humans.<ref>Template:Cite book</ref> Humans are sexually dimorphic; females select males using factors including voice pitch, facial shape, muscularity, and height.<ref>Template:Cite book</ref><ref name="Feinberg Jones 2006">Template:Cite journal</ref>

Among the many instances of sexual selection in mammals is extreme sexual dimorphism, with males as much as six times heavier than females, and male fighting for dominance among elephant seals. Dominant males establish large harems of several dozen females; unsuccessful males may attempt to copulate with a harem male's females if the dominant male is inattentive. This forces the harem male to defend his territory continuously, not feeding for as much as three months.<ref>Template:Cite encyclopedia</ref><ref name="McCann 1981">Template:Cite journal</ref>

Also seen in mammals is sex-role reversal, as in the highly social meerkats, where a large female is dominant within a pack, and female–female competition is observed. The dominant female produces most of the offspring; the subordinate females are nonbreeding, providing altruistic care to the young.<ref>Template:Cite journal</ref><ref name=coop>Template:Cite journal</ref>

In arthropods

{{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}}

Sexual selection occurs in a wide range of spider species, both before and after copulation.<ref name="Eberhard 2009">Template:Cite journal</ref> Post-copulatory sexual selection involves sperm competition and cryptic female choice. Sperm competition occurs where the sperm of more than one male competes to fertilise the egg of the female. Cryptic female choice involves the expelling of a male's sperm during or after copulations.<ref name="Peretti Eberhard 2010">Template:Cite journal</ref>

Many forms of sexual selection exist among the insects. Parental care is often provided by female insects, as in bees, but male parental care is found in belostomatid water bugs, where the male, after fertilizing the eggs, allows the female to glue her eggs onto his back. He broods them until the nymphs hatch 2–4 weeks later. The eggs are large and reduce the ability of the male to fertilise other females and catch prey, and increases its predation risk.<ref name="Gilbert Manica 2015">Template:Cite journal</ref>

Among the fireflies (Lampyrid beetles), males fly in darkness and emit a species-specific pattern of light flashes, which are answered by perching receptive females. The colour and temporal variation of the flashes contribute to success in attracting females.<ref name="Lewis Cratsley 2008">Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> Among the beetles, sexual selection is common. In the mealworm beetle, Tenebrio molitor, males release pheromones to attract females to mate.<ref>Template:Cite journal</ref> Females choose mates based on whether they are infected, and on their mass.<ref>Template:Cite journal</ref>

In molluscs

Postcopulatory intersexual selection occurs in Idiosepius paradoxus, the Japanese pygmy squid. Males place their spermatangia on an external location on the female's body. The female physically removes spermatangia of males she is presumed to favour less.<ref name="Sato-2016">Template:Cite journal</ref><ref name="Sato-2013">Template:Cite journal</ref>

In amphibians and reptiles

{{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}}

Many amphibians have annual breeding seasons with male–male competition. Males arrive at the water's edge first in large numbers, and produce a wide range of vocalizations to attract mates. Among frogs, the fittest males have the deepest croaks and the best territories; females select their mates at least partly based on the depth of croaking. This has led to sexual dimorphism, with females larger than males in 90% of species, and male fighting to access females.<ref>Template:Cite journal</ref><ref name="Wells Schwartz 2007">Template:Cite book</ref> Spikethumb frogs are suggested to engage in male-male competition with their elongated prepollex to maintain their mating site.<ref name="Gonzalez-Mollinedo-2020">Template:Cite journal</ref> The prepollex, which serves as a rudimentary digit, contains a projecting spine that may be used during this combat, leaving scars on the heads and forelimbs of other males.<ref name="Duellman-1992">Template:Cite journal</ref>

Many different tactics are used by snakes to acquire mates. Ritual combat between males for the females they want to mate with includes topping, a behaviour exhibited by most viperids, in which one male twists around the vertically elevated fore body of its opponent and forcing it downward. Neck biting is common while the snakes are entwined.<ref name="Shine Langkilde Mason 2004">Template:Cite journal</ref><ref name="Blouin-Demers Gibbs Weatherhead 2005">Template:Cite journal</ref>

In birds

{{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}}

Birds have evolved a wide variety of mating behaviours and many types of sexual selection. These include intersexual selection (female choice) and intrasexual competition, where individuals of the more abundant sex compete with each other for the privilege to mate. Many species, notably the birds-of-paradise, are sexually dimorphic; the differences such as in size and coloration are energetically costly attributes that signal competitive breeding. Conflicts between an individual's fitness and signalling adaptations ensure that sexually selected ornaments such as coloration of plumage and courtship behaviour are honest traits. Signals must be costly to ensure that only good-quality individuals can present these exaggerated sexual ornaments and behaviours. Males with the brightest plumage are favoured by females of multiple species of bird.<ref name=saino2013>Template:Cite journal</ref><ref name=edwards2012>Template:Cite journal</ref><ref name="Doutrelant 2012">Template:Cite journal</ref>

Many bird species make use of mating calls, the females preferring males with songs that are complex and varied in amplitude, structure, and frequency. Larger males have deeper songs and increased mating success.<ref>Template:Cite journal</ref><ref name="Pfaff 2007">Template:Cite journal</ref><ref name="Nemeth 2012">Template:Cite journal</ref><ref>Template:Cite journal</ref>

In plants and fungi

{{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}}

Flowering plants have many secondary sexual characteristics subject to sexual selection including floral symmetry if pollinators visit flowers assortatively by degree of symmetry,<ref name="Møller Eriksson 1995">Template:Cite journal</ref> nectar production, floral structure, and inflorescences, as well as sexual dimorphisms.<ref name="Ashman Delph 2006">Template:Cite journal</ref><ref name="Moore Pannell 2011">Template:Cite journal</ref><ref>Template:Cite journal</ref>

Fungi appear to make use of sexual selection, although they also often reproduce asexually. In the Basidiomycetes, the sex ratio is biased towards males, implying sexual selection there. Male–male competition to fertilise occurs in fungi including yeasts. Pheromone signaling is used by female gametes and by conidia, implying male choice in these cases. Female–female competition may also occur, indicated by the much faster evolution of female-biased genes in fungi.<ref name="Nieuwenhuis 2012">Template:Cite journal</ref><ref>Template:Cite journal</ref><ref name="Beekman Nieuwenhuis Ortiz-Barrientos Evans 2016">Template:Cite journal</ref><ref name="Whittle Johannesson 2013">Template:Cite journal</ref>

• Example sexual selection mechanisms in the named taxa
• Gorillafamily.JPG

  Among mammals, the male gorilla is much larger than female.

• Phidippus putnami male.jpg

  Males of many spiders, such as this Phidippus putnami, have elaborate courtship displays.

• Toe-Biter.jpg

  A male Abedus indentatus belostomatid bug carries eggs on its back.

• Fireflies, Georgia, US (detail).jpg

  Each firefly species attracts mates with its own flash pattern.

• Dendropsophus microcephalus - calling male (Cope, 1886).jpg

  Male Dendropsophus microcephalus calling

• Indian rat snake,Ptyas mucosa, Territorial Fight.jpg

  Territorial fight in the Indian rat snake, Ptyas mucosa

• Victoria's Riflebird courtship - Lake Eacham - Queensland S4E8070 (22198704599) (cropped).jpg

  Male Victoria's riflebird displaying to a female

• Satin Bowerbird nest.jpg

  A male satin bowerbird guards its bower from rival males in the hope of attracting females with its decorations.

• MacquarieIslandElephantSeal.JPG

  Male southern elephant seals fighting on Macquarie Island for the right to mate

• Lily Lilium 'Citronella' Flower.jpg

  Citronella flower's symmetry may have been subject to sexual selection by its pollinators.

• RanaArvalisBlueMale3.jpg

  Male moor frogs become blue to signal their fitness to females.

References

<ref name="Eens et al.">Template:Cite journal</ref>

<ref name="Emlen et al">Template:Cite journal</ref>

<ref name="Fritzsche et al">Template:Cite journal</ref>

<ref name="Gwynne">Template:Cite journal</ref>

<ref name="Merrell">Template:Cite journal</ref>

<ref name="Vincent et al">Template:Cite journal</ref>

<ref name="Williams et al">Template:Cite book</ref>

Further reading

• Template:Cite book

Template:Sociobiology Template:Speciation Template:Sex (biology) {{#invoke:Navbox|navbox}} Template:Patterns in nature {{#invoke:Navbox|navbox}} Template:Portal bar