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=== In birds ===
=== In birds ===
[[File:Victoria's Riflebird courtship - Lake Eacham - Queensland S4E8070 (22198704599) (cropped).jpg|thumb|upright|[[Bird-of-paradise|Birds-of-paradise]] such as this [[Victoria's riflebird]] have elaborate courtship displays in which the males show off their plumage and fitness to the females.]]
[[File:Victoria's Riflebird courtship - Lake Eacham - Queensland S4E8070 (22198704599) (cropped).jpg |thumb |upright |[[Bird-of-paradise |Birds-of-paradise]] such as this [[Victoria's riflebird]] have elaborate courtship displays in which the males show off their plumage and fitness to the females.]]


{{main|Sexual selection in birds}}
{{main |Sexual selection in birds}}


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 [[Bird-of-paradise|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.<ref name=edwards2012>{{cite journal |last=Edwards |first=D.B. |title=Immune investment is explained by sexual selection and pace-of-life, but not longevity in parrots (Psittaciformes) |journal=PLOS ONE |year=2012 |volume=7 |issue=12 |pages=e53066 |pmid=23300862 |doi=10.1371/journal.pone.0053066 |pmc=3531452 |bibcode=2012PLoSO...753066E |doi-access=free }}</ref> <ref name="Doutrelant 2012">{{cite journal |last=Doutrelant |first=C. |last2=Grégoire |first2=A. |last3=Midamegbe |first3=A. |last4=Lambrechts |first4=M. |last5=Perret |first5=P. |title=Female plumage coloration is sensitive to the cost of reproduction. An experiment in blue tits.|journal=[[Journal of Animal Ecology]]|date=January 2012 |volume=81 |issue=1 |pages=87–96 |pmid=21819397 |doi=10.1111/j.1365-2656.2011.01889.x |doi-access=free}}</ref>{{-}}
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 [[Bird-of-paradise |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.<ref name=edwards2012>{{cite journal |last=Edwards |first=D.B. |title=Immune investment is explained by sexual selection and pace-of-life, but not longevity in parrots (Psittaciformes) |journal=PLOS ONE |year=2012 |volume=7 |issue=12 |pages=e53066 |pmid=23300862 |doi=10.1371/journal.pone.0053066 |pmc=3531452 |bibcode=2012PLoSO...753066E |doi-access=free }}</ref> <ref name="Doutrelant 2012">{{cite journal |last=Doutrelant |first=C. |last2=Grégoire |first2=A. |last3=Midamegbe |first3=A. |last4=Lambrechts |first4=M. |last5=Perret |first5=P. |title=Female plumage coloration is sensitive to the cost of reproduction. An experiment in blue tits |journal=[[Journal of Animal Ecology]] |date=January 2012 |volume=81 |issue=1 |pages=87–96 |pmid=21819397 |doi=10.1111/j.1365-2656.2011.01889.x |doi-access=free}}</ref>

Many bird species make use of [[mating call]]s, 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>{{cite journal |last=Hall |first=. L. |author2=Kingma, S. A. |author3=Peters, A. |title=Male songbird indicates body size with low-pitched advertising songs. |journal=PLOS ONE |year=2013 |volume=8 |issue=2 |pages=e56717 |pmid=23437221 |doi=10.1371/journal.pone.0056717 |pmc=3577745 |bibcode=2013PLoSO...856717H |doi-access=free}}</ref><ref name="Pfaff 2007">{{cite journal |last=Pfaff |first=J. A. |author2=Zanette, L. |author3=MacDougall-Shackleton, S. A. |author4=MacDougall-Shackleton, E. A. |title=Song repertoire size varies with HVC volume and is indicative of male quality in song sparrows (Melospiza melodia) |journal=[[Proceedings of the Royal Society B]] |date=22 August 2007 |volume=274 |issue=1621 |pages=2035–40 |pmid=17567560 |doi=10.1098/rspb.2007.0170 |pmc=2275172}}</ref><ref name="Nemeth 2012">{{cite journal |last=Nemeth |first=E. |author2=Kempenaers, B. |author3=Matessi, G. |author4=Brumm, H. |title=Rock sparrow song reflects male age and reproductive success |journal=PLOS ONE |year=2012 |volume=7 |issue=8 |pages=e43259 |pmid=22927955 |doi=10.1371/journal.pone.0043259 |pmc=3426517 |bibcode=2012PLoSO...743259N |doi-access=free}}</ref>{{-}}


=== In mammals ===
=== In mammals ===

Revision as of 10:42, 16 March 2022

This article is about the evolutionary concept. For the artificial selection of the sex of offspring, see sex selection.

Sexual selection creates colourful differences between sexes (sexual dimorphism) in Goldie's bird-of-paradise. Male above; female below. Painting by John Gerrard Keulemans (d.1912)

Sexual selection is a mode of natural selection in which members of one biological 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.[1][2] Successful males benefit from frequent mating and monopolizing access to one or more fertile females. Females can have a limited number of offspring and maximize the return on the energy they invest in reproduction.

The concept was first articulated by Charles Darwin who wrote of a "second agency" of selection, in which competition between mate candidates could lead to speciation.[3] 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 such as birds of paradise and peafowl, or the antlers of deer, or the manes of lions, 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 also found in plants and fungi.[4][5][6]

History

Darwin

Sexual selection was first proposed by Charles Darwin in 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.

... depends, not on a struggle for existence, but on a struggle between the males for possession of the females; the result is not death to the unsuccessful competitor, but few or no offspring.[7]

... when the males and females of any animal have the same general habits ... but differ in structure, colour, or ornament, such differences have been mainly caused by sexual selection.[8]

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.[9]

Ronald Fisher

Ronald Fisher, the English statistician and evolutionary biologist developed a number of ideas about sexual selection in his 1930 book The Genetical Theory of Natural Selection including the sexy son hypothesis and Fisher's principle. 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. In a remark that was not widely understood[10] for another 50 years he said:

... plumage development in the male, and sexual preference for such developments in the female, must thus advance together, and so long as the process is unchecked by severe counterselection, will advance with ever-increasing speed. In the total absence of such checks, it is easy to see that the speed of development will be proportional to the development already attained, which will therefore increase with time exponentially, or in geometric progression. —Ronald Fisher, 1930

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.[11] 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.[10]

Richard Dawkins presents a non-mathematical explanation of the runaway sexual selection process in his book The Blind Watchmaker.[10] 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[12]

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.[10]

It is important to notice that the conditions of relative stability brought about by these or other means, will be far longer duration than the process in which the ornaments are evolved. In most existing species the runaway process must have been already checked, and we should expect that the more extraordinary developments of sexual plumage are not due like most characters to a long and even course of evolutionary progress, but to sudden spurts of change. —Ronald Fisher, 1930

After Fisher

Since Fisher's initial conceptual model of the 'runaway' process, Russell Lande[13] and Peter O'Donald[14] have provided detailed mathematical proofs that define the circumstances under which runaway sexual selection can take place. Alongside this, biologists have extended Darwin's formulation; Malte Andersson's widely-accepted[15] 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".[11][15] Despite some practical challenges for biologists, the concept of sexual selection is "straightforward".[15]

Theory

Reproductive success

Extinct Irish elk (Megaloceros giganteus). These antlers span 2.7 metres (8.9 ft) and have a mass of 40 kg (88 lb).

The reproductive success of an organism is measured by the number of offspring left behind, and their quality or probable fitness.[16]

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.[17] More recently, researchers have doubted whether Bateman was correct.[18]

Modern interpretation

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.[19] Only in the 21st century have they become more important in biology;[20] the theory is now seen as generally applicable and analogous to natural selection.[21]

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.[22]

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.[23][24][25] 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.[26]

In 1990, Michael Ryan and A.S. Rand, working with the Túngara frog, proposed the hypothesis of "Sensory Exploitation", where exaggerated male traits may provide a sensory stimulation that females find hard to resist.[27] In the late 1970s, Janzen and Mary Willson, noting that male flowers are often larger than female flowers, expanded the field of sexual selection into plants.[28]

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"[29] range of models variously attempt to relate sexual selection not only to the fundamental[29] questions of anisogamy and parental roles, but also to mechanisms such as sex ratios, parental care, having sexy sons, sexual conflict, and the "most-debated effect",[29] namely mate choice.[29]

Elaborated characteristics that might seem costly for their bearers (e.g., the tail of the 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.[30]

  • Male mountain gorilla, a tournament species
    Male mountain gorilla, a tournament species
  • Sexual selection protected flour beetles from extinction in a ten-year experiment.[22]
    Sexual selection protected flour beetles from extinction in a ten-year experiment.[22]
  • Túngara frog exemplified theory of "Sensory Exploitation", where exaggerated male traits supposedly made males irresistible to females.[27]
    Túngara frog exemplified theory of "Sensory Exploitation", where exaggerated male traits supposedly made males irresistible to females.[27]

Toolkit of natural selection

Reconstruction of Protarchaeopteryx, an early proto-bird

Sexual selection may explain how characteristics such as feathers had survival value at an early stage in their evolution. Geoffrey Miller proposes that the feathers of proto-birds like Archaeopteryx were originally sexual ornaments. The earliest proto-birds such as Protarchaeopteryx had well-developed feathers but no sign of the top/bottom asymmetry that gives wings lift. One proposal is that the feathers served as insulation, helping females incubate their eggs. But if proto-bird courtship combined displays of forelimb feathers with energetic jumps, then the transition from display to aerodynamic functions could have been relatively smooth.[31]

Sexual selection sometimes generates features that may help cause a species' extinction, as has been suggested[31] for the giant antlers of the Irish elk (Megaloceros giganteus) that became extinct in Pleistocene Europe.[32] Or it may do the opposite, driving species divergence—sometimes through elaborate changes in genitalia[33]—such that new species emerge.[34][35][36]

Sexual dimorphism

Main article: Sexual dimorphism

Sex differences directly related to reproduction and serving no direct purpose in courtship are called primary sexual characteristics. Traits amenable to sexual selection, which give an organism an advantage over its rivals (such as in courtship) without being directly involved in reproduction, are called secondary sex characteristics.

The rhinoceros beetle is a classic case of sexual dimorphism. Plate from Darwin's Descent of Man (male above)

In most sexual species the males and females have different equilibrium strategies, due to a difference in relative investment in producing offspring. As formulated in Bateman's principle, females have a greater initial investment in producing offspring (pregnancy in mammals or the production of the egg in birds and reptiles), and this difference in initial investment creates differences in variance in expected reproductive success and bootstraps the sexual selection processes. (Pipefish and Wilson's phalarope are classic examples of sex role reversal.[37]) Also, unlike a female, a male (except in monogamous species) has some uncertainty about whether or not he is the true parent of a child, and so is less interested in spending his energy helping to raise offspring that may or may not be related to him. As a result of these factors, males can be expected to be more willing to mate than females, while females are expected to be the ones doing the choosing (except in cases of forced copulations, which has been observed in numerous species, including mammals, birds, insects and fish[38]). The effects of sexual selection are thus often more pronounced in males than in females.

Differences in secondary sexual characteristics between males and females of a species are referred to as sexual dimorphisms. These can be as subtle as a size difference (sexual size dimorphism, often abbreviated as SSD) or as extreme as horns and colour patterns. Sexual dimorphisms abound in nature. Examples include the possession of antlers by only male deer, the brighter coloration of many male birds in comparison with females of the same species, or even more distinct differences in basic morphology, such as the drastically increased eye-span of the male stalk-eyed fly. The peacock, with its elaborate and colourful tail feathers, which the peahen lacks, is often referred to as perhaps the most extraordinary example of a dimorphism. Male and female black-throated blue warblers and Guianan cock-of-the-rocks also differ radically in their plumage. Early naturalists even believed the females to be a separate species. The largest sexual size dimorphism in vertebrates is the shell dwelling cichlid fish Neolamprologus callipterus in which males are up to 30 times the size of females.[39] Many other fish such as guppies are sexually dimorphic. Extreme sexual size dimorphism, with females larger than males, is quite common in spiders and birds of prey.

The maintenance of sexual reproduction in a highly competitive world is one of the major puzzles in biology given that asexual reproduction can reproduce much more quickly as 50% of offspring are not males, unable to produce offspring themselves. Many non-exclusive hypotheses have been proposed,[40] including the positive impact of an additional form of selection, sexual selection, on the probability of persistence of a species.[22]

Male intrasexual competition

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,[41] and the social situation[42] 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 due to their sheer strength and ability to ward off other males from taking over their females. For instance, in the fly Dryomyza anilis, size shows the strongest correlation to the outcome of male-male conflicts over resources like territory and females.[43]

Influencing factors

Sex ratio

Japanese medaka, Oryzias latipes

There are multiple types of male-male competition that may occur in a population at different times depending on the conditions. Competition variation occurs based on the frequency of various mating behaviours present in the population.[41] One factor that can influence the type of competition observed is the population density of males.[41] When there is a high density of males present in the population, competition tends to be less aggressive and therefore sneak tactics and disruptions techniques are more often employed.[41] These techniques often indicate a type of competition referred to as scramble competition.[41] In Japanese medaka, Oryzias latipes, sneaking behaviours refer to when a male interrupts a mating pair during copulation by grasping on to either the male or the female and releasing their own sperm in the hopes of being the one to fertilize the female.[41] Disruption is a technique which involves one male bumping the male that is copulating with the female away just before his sperm is released and the eggs are fertilized.[41]

However, all techniques are not equally successful when in competition for reproductive success. Disruption results in a shorter copulation period and can therefore disrupt the fertilization of the eggs by the sperm, which frequently results in lower rates of fertilization and smaller clutch size.[41]

Resource value and social ranking

Another factor that can influence male-male competition is the value of the resource to competitors. Male-male competition can pose many risks to a male's fitness, such as high energy expenditure, physical injury, lower sperm quality and lost paternity.[44] The risk of competition must therefore be worth the value of the resource. A male is more likely to engage in competition for a resource that improves their reproductive success if the resource value is higher. While male-male competition can occur in the presence or absence of a female, competition occurs more frequently in the presence of a female.[42] The presence of a female directly increases the resource value of a territory or shelter and so the males are more likely to accept the risk of competition when a female is present.[42] The smaller males of a species are also more likely to engage in competition with larger males in the presence of a female.[42] Due to the higher level of risk for subordinate males, they tend to engage in competition less frequently than larger, more dominant males and therefore breed less frequently than dominant males.[44] This is seen in many species, such as the Omei treefrog, Rhacophorus omeimontis, where larger males obtain more mating opportunities and mate with larger females.[45]

Winner–loser effects

A third factor that can impact the success of a male in competition is winner-loser effects.[46] Burrowing crickets, Velarifictorus aspersus, compete for burrows to attract females using their large mandibles for fighting.[46] Female burrowing crickets are more likely to choose winner of a competition in the 2 hours after the fight.[46] The presence of a winning male suppresses mating behaviours of the losing males because the winning male tends to produce more frequent and enhanced mating calls in this period of time.[46]

Effect on female fitness

Male-male competition can both positively and negatively affect female fitness. When there is a high density of males in a population and a large number of males attempting to mate with the female, she is more likely to resist mating attempts, resulting in lower fertilization rates.[41] High levels of male-male competition can also result in a reduction in female investment in mating.[44] Many forms of competition can also cause significant distress for the female negatively impacting her ability to reproduce.[41] An increase in male-male competition can affect a female's ability to select the best mates, and therefore decrease the likelihood of successful reproduction.[47]

However, group mating in Japanese medaka increases the fitness of females due to an increase in genetic variation, a higher likelihood of paternal care, and a higher likelihood of successful fertilization.[41] Exposure to environmental estrogens, such as some herbicides, can confuse female choice of males.[48]

In different taxa

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 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.[5]

In humans

Main article: Sexual selection in humans

Darwin conjectured that heritable traits such as beards, hairlessness, and steatopygia in different human populations are results of sexual selection in humans.[49] Geoffrey Miller has suggested that many human behaviours not clearly tied to survival benefits, such as humour, music, visual art, verbal creativity, and some forms of altruism, are courtship adaptations that have been favoured through sexual selection. In that view, many human artefacts could be considered subject to sexual selection as part of the extended phenotype, for instance clothing that enhances sexually selected traits.[50] Some argue that the evolution of human intelligence is a sexually selected trait, as it would not confer enough fitness in itself relative to its high maintenance costs.[51]

In spiders

Males of many spiders, such as this Phidippus putnami, have elaborate courtship displays.
Main article: Sexual selection in spiders

Sexual selection occurs in a wide range of spider species, both before and after copulation.[52] Post-copulatory sexual selection involves sperm competition and cryptic female choice. Sperm competition occurs where the sperm of more than one male competes to fertilize the egg of the female. Cryptic female choice involves the expelling of a male's sperm during or after copulations.[53]

In insects

Parental care may be provided by either sex. Here a male Abedus indentatus belostomatid bug carries eggs on its back.
Main article: Sexual selection in insects

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 fertilize other females and catch prey, and increases its predation risk.[54]

In amphibians

Male Dendropsophus microcephalus calling
Main article: Sexual selection in amphibians

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.[55][56]

In scaled reptiles

Main article: Sexual selection in scaled reptiles

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 behavior exhibited by most viperids in which one male will twist around the vertically elevated fore body of its opponent and forcing it downward. It is common for neck biting to occur while the snakes are entwined.[57][58]

In birds

Birds-of-paradise such as this Victoria's riflebird have elaborate courtship displays in which the males show off their plumage and fitness to the females.
Main article: Sexual selection in birds

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.[59] [60]

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.[61][62][63]

In mammals

Male southern elephant seals fighting on Macquarie Island for the right to mate
Main article: Sexual selection in mammals

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 males to defend his territory continuously, not feeding for as much as three months.[64][65]

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.[66][67]

References

  1. ^ Cecie Starr (2013). Biology: The Unity & Diversity of Life (Ralph Taggart, Christine Evers, Lisa Starr ed.). Cengage Learning. p. 281.
  2. ^ Vogt, Yngve (January 29, 2014). "Large testicles are linked to infidelity". Phys.org. Archived from the original on January 31, 2014. Retrieved January 31, 2014.
  3. ^ Darwin, Charles (1858). "On the Tendency of Species to form Varieties; and on the Perpetuation of Varieties and Species by Natural Means of Selection" (PDF). Journal of the Proceedings of the Linnean Society of London. Zoology. 3 (9): 46–50. doi:10.1111/j.1096-3642.1858.tb02500.x. Archived (PDF) from the original on 2012-10-22.
  4. ^ Moore, Jamie C.; Pannell, John R. (2011). "Sexual selection in plants". Current Biology. 21 (5): R176–R182. doi:10.1016/j.cub.2010.12.035. PMID 21377091. S2CID 18044399.
  5. ^ a b Nieuwenhuis, B. P. S.; Aanen, D.K. (2012). "Sexual selection in fungi". Journal of Evolutionary Biology. 25 (12): 2397–2411. doi:10.1111/jeb.12017. PMID 23163326. S2CID 5657743.
  6. ^ Leonard, Janet L. (2006-08-01). "Sexual selection: lessons from hermaphrodite mating systems". Integrative and Comparative Biology. 46 (4): 349–367. doi:10.1093/icb/icj041. ISSN 1540-7063. PMID 21672747.
  7. ^ Darwin, Charles (1859). On the Origin of Species (1st edition). Chapter 4, p. 88. "And this leads me to say a few words on what I call Sexual Selection. This depends ..." "Archived copy". Archived from the original on 2011-11-05. Retrieved 2011-05-22.{{cite web}}: CS1 maint: archived copy as title (link)
  8. ^ Darwin, Charles (1859). On the Origin of Species (1st edition). Chapter 4, p. 89. "Archived copy". Archived from the original on 2011-11-05. Retrieved 2011-05-22.{{cite web}}: CS1 maint: archived copy as title (link)
  9. ^ Wallace, Alfred Russel (1892). "Note on Sexual Selection (S459: 1892)". Charles Smith. Archived from the original on 17 February 2017. Retrieved 13 January 2017.
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