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The sciences of evolutionary psychologyhuman behavioural ecologyand sociobiology study the influence of sexual selection in humans. Darwin's ideas on sexual selection were met with scepticism by his contemporaries and not considered of great importance in the early 20th century, until in the s biologists decided to include sexual selection as a mode of natural selection.
Hamiltonholds that the fact that the male is able to survive until and through the age of reproduction with such a seemingly maladaptive trait is taken by the female to be a testament to his overall fitness.
Such handicaps might prove he is either free of or resistant to diseaseor that he possesses more speed or a greater physical strength that is used to combat the troubles brought on by the exaggerated trait. Zahavi's work spurred a re-examination of the field, which has produced an ever-accelerating number of theories.
InHamilton 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.
InMichael Ryan and A. Rand, working with the tungara frogproposed the hypothesis of "Sensory Exploitation", where exaggerated male traits may provide a sensory stimulation that females find hard to resist.
Rice have been added. In the late s, Janzen and Mary Willson, noting that male flowers are often larger than female flowers, expanded the field of sexual selection into plants.
These include cuckoldryEnglish is the natural choice for gifts, sperm competitioninfanticide especially in primates English is the natural choice for, physical beautymating by subterfuge, species isolation mechanisms, male parental care, ambiparental care, mate location, polygamy, and homosexual rape in certain male animals.
Sexual conflict leads to an antagonistic co-evolution in which one sex tends to control the other, resulting in a tug of war.
Besides, the sexual propaganda theory only argued that mates were opportunistically lead, on the basis of various factors determining the choice such as phenotypic characteristics, apparent vigour of individuals, strength of mate signals, trophic resources, territoriality etc.
One possible explanation for the apparent lack of costs is that "compensatory traits" have evolved in concert with the sexually selected traits. Geoffrey Miller proposes that sexual selection might have contributed by creating evolutionary modules such as Archaeopteryx feathers as sexual ornaments, at first.
Some have suggested that the feathers served as insulation, helping females incubate their eggs. But perhaps the feathers served as the kinds of sexual ornaments still common in most bird species, and especially in birds such as peacocks and birds-of-paradise today.
If proto-bird courtship displays combined displays of forelimb feathers with energetic jumps, then the transition from display to aerodynamic functions could have been relatively smooth.
Please help improve this section by adding citations to reliable sources. Unsourced material may be challenged and removed. November 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 reproductionare called secondary sex characteristics.
The rhinoceros beetle is a classic case of sexual dimorphism. Plate from Darwin's Descent of Manmale at top, female at bottom 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 reptilesand this difference in initial investment creates differences in variance in expected reproductive success and bootstraps the sexual selection processes.
Classic examples of reversed sex-role species include the pipefishand Wilson's phalarope . 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 are typically more willing to mate than females, and so females are typically the ones doing the choosing except in cases of forced copulationswhich can occur in certain species of primatesducksand others.
The effects of sexual selection are thus held to typically be 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 deerthe 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 peacockwith 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 .
Many other fish such as guppies also exhibit sexual dimorphism. Extreme sexual size dimorphism, with females larger than males, is quite common in spiders and birds of prey. The Role of Male-Male Competition in Sexual Selection[ edit ] Male-male competition is when two males of the same species compete for the opportunity to mate with a female.
Sexually dimorphic traits, size, sex ratio  and the social situation  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.
There are multiple types of male-male competition that may occur in a population at different times depending on the conditions.English-German online dictionary developed to help you share your knowledge with others.
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