Sexual dimorphism
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[[WP:UNDO|Undid]] revision 225615622 by [[Special:Contributions/124.217.52.252|124.217.52.252]] ([[User talk:124.217.52.252|talk]]): "Peacock" is a bird, and not a sexual innuendo.
[[Image:Male and female pheasant.jpg|thumb|250px|Female (left) and male [[Common Pheasant]], illustrating the dramatic difference in both color and size, between the sexes]]
'''Sexual dimorphism''' is the systematic difference in form between individuals of different [[sex]] in the same [[species]]. Examples include size, color, and the presence or absence of parts of the body used in courtship displays or fights, such as ornamental feathers, horns, antlers or tusks.
Dimorphism comes from two Greek words, ''di'' meaning two and ''morph'' meaning form. Thus, sexual dimorphism basically refers to two forms of sex, male and female.
==Examples==
[[Image:Peacock courting peahen.jpg|thumb|[[Peacock]] courting [[peahen]]]]
In many species, including most [[mammal]]s, the [[male]] is larger than the [[female]]<ref name="Lindenfors">Lindenfors P, Gittleman JL & Jones KE, Sexual size dimorphism in mammals, ''in: Fairbairn DJ, Blanckenhorn WU & Szekely T (eds) Sex, size and gender roles: evolutionary studies of sexual size dimorphism ''(Oxford: Oxford University Press, 2007), pages 19-26</ref>. In others, such as most [[insect]]s, [[spider#Reproduction|spider]]s, [[birds]], [[reptiles]] and [[amphibian]]s, many [[fish]], and certain mammals such as the [[spotted hyena]], the female is larger than the male. Other sex-specific differences include differences in colouration (''sexual dichromatism''), presence vs. absence of certain body parts such as horns, antlers, tusks or display feathers; size of the eyes (some insects); possession of stings (various kinds of [[Hymenoptera]]), and different thresholds for certain behaviors (aggression, infant care, etc).[[Image:Male and female A. appensa.jpg|thumb|left|160px|Female and male ''[[Argiope appensa]]'', displaying reverse sexual dimorphism]]
Among vertebrates, sexual dimorphism is particularly apparent in [[duck]]s, and most [[gamefowl]] perhaps most dramatically including [[peafowl]]. Male pheasants are notably larger than females and possess bright plumage; females are usually a brown irrespective of the particular [[species]]. In some birds (most of which are [[wader]]s such as the [[phalarope]]s and [[painted snipe]]s), females have brighter colors than males. As this is the opposite of the usual sexual dichromatism, it is termed ''reverse sexual dimorphism''. In many predatory birds, females are larger than males and often considerably so. This seems to reduce competition between members of a pair, as they have different optimal prey sizes. Some cases of sexual dimorphism in birds are so striking that males and females of the same species were originally taken to be members of entirely different species, as in the case of the [[Eclectus Parrot]] ''(Eclectus roratus)'', where the male is predominantly green with an orange beak and the female scarlet and deep blue with a black beak.
The [[Huia]] ''(Heteralocha acutirostris)'', a New Zealand bird species (now extinct), was another striking example of sexual dimorphism<!-- Note: this is not reverse dimorphism, which is only where the dimorphism is the reverse of normal.-->. The male's bill was short, sharp and stout while the female's was long, thin and crescent shaped. This beak dimorphism allowed mated pairs of Huia to avoid competing for the same food source, with males chiseling into and breaking apart rotting logs, while females were adept at probing into fresher wood for [[grub]]s.
[[Image:Anas platyrhynchosZZ.jpg|thumb|right|Male (front) and female [[mallard]]s. The male mallard has an unmistakable green head.]]
Certain sexual dimorphisms have obvious utility beyond mate attraction, such as the [[Blue Wildebeest]] (and many other [[biungulates]]). The [[horn (anatomy)|horns]] of the male are much larger, allowing the male to engage in combat more effectively as he competes with other bucks for mating privileges.
An extreme example of sexual dimorphism <!-- Note: this is not reverse dimorphism, which is only where the dimorphism is the reverse of normal. Larger females are not unusual. --> is found the genus ''[[Osedax]]'' of [[polychaete]] worms, which lives on [[whale fall]]s. The females feed on the bones of the dead whale, but the males live inside the females and do not develop past their larval stage except to produce large amounts of sperm. In the [[echiura]]n ''[[Bonellia viridis]]'', exposure to adult females causes larvae which encounter them to develop into tiny, semi-parasitic males which are swallowed and live inside the female's genital sac. The [[argonaut (animal)|argonauts]] also have males which are tiny compared to the female. In the parasitic [[barnacle]]s ''[[Sacculina]]'', the males are tiny, free-ranging animals, whereas the females only exist as a web-like tissue inside their hosts. In the majority of [[scale insect]]s, females are highly modified (eyeless and wingless, with non-functional appendages and reduced segmentation), attached permanently to their host plants, while males are rather ordinary though delicate insects, smaller and winged.[[Image:Cryptopsaras couesii (triplewart seadevil).gif|thumb|250px|right|Female [[Triplewart seadevil]], an anglerfish, with male attached near vent (arrow)]]
Some species of [[anglerfish]] also display extreme <!-- Note: this is not reverse dimorphism, which is only where the dimorphism is the reverse of normal. Larger females are not unusual. --> sexual dimorphism. Females are typical anglerfish, while males are tiny rudimentary creatures with no digestive systems. The males must find a female and fuse with her – he then lives parasitically, becoming little more than a sperm-producing body. A similar situation is found in the Zeus water bug ''[[Phoreticovelia disparata]]'' where the female has a glandular area on her back that can serve to feed a male that clings to her (though males can survive away from females, they generally are not free-living).<ref>Arnqvist, Göran , Therésa M. Jones, Mark A. Elgar (2003)Reversal of sex roles in nuptial feeding. Nature 424:387 [http://www.ebc.uu.se/zooeko/GoranA/nature2003.pdf]</ref>
===Psychological and behavioral differentiation===
[[Sex steroid]]-induced differentiation of adult reproductive and other behavior has been demonstrated experimentally in many animals. In some [[mammals]], adult sex-dimorphic reproductive behavior (e.g., [[mating|mounting]] or [[lordosis behavior|receptive lordosis]]) can be shifted to that of the other sex by supplementation or deprivation of [[androgen]]s in fetal life or early infancy, even if adult levels are normal.
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==Evolution of sexual dimorphism==
===Handicap principle===
{{Main|Handicap principle}}
The handicap principle is the proposed evolutionary force that gives males of some species traits that at first glance seem to place the organism at a ''dis''advantage.
For example, the bright colouration of male game birds makes them highly visible targets for predators, while the drab females are better [[camouflage]]d. Other examples are [[bird of paradise]] and [[lyrebird]], whose males have such large plumes that their flight is inhibited. Strong smells, loud cries and singing can also attract predators.
The answer that has been suggested to this apparent paradox is that, at a biological level, the reproductive success of an organism is often more important than duration of life. This is particularly apparent in the case of game birds: a male [[Common Pheasant]] in the wild often lives no more than 10 months, with females living twice as long. However, a male pheasant's ability to reproduce depends not on how long he lives but whether females will select him to be their mate.
A brightly coloured and heavily plumed male presumably demonstrates to the female that he is [[Fitness (biology)|fit]] in evolutionary terms – [[Image:See elefanten edit.jpg|thumb|200px|right|Sexual dimorphism of the polygamous [[Northern elephant seal]]]] he has been able to survive in spite of impediments and must therefore be a good choice to father her chicks (especially her daughters, who will have his "fit" genes, but will not be hampered by male plumage). This explanation was first proposed by [[Amotz Zahavi]].
Development of such characters could not at first be explained in terms of simple [[natural selection]]. In 1871 [[Charles Darwin]] advanced the [[theory of sexual selection]], which related sexual dimorphism with [[sexual selection]].
===Polygamy===
Comparison of sexual dimorphism in birds and their mating habits shows that the time spent in search for mates, staking territories and mating competes with the demands of taking care of young. For birds and in general, it can be stated that the stronger the dimorphism in a species, the more likely is it to be [[polygamous]] and the less is the task of caring for offspring shared among the sexes. This theory is developed by [[Robert Trivers|R. L. Trivers']] in the [[Parental investment|parental investment theory]]. It applies to all ecology.
==Sexual dimorphism in humans==
{| class="wikitable" width="200" align="right"
| colspan="2" | [[Image:Human.svg|center|200px|Pioneer plaque]]
|-
| [[Image:Gray241.png|center|100px|Male pelvis]]
| [[Image:Gray242.png|center|100px|Female pelvis]]
|-
| colspan="2" |
'''Top:''' Stylised illustration of humans on the [[Pioneer plaque]], showing both male (left) and female (right).
<br />'''Above:''' Comparison between a male (left) and a female [[pelvis]] (right).
|}
{{main|Sex differences in humans}}
Sexual dimorphism in humans is the subject of much controversy, especially relating to mental ability and psychological gender. (For a discussion, see [[biology of gender]], [[sex and intelligence]], [[gender]], and [[transgender]].) Obvious differences between men and women include all the features related to reproductive role, notably the endocrine (hormonal) systems and their physiological and behavioural effects. Such undisputed sexual dimorphism include gonadal differentiation, internal genital differentiation, external genital differentiation, breast differentiation, muscle mass differentiation, and hair differentiation.
The basal [[Metabolism|metabolic]] rate is about 6 percent higher in adolescent boys than girls and increases to about 10 percent higher after puberty. Women tend to convert more food into [[fat]], while men convert more into [[muscle]] and expendable circulating energy reserves. At age eighteen, men (on average) have about 50 percent more muscle mass than women in the upper body, 10 to 15 percent more in the lower. Men, on average, have denser, stronger [[bone]]s, [[tendon]]s, and [[ligament]]s. This allows for heavier work.<ref name=Glucksman>
A Glucksman, ''Sexual Dimorphism in Human and Mammalian Biology and Pathology'', ([[Academic Press]], 1981).</ref>
Men dissipate heat faster than women through their [[sweat gland]]s.{{Fact|date=June 2008}} Women have a greater insulation and energy reserves stored in [[subcutaneous fat]], withstanding cold better.{{Fact|date=June 2008}} Sex differences in endurance events are less significant than for sprinting events.
Men typically have larger [[Vertebrate trachea|tracheae]] and branching [[Bronchus|bronchi]], with about 30 percent greater [[Lung volumes|lung volume]] per [[body mass]]. They have larger [[heart]]s, 10 percent higher [[red blood cell]] count, higher [[hemoglobin]], hence greater oxygen-carrying capacity. They also have higher circulating [[Coagulation|clotting factors]] ([[vitamin K]], pro[[thrombin]] and [[platelet]]s). These differences lead to faster healing of [[wound]]s and higher peripheral pain tolerance.<ref name="Glucksman" />
Women typically have more [[white blood cell]]s (stored and circulating), more [[granulocyte]]s and B and T [[lymphocyte]]s. Additionally, they produce more [[Antibody|antibodies]] at a faster rate than men. Hence they develop fewer [[Infection|infectious]] diseases and succumb for shorter periods.<ref name="Glucksman" /> [[Ethology|Ethologists]] argue that women, interacting with other women and multiple offspring in social groups, have experienced such traits as a [[Natural selection|selective]] advantage.<ref>J Durden-Smith and D Desimone, ''Sex and the Brain'', (New York: Arbor House, 1983).</ref><ref>ES Gersh and I Gersh, ''Biology of Women'', (Baltimore: University Park Press, 1981).</ref><ref>J Stein (editor), ''Internal Medicine'', 2nd edition., (Boston: [[Little, Brown and Company]], 1987).</ref><ref>M McLaughlin and T Shryer, 'Men vs Women: The New Debate Over Sex Differences', ''[[U.S. News & World Report]]'' '''8 August''' (1988): pp. 50-58.</ref><ref>BS McEwen, 'Neural Gonadal Steroid Action', ''[[Science (journal)|Science]]'' '''211''' (1981): 1303–1311.</ref> Note that almost all examples of sexual dimorphism in humans are quantitative, and have some degree of overlap.
Some biologists theorise that a species' degree of sexual dimorphism is inversely related to the degree of paternal investment in [[parenting]]. Species with the highest sexual dimorphism, such as the pheasant, tend to be those species in which the care and raising of offspring is done only by the mother, with no involvement of the father (low degree of paternal investment).
Although there are many biologically-determined, sexually-dimorphic behaviours in other species, these have few, if any, implications for human society. However, analysis of sexually dimorphic human behavior naturally provokes controversy. One less controversial, but still hypothetical, area with considerable discussion in academic literature concerns potential evolutionary advantages associated with sexual competition (both intrasexual and intersexual) and short- and long-term sexual strategies.<ref>(David M Buss, 2007)<!-- Need to check this reference, it was used to support a totally different statement in a previous version of this entry. --></ref>
According to Daly and WIlson, "The sexes differ more in human beings than in monogamous mammals, but much less than in extremely polygamous mammals."<ref>Martin Daly and Margo Wilson, 'Evolutionary Psychology and Marital Conflict', in ''Sex, Power, Conflict: Evolutionary and Feminist Perspectives'', edited by DM Buss and Neil M Malamuth, (Oxford University Press, 1996), p. 13.</ref>
D.M. Buss stated that "Males should prefer attributes in potential mates associated with reproductive value ''or'' fertility, depending on whether males in human evolutionary history have tended to seek long-term or short-term mating partners.<ref>(Buss 1987; Symons 1979; Williams 1975)</ref> Specifically, if males in our evolutionary past have tended to seek short-term mating partners, selection should have favoured male preferences for females in their early 20s who show cues positively correlated with fertility. If males in our evolutionary past tended to seek long-term mating partners, selection should have favoured preferences for females in their mid-teens who show cues indicative of reproductive value. Evolutionary theorists differ on which of these they judge to be most likely."<ref>DM Buss, 'Evolutionary theory', in ''Personality: Critical Concepts'', edited by Cary L Cooper and Lawrence A Pervin, (Routledge, 1998), p. 422.</ref>
==See also==
{{wiktionary|sexual dimorphism|sexual dimorphism}}
{{Commonscat|sexual dimorphism}}
*[[Sexual dimorphism in non-human primates]]
*[[Bateman's principle]]
*[[Digit ratio]]
*[[Operational sex ratio]]
*[[Sexual selection]]
*[[Sexual differentiation]]
*[[Sexually dimorphic nucleus]]
*[[Sexual dimorphism measures]]
*[[Sexual reproduction]]
*[[Sex-limited genes]]
*[[Gender differences]]
*[[List of homologues of the human reproductive system]]
==References==
{{reflist}}
*Bonduriansky, R. (2007) The evolution of condition-dependent sexual dimorphism. The American Naturalist, 169:1 pp9-19.
*Biological Journal of the Linnean Society (1999), 67: 1–18.
==External links==
* {{MeshName|Sex+dimorphism}}
{{Reproductive physiology}}
[[Category:Sex]]
[[Category:Animal anatomy]]
[[Category:Biology of gender]]
[[Category:Sexual selection]]
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