View source for Sexual dimorphism

{{Short description|Evolved difference in sex-specific characteristics}}
{{Redirect|Sex differences|sexual dimorphism in humans|Sex differences in humans}}
{{Distinguish|Gonochorism}}
{{Use dmy dates|date=October 2022}}
[[File:Pair of mandarin ducks.jpg|thumb|260 px|[[Mandarin duck]]s, male (left) and female (right), illustrating the dramatic difference in plumage between sexes, a manifestation of sexual dimorphism]]
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'''Sexual dimorphism''' is the condition where [[sex]]es of the same [[species]] exhibit different [[Morphology (biology)|morphological]] characteristics, including characteristics not directly involved in [[reproduction]].<ref>{{Cite book|url=https://books.google.com/books?id=O5lnDwAAQBAJ&q=sexual+dimorphism+encyclopedia&pg=RA1-PA7|title=Encyclopedia of Animal Behaviour|date=21 January 2019|publisher=Academic Press|isbn=978-0-12-813252-4|volume=2|pages=7|language=en}}</ref> The condition occurs in most [[dioecy|dioecious]] species, which consist of most animals and some plants. Differences may include [[secondary sex characteristic]]s, size, weight, color, markings, or behavioral or cognitive traits. Male-male reproductive competition has evolved a diverse array of sexually dimorphic traits. Aggressive utility traits such as "battle" teeth and blunt heads reinforced as battering rams are used as weapons in aggressive interactions between rivals. Passive displays such as ornamental feathering or song-calling have also evolved mainly through sexual selection.<ref>{{cite book | vauthors = Ralls K, Mesnick S | title=Encyclopedia of Marine Mammals | chapter=Sexual dimorphism | chapter-url=https://www.sciencedirect.com/science/article/pii/B9780123735539002339 | publisher=Academic Press | year=2009 | pages=1005–1011 | doi=10.1016/B978-0-12-373553-9.00233-9 | isbn=9780123735539 }}</ref> These differences may be subtle or exaggerated and may be subjected to [[sexual selection]] and [[natural selection]]. The opposite of dimorphism is ''monomorphism'', when both biological sexes are [[phenotype|phenotypically]] indistinguishable from each other.<ref>{{cite web|url=http://human-biology.key-spot.ru/search.php?key=sexual+monomorphism|title=Dictionary of Human Evolution and Biology|website=Human-biology.key-spot.ru|access-date=3 November 2017|archive-date=7 November 2017|archive-url=https://web.archive.org/web/20171107023759/http://human-biology.key-spot.ru/search.php?key=sexual+monomorphism|url-status=dead}}</ref>

==Overview==
[[File:Peacock courting peahen.jpg|thumb|260 px|The [[peacock]], on the right, is courting the [[peahen]], on the left.]]
[[File:Anas_platyrhynchos_male_female_quadrat.jpg|thumb|260 px|Male (bottom) and female [[mallard]]s. The male mallard has an unmistakable bottle-green head when his [[Nuptial plumage|breeding plumage]] is present.]]

===Ornamentation and coloration===
[[File:Orgyia antiqua MHNT.CUT.2012.0.356.Gières.jpg|thumb|260 px|''[[Orgyia antiqua]]'' male (left) and female (right)]]Common and easily identified types of dimorphism consist of [[Biological ornament|ornamentation]] and coloration, though not always apparent. A difference in the coloration of sexes within a given species is called sexual dichromatism, commonly seen in many species of birds and reptiles.<ref>{{cite journal | vauthors = Armenta JK, Dunn PO, Whittingham LA | title = Quantifying avian sexual dichromatism: a comparison of methods | journal = The Journal of Experimental Biology | volume = 211 | issue = Pt 15 | pages = 2423–30 | date = August 2008 | pmid = 18626076 | doi = 10.1242/jeb.013094 | doi-access = free | bibcode = 2008JExpB.211.2423A }}</ref> [[Sexual selection]] leads to exaggerated dimorphic traits that are used predominantly in competition over mates.<ref name="Andersson-1994">{{harvnb|Andersson|1994|p=8}}</ref> The increased fitness resulting from ornamentation offsets its cost to produce or maintain, suggesting complex evolutionary implications, but the costs and evolutionary implications vary from species to species.<ref>{{cite journal | vauthors = Zahavi A | title = Mate selection-a selection for a handicap | journal = Journal of Theoretical Biology | volume = 53 | issue = 1 | pages = 205–14 | date = September 1975 | pmid = 1195756 | doi = 10.1016/0022-5193(75)90111-3 | bibcode = 1975JThBi..53..205Z | url = http://eebweb.arizona.edu/faculty/dornhaus/courses/materials/papers/other/Zahavi%20sexual%20selection%20handicap%20model%20signal.pdf | citeseerx = 10.1.1.586.3819 | author-link = Amotz Zahavi | access-date = 14 May 2011 | archive-date = 10 August 2017 | archive-url = https://web.archive.org/web/20170810123757/http://eebweb.arizona.edu/Faculty/Dornhaus/courses/materials/papers/other/Zahavi%20sexual%20selection%20handicap%20model%20signal.pdf | url-status = dead }}</ref>

The [[peafowl]] constitute conspicuous illustrations of the principle. The ornate [[plumage]] of peacocks, as used in the courting display, attracts [[peahen]]s. At first sight, one might mistake peacocks and peahens for completely different species because of the vibrant colours and the sheer size of the male's plumage; the peahen is of a subdued brown coloration.<ref name="Zi-2003">{{cite journal | vauthors = Zi J, Yu X, Li Y, Hu X, Xu C, Wang X, Liu X, Fu R | display-authors = 6 | title = Coloration strategies in peacock feathers | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 100 | issue = 22 | pages = 12576–8 | date = October 2003 | pmid = 14557541 | pmc = 240659 | doi = 10.1073/pnas.2133313100 | bibcode = 2003PNAS..10012576Z | doi-access = free }}</ref> The plumage of the peacock increases its vulnerability to predators because it is a hindrance in flight, and it renders the bird conspicuous in general.<ref name="Zi-2003" /> Similar examples are manifold, such as in [[bird-of-paradise|birds of paradise]]<ref>{{cite web |url= https://education.nationalgeographic.org/resource/birds-paradise-beauty-kings/|title=Birds-of-Paradise: Beauty Kings |author=<!--Not stated--> |date=19 October 2023 |website= |publisher=National Geographic Society |access-date=22 November 2023 |quote=}}</ref> and [[argus (bird)|argus pheasants]].{{citation needed|date=May 2022}}

Another example of sexual dichromatism is that of nestling [[blue tit]]s. Males are chromatically more yellow than females. It is believed that this is obtained by the ingestion of green [[Lepidoptera]]n larvae, which contain large amounts of the [[carotenoid]]s [[lutein]] and [[zeaxanthin]].<ref>{{cite journal | vauthors = Slagsvold T, Lifjeld JT |year=1985 |title=Variation in plumage colour of the Great tit ''Parus major'' in relation to habitat, season and food |journal=[[Journal of Zoology]] |volume=206 |issue=3 |pages=321–328 |doi=10.1111/j.1469-7998.1985.tb05661.x}}</ref> This diet also affects the sexually dimorphic colours in the human-invisible [[ultraviolet]] spectrum.<ref>{{cite journal | doi=10.1016/s0042-6989(97)00026-6 | title=Visual pigments and oil droplets from six classes of photoreceptor in the retinas of birds | date=1997 | last1=Bowmaker | first1=J.K. | last2=Heath | first2=L.A. | last3=Wilkie | first3=S.E. | last4=Hunt | first4=D.M. | journal=Vision Research | volume=37 | issue=16 | pages=2183–2194 | pmid=9578901 }}</ref> Hence, the male birds, although appearing yellow to humans, actually have a violet-tinted plumage that is seen by females. This plumage is thought to be an indicator of male parental abilities.<ref>{{cite journal | vauthors = Senar JC, Figuerola J, Pascual J | title = Brighter yellow blue tits make better parents | journal = Proceedings. Biological Sciences | volume = 269 | issue = 1488 | pages = 257–61 | date = February 2002 | pmid = 11839194 | pmc = 1690890 | doi = 10.1098/rspb.2001.1882 }}</ref> Perhaps this is a good indicator for females because it shows that they are good at obtaining a food supply from which the carotenoid is obtained. There is a positive correlation between the chromas of the tail and breast feathers and body condition.<ref name="Johnsen-2003">{{cite journal | vauthors = Johnsen A, Delhey K, Andersson S, Kempenaers B | title = Plumage colour in nestling blue tits: sexual dichromatism, condition dependence and genetic effects | journal = Proceedings. Biological Sciences | volume = 270 | issue = 1521 | pages = 1263–70 | date = June 2003 | pmid = 12816639 | pmc = 1691364 | doi = 10.1098/rspb.2003.2375 | jstor = 3558810 }}</ref> Carotenoids play an important role in [[immune function]] for many animals, so carotenoid dependent signals might indicate health.<ref>{{cite journal | vauthors = Lozano GA |year=1994 |title=Carotenoids, parasites, and sexual selection |journal=[[Oikos (journal)|Oikos]] |volume=70 |issue=2 |pages=309–311 |url=http://www.georgealozano.com/papers/mine/Lozano1994Carotenoids.pdf |jstor=3545643 | doi=10.2307/3545643|bibcode=1994Oikos..70..309L }}</ref>

Frogs constitute another conspicuous illustration of the principle. There are two types of dichromatism for frog species: ontogenetic and dynamic. Ontogenetic frogs are more common and have permanent color changes in males or females. [[Litoria lesueuri|''Ranoidea lesueuri'']] is an example of a dynamic frog with temporary color changes in males during the breeding season.<ref>Donnellan, S. C., & Mahony, M. J. (2004). Allozyme, chromosomal, and morphological variability in the Litoria lesueuri species group (Anura : Hylidae), including a description of a new species. Australian Journal of Zoology</ref> ''[[Hyperolius ocellatus]]'' is an ontogenetic frog with dramatic differences in both color and pattern between the sexes. At sexual maturity, the males display a bright green with white dorsolateral lines.<ref>Bell, R. C., & Zamudio, K. R. (2012). Sexual dichromatism in frogs: natural selection, sexual selection, and unexpected diversity. Proceedings of the Royal Society B: Biological Sciences.</ref> In contrast, the females are rusty red to silver with small spots. The bright coloration in the male population attracts females and is an [[Aposematism|aposematic]] sign to potential predators.

Females often show a preference for exaggerated male [[secondary sexual characteristics]] in mate selection.<ref>{{cite journal | vauthors = Ryan MJ, Rand AS | title = Species Recognition and Sexual Selection as a Unitary Problem in Animal Communication | journal = Evolution; International Journal of Organic Evolution | volume = 47 | issue = 2 | pages = 647–657 | date = April 1993 | pmid = 28568715 | doi = 10.2307/2410076 | jstor = 2410076 }}</ref> The [[sexy son hypothesis]] explains that females prefer more elaborate males and select against males that are dull in color, independent of the species' vision.<ref name="Rubolini-2004">{{cite journal | vauthors = Rubolini D, Spina F, Saino N | year = 2004 | title = Protandry and sexual dimorphism in trans-Saharan migratory birds | journal = Behavioral Ecology | volume = 15 | issue = 4| pages = 592–601 | doi=10.1093/beheco/arh048| citeseerx = 10.1.1.498.7541 }}</ref>

Similar sexual dimorphism and mating choice are also observed in many fish species. For example, male [[guppy|guppies]] have colorful spots and ornamentations, while females are generally grey. Female guppies prefer brightly colored males to duller males.<ref>{{cite book| url=https://books.google.com/books?id=zunYrumtsR8C&q=sexuaql+dimorphism+guppies&pg=PA61|title=The Differences Between the Sexes| vauthors = Short RV, Balaban E |date=4 August 1994|publisher=Cambridge University Press|access-date=3 November 2017|via= Google Books|isbn=9780521448789}}</ref>{{Page needed|date=August 2021}}

In [[Ophioblennius atlanticus|redlip blennies]], only the male fish develops an organ at the anal-urogenital region that produces antimicrobial substances. During parental care, males rub their anal-urogenital regions over their nests' internal surfaces, thereby protecting their eggs from microbial infections, one of the most common causes for mortality in young fish.<ref>{{cite journal | vauthors = Giacomello E, Marchini D, Rasotto MB | title = A male sexually dimorphic trait provides antimicrobials to eggs in blenny fish | journal = Biology Letters | volume = 2 | issue = 3 | pages = 330–3 | date = September 2006 | pmid = 17148395 | pmc = 1686180 | doi = 10.1098/rsbl.2006.0492 }}</ref>

==Plants==
Most [[flowering plant]]s are [[hermaphroditic]] but approximately 6% of species have separate males and females ([[dioecy]]).<ref name="Renner-1995">{{cite journal| vauthors = Renner SS, Ricklefs RE |title=Dioecy and its correlates in the flowering plants|journal=American Journal of Botany|year=1995|volume=82|issue=5|pages=596–606|doi=10.2307/2445418|jstor=2445418|url=http://nbn-resolving.de/urn:nbn:de:bvb:19-epub-14619-6}}</ref> Sexual dimorphism is common in dioecious plants<ref>{{Cite book| vauthors = Behnke H, Lüttge U, Esser K, Kadereit JW, Runge M |url=https://books.google.com/books?id=-rPtCAAAQBAJ&dq=hermaphroditism+botany&pg=PA403|title=Progress in Botany / Fortschritte der Botanik: Structural Botany Physiology Genetics Taxonomy Geobotany / Struktur Physiologie Genetik Systematik Geobotanik |date=6 December 2012|publisher=Springer Science & Business Media|isbn=978-3-642-79844-3|language=en}}</ref>{{Rp|403}} and [[Dioicy|dioicous]] species.<ref name="Ramawat-2016">{{Cite book|url=https://books.google.com/books?id=qXvSBQAAQBAJ&q=Sexual+systems+in+bryophytes&pg=PA62|title=Reproductive Biology of Plants|vauthors=Ramawat KG, Merillon JM, Shivanna KR|date=19 April 2016|publisher=CRC Press|isbn=978-1-4822-0133-8|language=en}}</ref>{{rp|71}}

Males and females in [[insect-pollinated]] species generally look similar to one another because plants provide rewards (e.g. [[nectar]]) that encourage [[pollinators]] to visit another similar [[flower]], completing [[pollination]]. ''[[Catasetum]]'' orchids are one interesting exception to this rule. Male ''[[Catasetum]]'' [[orchids]] violently attach [[pollinia]] to [[euglossine]] bee pollinators. The bees will then avoid other male flowers but may visit the female, which looks different from the males.<ref name="Romero-1986">{{cite journal | vauthors = Romero GA, Nelson CE | title = Sexual dimorphism in Catasetum orchids: forcible pollen emplacement and male flower competition | journal = Science | volume = 232 | issue = 4757 | pages = 1538–40 | date = June 1986 | pmid = 17773505 | doi = 10.1126/science.232.4757.1538 | bibcode = 1986Sci...232.1538R | s2cid = 31296391 | jstor = 1698050 }}</ref>

Various other dioecious exceptions, such as ''[[Loxostylis alata]]'', have visibly different sexes, with the effect of eliciting the most efficient behavior from pollinators, who then use the most efficient strategy in visiting each gender of flower instead of searching, say, for pollen in a nectar-bearing female flower.{{Citation needed|date=November 2021}}

Some plants, such as some species of ''[[Geranium]]'', have what amounts to serial sexual dimorphism. The flowers of such species might, for example, present their [[anthers]] on opening, then shed the exhausted anthers after a day or two and perhaps change their colours as well while the [[pistil]] matures; specialist pollinators are very much inclined to concentrate on the exact appearance of the flowers they serve, which saves their time and effort and serves the interests of the plant accordingly. Some such plants go even further and change their appearance once fertilized, thereby discouraging further visits from pollinators. This is advantageous to both parties because it avoids damaging the developing fruit and wasting the pollinator's effort on unrewarding visits. In effect, the strategy ensures that pollinators can expect a reward every time they visit an appropriately advertising flower.{{citation needed|date=May 2022}}

Females of the aquatic plant ''[[Vallisneria americana]]'' have floating flowers attached by a long [[scape (botany)|flower stalk]] that are fertilized if they contact one of the thousands of free-floating flowers released by a male.<ref name="eelgrass">{{cite web|url=http://www.bio.umass.edu/biology/conn.river/eelgr.html|title=Eel Grass (aka wild celery, tape grass)|website=[[University of Massachusetts]]|url-status=dead|archive-url=https://web.archive.org/web/20110712075849/http://www.bio.umass.edu/biology/conn.river/eelgr.html|archive-date=12 July 2011}}</ref>{{Better source needed|date=August 2021}} Sexual dimorphism is most often associated with [[Pollination syndrome|wind-pollination]] in plants due to selection for efficient [[pollen]] dispersal in males vs pollen capture in females, e.g. ''Leucadendron rubrum''.<ref name="Friedman-2009">{{cite journal | vauthors = Friedman J, Barrett SC | title = Wind of change: new insights on the ecology and evolution of pollination and mating in wind-pollinated plants | journal = Annals of Botany | volume = 103 | issue = 9 | pages = 1515–27 | date = June 2009 | pmid = 19218583 | pmc = 2701749 | doi = 10.1093/aob/mcp035 }}</ref>

Sexual dimorphism in plants can also be dependent on reproductive development. This can be seen in ''[[Cannabis sativa]]'', a type of hemp, which have higher photosynthesis rates in males while growing but higher rates in females once the plants become sexually mature.<ref name="Geber-1999">{{Cite book | vauthors = Geber MA |year=1999 |title=Gender and sexual dimorphism in flowering plants |url=https://books.google.com/books?id=pUo2T34ppKUC&pg=PA206 |isbn=978-3-540-64597-9 |publisher=Springer |location=Berlin }} p. 206</ref>

Every sexually reproducing extant species of the vascular plant has an alternation of generations; the plants we see about us generally are [[Ploidy|diploid]] [[sporophyte]]s, but their offspring are not the seeds that people commonly recognize as the new generation. The seed actually is the offspring of the [[Ploidy|haploid]] generation of [[Gametophyte|microgametophytes]] ([[pollen]]) and [[Gametophyte|megagametophytes]] (the [[embryo sac]]s in the [[ovules]]). Each pollen grain accordingly may be seen as a male plant in its own right; it produces a sperm cell and is dramatically different from the female plant, the megagametophyte that produces the female gamete.{{Citation needed|date=August 2021}}

==Insects==
[[File:Colias dimera copulating.jpg|thumb|260 px|''[[Colias dimera]]'' mating. The male is a brighter yellow than the female.]]
Insects display a wide variety of sexual dimorphism between taxa including size, ornamentation and coloration.<ref>{{cite journal | vauthors = Bonduriansky R | title = The evolution of condition-dependent sexual dimorphism | journal = The American Naturalist | volume = 169 | issue = 1 | pages = 9–19 | date = January 2007 | pmid = 17206580 | doi = 10.1086/510214 | bibcode = 2007ANat..169....9B | s2cid = 17439073 }}</ref> The female-biased sexual size dimorphism observed in many taxa evolved despite intense male-male competition for mates.<ref>{{cite journal | vauthors = Barreto FS, Avise JC | title = The genetic mating system of a sea spider with male-biased sexual size dimorphism: evidence for paternity skew despite random mating success | journal = Behavioral Ecology and Sociobiology | volume = 65 | issue = 8 | pages = 1595–1604 | date = August 2011 | pmid = 21874083 | pmc = 3134710 | doi = 10.1007/s00265-011-1170-x | bibcode = 2011BEcoS..65.1595B }}</ref> In ''[[Osmia rufa]]'', for example, the female is larger/broader than males, with males being 8–10&nbsp;mm in size and females being 10–12&nbsp;mm in size.<ref>{{Cite journal|title = On managing the red mason bee (Osmia bicornis) in apple orchards|journal = Apidologie|date = 30 June 2011|issn = 0044-8435|pages = 564–576|volume = 42|issue = 5|doi = 10.1007/s13592-011-0059-z| vauthors = Gruber B, Eckel K, Everaars J, Dormann CF |s2cid = 22935710|url = https://hal.archives-ouvertes.fr/hal-01003594/file/hal-01003594.pdf}}</ref> In the [[Asterocampa celtis|hackberry emperor]] females are similarly larger than males.<ref>{{Cite web|url=http://entnemdept.ufl.edu/creatures/bfly/hackberry_emperor.htm|title=hackberry emperor – Asterocampa celtis (Boisduval & Leconte)|website=entnemdept.ufl.edu|access-date=15 November 2017}}</ref> The reason for the sexual dimorphism is due to provision size mass, in which females consume more pollen than males.<ref>{{Cite journal|title = Late embryogenesis and immature development of Osmia rufa cornigera (Rossi) (Hymenoptera : Megachilidae)|journal = Apidologie|pages = 359–367|volume = 20|issue = 4|doi = 10.1051/apido:19890408| vauthors = Rust R, Torchio P, Trostle G |year = 1989|doi-access = free}}</ref>

In some species, there is evidence of male dimorphism, but it appears to be for distinctions of roles. This is seen in the bee species ''[[Macrotera portalis]]'' in which there is a small-headed morph, capable of flight, and large-headed morph, incapable of flight, for males.<ref name="Danforth-1991">{{cite journal | vauthors = Danforth B |title=The morphology and behavior of dimorphic males in Perdita portalis (Hymenoptera : Andrenidae) |journal=Behavioral Ecology and Sociobiology |volume=29 |issue=4 |pages=235–247 |year=1991 |doi=10.1007/bf00163980|bibcode=1991BEcoS..29..235D |s2cid=37651908 }}</ref>  ''[[Anthidium manicatum]]'' also displays male-biased sexual dimorphism. The selection for larger size in males rather than females in this species may have resulted due to their aggressive territorial behavior and subsequent differential mating success.<ref>{{cite journal | vauthors = Jaycox Elbert R | year = 1967 | title = Territorial Behavior Among Males of Anthidium Bamngense | journal = Journal of the Kansas Entomological Society | volume = 40 | issue = 4| pages = 565–570 }}</ref> Another example is ''[[Lasioglossum hemichalceum]]'', which is a species of sweat bee that shows drastic physical dimorphisms between male offspring.<ref>{{Cite journal|title = Male Dimorphism in Lasioglossum (Chilalictus) hemichalceum: The Role of Larval Nutrition|journal = Journal of the Kansas Entomological Society|date = 1 October 1996|pages = 147–157|volume = 69|issue = 4| vauthors = Kukuk PF |jstor=25085712}}</ref> Not all dimorphism has to have a drastic difference between the sexes. ''[[Andrena agilissima]]'' is a mining bee where the females only have a slightly larger head than the males.<ref>{{Cite journal|title = Mating in a communal bee, Andrena agilissima (Hymenoptera Andrenidae)|journal = Ethology Ecology & Evolution|date = 1 October 1999|issn = 0394-9370|pages = 371–382|volume = 11|issue = 4|doi = 10.1080/08927014.1999.9522820| vauthors = Paxton RJ, Giovanetti M, Andrietti F, Scamoni E, Scanni B | bibcode=1999EtEcE..11..371P }}</ref>

Weaponry leads to increased fitness by increasing success in male–male competition in many insect species.<ref>{{cite journal | vauthors = Wang MQ, Yang D | year = 2005 | title = Sexual dimorphism in insects | journal = Chinese Bulletin of Entomology | volume = 42 | pages = 721–725 }}</ref> The beetle horns in ''[[Onthophagus taurus]]'' are enlarged growths of the head or thorax expressed only in the males. ''Copris ochus'' also has distinct sexual and male dimorphism in head horns.<ref name="Sugiura-2007">{{cite journal | vauthors = Sugiura S, Yamaura Y, Makihara H | title = Sexual and male horn dimorphism in Copris ochus (Coleoptera: Scarabaeidae) | journal = Zoological Science | volume = 24 | issue = 11 | pages = 1082–1085 | date = November 2007 | pmid = 18348608 | doi = 10.2108/zsj.24.1082 | s2cid = 34705415 | doi-access = free }}</ref> Another beetle with a distinct horn-related sexual dimorphism is ''Allomyrina dichotoma,'' also known as the [[Japanese rhinoceros beetle]].<ref>{{Cite journal |last=Hongo |first=Yoshihito |date=2007-12-01 |title=Evolution of male dimorphic allometry in a population of the Japanese horned beetle Trypoxylus dichotomus septentrionalis |url=https://doi.org/10.1007/s00265-007-0459-2 |journal=Behavioral Ecology and Sociobiology |language=en |volume=62 |issue=2 |pages=245–253 |doi=10.1007/s00265-007-0459-2 |bibcode=2007BEcoS..62..245H |issn=1432-0762|url-access=subscription }}</ref> These structures are impressive because of the exaggerated sizes.<ref name="Emlen-2005">{{cite journal | vauthors = Emlen DJ, Marangelo J, Ball B, Cunningham CW | title = Diversity in the weapons of sexual selection: horn evolution in the beetle genus Onthophagus (Coleoptera: Scarabaeidae) | journal = Evolution; International Journal of Organic Evolution | volume = 59 | issue = 5 | pages = 1060–1084 | date = May 2005 | pmid = 16136805 | doi = 10.1111/j.0014-3820.2005.tb01044.x | bibcode = 2005Evolu..59.1060E | citeseerx = 10.1.1.133.7557 | s2cid = 221736269 }}</ref> There is a direct correlation between male horn lengths and body size and higher access to mates and fitness.<ref name="Emlen-2005" /> In other beetle species, both males and females may have ornamentation such as horns.<ref name="Sugiura-2007" />
Generally, insect sexual size dimorphism (SSD) within species increases with body size.<ref>Teder, T., & Tammaru, T. (2005). "Sexual size dimorphism within species increases with body size in insects". ''Oikos'' {{ISBN?}}</ref>

Sexual dimorphism within insects is also displayed by dichromatism. In butterfly genera ''[[Bicyclus]]'' and ''[[Junonia]]'', dimorphic wing patterns evolved due to sex-limited expression, which mediates the [[intralocus sexual conflict]] and leads to increased fitness in males.<ref>{{cite journal | vauthors = Oliver JC, Monteiro A | title = On the origins of sexual dimorphism in butterflies | journal = Proceedings. Biological Sciences | volume = 278 | issue = 1714 | pages = 1981–1988 | date = July 2011 | pmid = 21123259 | pmc = 3107650 | doi = 10.1098/rspb.2010.2220 }}</ref> The sexual dichromatic nature of ''[[Bicyclus anynana]]'' is reflected by female selection on the basis of dorsal UV-reflective eyespot pupils.<ref>{{cite journal | vauthors = Robertson KA, Monteiro A | title = Female Bicyclus anynana butterflies choose males on the basis of their dorsal UV-reflective eyespot pupils | journal = Proceedings. Biological Sciences | volume = 272 | issue = 1572 | pages = 1541–1546 | date = August 2005 | pmid = 16048768 | pmc = 1559841 | doi = 10.1098/rspb.2005.3142 }}</ref> The [[Gonepteryx rhamni|common brimstone]] also displays sexual dichromatism; males have yellow and iridescent wings, while female wings are white and non-iridescent.<ref>{{Cite journal |doi=10.2307/3546246 |jstor=3546246|title=Early Male Emergence and Reproductive Phenology of the Adult Overwintering Butterfly Gonepteryx rhamni in Sweden|journal=Oikos|volume=75|issue=2|pages=227–240|year=1996| vauthors = Wiklund C, Lindfors V, Forsberg J |bibcode=1996Oikos..75..227W }}</ref> Naturally selected deviation in protective female coloration is displayed in mimetic butterflies.<ref>{{cite journal | vauthors = Kunte K | title = Mimetic butterflies support Wallace's model of sexual dimorphism | journal = Proceedings. Biological Sciences | volume = 275 | issue = 1643 | pages = 1617–1624 | date = July 2008 | pmid = 18426753 | pmc = 2602815 | doi = 10.1098/rspb.2008.0171 }}</ref>

==Spiders and sexual cannibalism==
[[File:Male and female A. appensa.jpg|thumb|right|Female (left) and male (right) ''[[Argiope appensa]]'', displaying typical sexual differences in spiders, with dramatically smaller males]]
[[File:Hammock Spiders (Pityohyphantes).jpg|alt=Hammock Spiders (Pityohyphantes spp.) courting. Female left and male right.|thumb|Hammock spiders (''[[Pityohyphantes]]'' ''[[Species affinis|sp.]]'') courting. Female left and male right.]]
Many [[arachnid]] groups exhibit sexual dimorphism,<ref name="McLean-2018">{{cite journal | vauthors = McLean CJ, Garwood RJ, Brassey CA | title = Sexual dimorphism in the Arachnid orders | journal = PeerJ | volume = 6 | pages = e5751 | year = 2018 | pmid = 30416880 | pmc = 6225839 | doi = 10.7717/peerj.5751 | doi-access = free }}</ref> but it is most widely studied in the spiders. In the orb-weaving spider ''[[Zygiella x-notata]]'', for example, adult females have a larger body size than adult males.<ref>{{Cite thesis| vauthors = Smith T | degree = MS |title=Discovering the daily activity pattern of Zygiella x-notata and its relationship to light|url=https://fse.studenttheses.ub.rug.nl/22461/1/MSc_thesis_Smith.pdf}}</ref> Size dimorphism shows a correlation with [[sexual cannibalism]],<ref name="Prenter-1999">{{cite journal | vauthors = Prenter J, Elwood RW, Montgomery WI | title = Sexual Size Dimorphism and Reproductive Investment by Female Spiders: A Comparative Analysis | journal = Evolution; International Journal of Organic Evolution | volume = 53 | issue = 6 | pages = 1987–1994 | date = December 1999 | pmid = 28565440 | doi = 10.2307/2640458 | jstor = 2640458 }}</ref> which is prominent in spiders (it is also found in insects such as [[praying mantis]]es). In the size dimorphic [[wolf spider]] ''[[Tigrosa helluo]]'', food-limited females cannibalize more frequently.<ref name="Wilder-2008">{{cite journal | vauthors = Wilder SM, Rypstra AL | year = 2008 | title = Sexual size dimorphism mediates the occurrence of state-dependent sexual cannibalism in a wolf spider | journal = Animal Behaviour | volume = 76 | issue = 2| pages = 447–454 | doi=10.1016/j.anbehav.2007.12.023|s2cid=54373571 }}</ref> Therefore, there is a high risk of low fitness for males due to pre-copulatory cannibalism, which led to male selection of larger females for two reasons: higher [[fecundity]] and lower rates of cannibalism.<ref name="Wilder-2008"/> In addition, female fecundity is positively correlated with female body size and large female body size is selected for, which is seen in the family [[Orb-weaver spider|Araneidae]]. All ''Argiope'' species, including ''[[Argiope bruennichi]]'', use this method. Some males evolved ornamentation{{vague|date=July 2018}} including binding the female with silk, having proportionally longer legs, modifying the female's web, mating while the female is feeding, or providing a nuptial gift in response to sexual cannibalism.<ref name="Wilder-2008"/> Male body size is not under selection due to cannibalism in all spider species such as ''[[Nephila pilipes]]'', but is more prominently selected for in less dimorphic species of spiders, which often selects for larger male size.<ref>{{cite journal | vauthors = Foellmer MW, Fairbairn DJ | year = 2004 | title = Males under attack: Sexual cannibalism and its consequences for male morphology and behaviour in an orb-weaving spider | url = https://www.researchgate.net/publication/232713769| journal = Evolutionary Ecology Research | volume = 6 | pages = 163–181 }}</ref> In the species ''[[Maratus volans]]'', the males are known for their characteristic colorful fan which attracts the females during mating.<ref>{{cite journal | vauthors = Girard MB, Elias DO, Kasumovic MM | title = Female preference for multi-modal courtship: multiple signals are important for male mating success in peacock spiders | journal = Proceedings. Biological Sciences | volume = 282 | issue = 1820 | pages = 20152222 | date = December 2015 | pmid = 26631566 | pmc = 4685782 | doi = 10.1098/rspb.2015.2222 }}</ref>

==Fish==
Ray-finned fish are an ancient and diverse class, with the widest degree of sexual dimorphism of any animal class. Fairbairn notes that "females are generally larger than males but males are often larger in species with male–male combat or male paternal care ... [sizes range] from dwarf males to males more than 12 times heavier than females."<ref>{{cite book| vauthors = Fairbairn D |title=Odd Couples: Extraordinary Differences between the Sexes in the Animal Kingdom|date=28 April 2013|publisher=Princeton|isbn=978-0691141961}}</ref>{{Page needed|date=August 2021}}

There are cases where males are substantially larger than females. An example is ''[[Lamprologus callipterus]]'', a type of cichlid fish. In this fish, the males are characterized as being up to 60 times larger than the females. The male's increased size is believed to be advantageous because males collect and defend empty snail shells in each of which a female breeds.<ref>{{cite journal | vauthors = Ota K, Kohda M, Sato T | title = Unusual allometry for sexual size dimorphism in a cichlid where males are extremely larger than females | journal = Journal of Biosciences | volume = 35 | issue = 2 | pages = 257–65 | date = June 2010 | pmid = 20689182 | doi = 10.1007/s12038-010-0030-6 |  s2cid = 12396902 }}</ref> Males must be larger and more powerful in order to collect the largest shells. The female's body size must remain small because in order for her to breed, she must lay her eggs inside the empty shells. If she grows too large, she will not fit in the shells and will be unable to breed. The female's small body size is also likely beneficial to her chances of finding an unoccupied shell. Larger shells, although preferred by females, are often limited in availability.<ref>{{Cite journal | vauthors = Sato T |year=1994 |title=Active accumulation of spawning substrate: a determinant of extreme polygyny in a shell-brooding cichlid fish |journal=[[Animal Behaviour (journal)|Animal Behaviour]] |volume=48 |issue=3 |pages=669–678 |doi=10.1006/anbe.1994.1286|s2cid=53192909 }}</ref> Hence, the female is limited to the growth of the size of the shell and may actually change her growth rate according to shell size availability.<ref>{{cite journal |vauthors=Schütz D, Taborsky M |year=2005 |title=Mate choice and sexual conflict in the size dimorphic water spider ''Argyroneta aquatica'' (Araneae: Argyronetidae) |journal=[[Journal of Arachnology]] |volume=33 |issue=3 |pages=767–775 |doi=10.1636/S03-56.1 |s2cid=26712792 |url=http://www.americanarachnology.org/JoA_free/JoA_v33_n3/arac-033-03-0767.pdf |access-date=14 May 2011 |archive-date=20 March 2012 |archive-url=https://web.archive.org/web/20120320072921/http://www.americanarachnology.org/JoA_free/JoA_v33_n3/arac-033-03-0767.pdf |url-status=dead }}</ref> In other words, the male's ability to collect large shells depends on his size. The larger the male, the larger the shells he is able to collect. This then allows for females to be larger in his brooding nest which makes the difference between the sizes of the sexes less substantial. Male–male competition in this fish species also selects for large size in males. There is aggressive competition by males over territory and access to larger shells. Large males win fights and steal shells from competitors. Another example is the [[dragonet]], in which males are considerably larger than females and possess longer fins.

Sexual dimorphism also occurs in hermaphroditic fish. These species are known as [[sequential hermaphrodites]]. In fish, [[Life history theory|reproductive histories]] often include the sex-change from female to male where there is a strong connection between growth, the sex of an individual, and the mating system within which it operates.<ref>{{cite journal | vauthors = McCormick MI, Ryen CA, Munday PL, Walker SP | title = Differing mechanisms underlie sexual size-dimorphism in two populations of a sex-changing fish | journal = PLOS ONE| volume = 5 | issue = 5 | pages = e10616 | date = May 2010 | pmid = 20485547 | pmc = 2868897 | doi = 10.1371/journal.pone.0010616 | veditors = Briffa M | bibcode = 2010PLoSO...510616M | doi-access = free }}</ref> In protogynous mating systems where males dominate mating with many females, size plays a significant role in male reproductive success.<ref>{{cite journal | vauthors = Warner RR | title = Sex change and the size-advantage model | journal = Trends in Ecology & Evolution | volume = 3 | issue = 6 | pages = 133–6 | date = June 1988 | pmid = 21227182 | doi = 10.1016/0169-5347(88)90176-0 | bibcode = 1988TEcoE...3..133W }}</ref> Males have a propensity to be larger than females of a comparable age but it is unclear whether the size increase is due to a growth spurt at the time of the sexual transition or due to the history of faster growth in sex changing individuals.<ref>{{cite journal | vauthors = Adams S, Williams AJ |year=2001 |title=A preliminary test of the transitional growth spurt hypothesis using the protogynous coral trout ''Plectropomus maculatus'' |journal=[[Journal of Fish Biology]] |volume=59 |issue=1 |pages=183–185 |doi=10.1111/j.1095-8649.2001.tb02350.x|bibcode=2001JFBio..59..183A }}</ref> Larger males are able to stifle the growth of females and control environmental resources.{{citation needed|date=May 2022}}

Social organization plays a large role in the changing of sex by the fish. It is often seen that a fish will change its sex when there is a lack of a dominant male within the social hierarchy. The females that change sex are often those who attain and preserve an initial size advantage early in life. In either case, females which change sex to males are larger and often prove to be a good example of dimorphism.

In other cases with fish, males will go through noticeable changes in body size, and females will go through morphological changes that can only be seen inside of the body.  For example, in [[sockeye salmon]], males develop larger body size at maturity, including an increase in body depth, hump height, and snout length.  Females experience minor changes in snout length, but the most noticeable difference is the huge increase in [[gonad]] size, which accounts for about 25% of body mass.<ref name="Hendry-1999">{{cite journal| vauthors = Hendry A, Berg OK |title=Secondary sexual characters, energy use, senescence, and the cost of reproduction in sockeye salmon|journal=Canadian Journal of Zoology|year=1999|volume=77|issue=11|pages=1663–1675|doi=10.1139/cjz-77-11-1663}}</ref>

Sexual selection was observed for female ornamentation in ''[[Gobiusculus flavescens]]'', known as two-spotted gobies.<ref name="Amundsen-2001">{{cite journal | vauthors = Amundsen T, Forsgren E | title = Male mate choice selects for female coloration in a fish | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 98 | issue = 23 | pages = 13155–60 | date = November 2001 | pmid = 11606720 | pmc = 60840 | doi = 10.1073/pnas.211439298 | bibcode = 2001PNAS...9813155A | doi-access = free }}</ref> Traditional hypotheses suggest that male–male competition drives selection. However, selection for ornamentation within this species suggests that showy female traits can be selected through either female–female competition or male mate choice.<ref name="Amundsen-2001"/> Since carotenoid-based ornamentation suggests mate quality, female two-spotted guppies that develop colorful orange bellies during breeding season are considered favorable to males.<ref name="Svensson-2006">{{cite journal | vauthors = Svensson PA, Pélabon C, Blount JD, Surai PF, Amundsen T | year = 2006 | title = Does female nuptial coloration reflect egg carotenoids and clutch quality in the Two-Spotted Goby (Gobiusculus flavescens, Gobiidae)? | journal = Functional Ecology | volume = 20 | issue = 4| pages = 689–698 | doi=10.1111/j.1365-2435.2006.01151.x| doi-access = free | bibcode = 2006FuEco..20..689S | hdl = 10536/DRO/DU:30038904 | hdl-access = free }}</ref> The males invest heavily in offspring during incubation, which leads to the sexual preference in colorful females due to higher egg quality.<ref name="Svensson-2006"/>

==Amphibians and non-avian reptiles==
[[File:Mississippi map turtles (Graptemys pseudogeographica kohni) female & male.jpg|thumb|260 px|[[Mississippi map turtle]]s (''Graptemys pseudogeographica kohni'') adult female (left) and adult male (right)]]
In amphibians and reptiles, the degree of sexual dimorphism varies widely among [[taxonomic groups]]. The sexual dimorphism in amphibians and reptiles may be reflected in any of the following: anatomy; relative length of tail; relative size of head; overall size as in many species of [[viperidae|vipers]] and [[lizards]]; coloration as in many [[amphibians]], [[snakes]], and lizards, as well as in some [[turtles]]; an ornament as in many [[newts]] and lizards; the presence of specific sex-related behaviour is common to many lizards; and vocal qualities which are frequently observed in [[frogs]].{{citation needed|date=May 2022}}

[[Anole]] lizards show prominent size dimorphism with males typically being significantly larger than females. For instance, the average male ''[[Anolis sagrei]]'' was 53.4&nbsp;mm vs. 40&nbsp;mm in females.<ref>{{cite journal | vauthors = Butler MA, Schoener TW, Losos JB | title = The relationship between sexual size dimorphism and habitat use in Greater Antillean Anolis lizards | journal = Evolution; International Journal of Organic Evolution | volume = 54 | issue = 1 | pages = 259–72 | date = February 2000 | pmid = 10937202 | doi = 10.1111/j.0014-3820.2000.tb00026.x | s2cid = 7887284 | doi-access = free }}</ref> Different sizes of the heads in anoles have been explained by differences in the estrogen pathway.<ref>{{cite journal | vauthors = Sanger TJ, Seav SM, Tokita M, Langerhans RB, Ross LM, Losos JB, Abzhanov A | title = The oestrogen pathway underlies the evolution of exaggerated male cranial shapes in Anolis lizards | journal = Proceedings. Biological Sciences | volume = 281 | issue = 1784 | pages = 20140329 | date = June 2014 | pmid = 24741020 | pmc = 4043096 | doi = 10.1098/rspb.2014.0329 }}</ref> The sexual dimorphism in lizards is generally attributed to the effects of sexual selection, but other mechanisms including ecological divergence and fecundity selection provide alternative explanations.<ref name="Pinto-2005">Pinto, A., Wiederhecker, H., & Colli, G. (2005). Sexual dimorphism in the Neotropical lizard, Tropidurus torquatus (Squamata, Tropiduridae). Amphibia-Reptilia.</ref> The development of color dimorphism in lizards is induced by hormonal changes at the onset of sexual maturity, as seen in ''Psamodromus algirus'', ''Sceloporus gadoviae'', and ''S. undulates erythrocheilus''.<ref name="Pinto-2005"/> Sexual dimorphism in size is also seen in frog species like ''[[Bibron's toadlet|P. bibroni]]<nowiki/>i''.

Male painted dragon lizards, ''[[Ctenophorus pictus]]''. are brightly conspicuous in their breeding coloration, but male colour declines with [[ageing|aging]].  Male coloration appears to reflect innate anti-oxidation capacity that protects against [[DNA oxidation|oxidative DNA damage]].<ref name="Olsson-2012">{{cite journal | vauthors = Olsson M, Tobler M, Healey M, Perrin C, Wilson M | title = A significant component of ageing (DNA damage) is reflected in fading breeding colors: an experimental test using innate antioxidant mimetics in painted dragon lizards | journal = Evolution; International Journal of Organic Evolution | volume = 66 | issue = 8 | pages = 2475–83 | date = August 2012 | pmid = 22834746 | doi = 10.1111/j.1558-5646.2012.01617.x | s2cid = 205783815 | doi-access = free }}</ref> Male breeding coloration is likely an indicator to females of the underlying level of oxidative DNA damage (a significant component of aging) in potential mates.<ref name="Olsson-2012" />

== Birds ==
[[File:Golden-shouldered Parrot 0A2A7450.jpg|thumb|upright=1.1|Female (left) and male (right) [[golden-shouldered parrot]], showing that the male is much more colorful than the female]]
Possible mechanisms have been proposed to explain macroevolution of sexual size dimorphism in birds. These include sexual selection, selection for fecundity in females, niche divergence between the sexes, and allometry, but their relative importance is still not fully understood .<ref name="Caron-2024">{{cite journal | vauthors = Caron FS, Pie MR |year=2024 |title=The macroevolution of sexual size dimorphism in birds |journal=[[Biological Journal of the Linnean Society]] |volume=141 |issue=5 |pages=in press |doi=10.1093/biolinnean/blad168 |doi-access=free }}</ref><ref name="Bravo-2024">{{cite journal | vauthors = Bravo CR, Bautista-Sopelana LM, Alonso JC |year=2024 |title=Revisiting niche divergence hypothesis in sexually dimorphic birds: Is diet overlap correlated with sexual size dimorphism? |journal=[[Journal of Animal Ecology]] |volume=93 |issue=4 |pages=460–474 |doi=10.1111/1365-2656.14058 |pmid=38462717 |bibcode=2024JAnEc..93..460B |hdl=10261/360371 |hdl-access=free }}</ref> Sexual dimorphism in birds can be manifested in size or plumage differences between the sexes. Sexual size dimorphism varies among taxa, with males typically being larger, though this is not always the case, e.g. [[birds of prey]], [[hummingbird]]s, and some species of flightless birds.<ref>{{harvnb|Andersson|1994|p=269}}</ref><ref>{{Cite journal|title = Becoming Different But Staying Alike: Patterns of Sexual Size and Shape Dimorphism in Bills of Hummingbirds|journal = Evolutionary Biology|date = 11 November 2012|issn = 0071-3260|pages = 246–260|volume = 40|issue = 2|doi = 10.1007/s11692-012-9206-3 | vauthors = Berns CM, Adams DC |s2cid = 276492}}</ref> Plumage dimorphism, in the form of ornamentation or coloration, also varies, though males are typically the more ornamented or brightly colored sex.<ref>{{cite journal | vauthors = McGraw KJ, Hill GE, Stradi R, Parker RS | title = The effect of dietary carotenoid access on sexual dichromatism and plumage pigment composition in the American goldfinch | journal = Comparative Biochemistry and Physiology. Part B, Biochemistry & Molecular Biology | volume = 131 | issue = 2 | pages = 261–9 | date = February 2002 | pmid = 11818247 | doi = 10.1016/S1096-4959(01)00500-0 | url = http://www.aces.edu/dept/hilllab/81.pdf | archive-url = https://web.archive.org/web/20050828183926/http://www.aces.edu/dept/hilllab/81.pdf | archive-date = 28 August 2005 }}</ref> Such differences have been attributed to the unequal reproductive contributions of the sexes.<ref>{{cite journal | doi=10.1126/science.250.4986.1394 | title=Realized Reproductive Success of Polygynous Red-Winged Blackbirds Revealed by DNA Markers | date=1990 | last1=Gibbs | first1=H. Lisle | last2=Weatherhead | first2=Patrick J. | last3=Boag | first3=Peter T. | last4=White | first4=Bradley N. | last5=Tabak | first5=Lisa M. | last6=Hoysak | first6=Drew J. | journal=Science | volume=250 | issue=4986 | pages=1394–1397 | pmid=17754986 | bibcode=1990Sci...250.1394L }}</ref> This difference produces a stronger female choice since they have more risk in producing offspring. In some species, the male's contribution to reproduction ends at copulation, while in other species the male becomes the main (or only) caregiver. Plumage polymorphisms have evolved to reflect these differences and other measures of reproductive fitness, such as body condition<ref name="Lindsay-2009">{{cite journal | vauthors = Lindsay WR, Webster MS, Varian CW, Schwabl H |year=2009 |title=Plumage colour acquisition and behaviour are associated with androgens in a phenotypically plastic bird |journal=[[Animal Behaviour (journal)|Animal Behaviour]] |volume=77 |issue=6 |pages=1525–1532 |doi=10.1016/j.anbehav.2009.02.027|s2cid=15799876 }}</ref> or survival.<ref>{{cite journal | vauthors = Petrie M |year=1994 |title=Improved growth and survival of offspring of peacocks with more elaborate trains |journal=[[Nature (journal)|Nature]] |volume=371 |issue=6498 |pages=598–599 |doi=10.1038/371598a0|bibcode = 1994Natur.371..598P |s2cid=4316752 }}</ref> The male phenotype sends signals to females who then choose the 'fittest' available male.

Sexual dimorphism is a product of both genetics and environmental factors. An example of [[sexual polymorphism]] determined by environmental conditions exists in the [[red-backed fairywren]]. Red-backed fairywren males can be classified into three categories during [[breeding season]]: black breeders, brown breeders, and brown auxiliaries.<ref name="Lindsay-2009"/> These differences arise in response to the bird's body condition: if they are healthy they will produce more androgens thus becoming black breeders, while less healthy birds produce less androgens and become brown auxiliaries.<ref name="Lindsay-2009"/> The [[reproductive success]] of the male is thus determined by his success during each year's non-breeding season, causing reproductive success to vary with each year's environmental conditions.

[[File:Eclectus roratus-20030511.jpg|thumb|left|upright=1.1|The [[eclectus parrot]] is an example of a bird where the female (right) is more colorful than the male (left).]]

Migratory patterns and behaviors also influence sexual dimorphisms. This aspect also stems back to size dimorphism in species. It has been shown that the larger males are better at coping with the difficulties of migration and thus are more successful in reproducing when reaching the breeding destination.<ref>{{cite journal | vauthors = Rubolini D, Spina F, Saino N | year = 2004 | title = Protandry and sexual dimorphism in trans-saharan migratory birds | journal = Behavioral Ecology | volume = 15 | issue = 4| pages = 592–601 | doi=10.1093/beheco/arh048| doi-access = free }}</ref> When viewing this from an evolutionary standpoint, many theories and explanations come into consideration. If these are the result for every migration and breeding season, the expected results should be a shift towards a larger male population through sexual selection. Sexual selection is strong when the factor of environmental selection is also introduced. Environmental selection may support a smaller chick size if those chicks were born in an area that allowed them to grow to a larger size, even though under normal conditions they would not be able to reach this optimal size for migration. When the environment gives advantages and disadvantages of this sort, the strength of selection is weakened and the environmental forces are given greater morphological weight. The sexual dimorphism could also produce a change in timing of migration leading to differences in mating success within the bird population.<ref>{{cite journal | vauthors = Kissner KJ, Weatherhead PJ, Francis CM | title = Sexual size dimorphism and timing of spring migration in birds | journal = Journal of Evolutionary Biology | volume = 16 | issue = 1 | pages = 154–62 | date = January 2003 | pmid = 14635890 | doi = 10.1046/j.1420-9101.2003.00479.x | citeseerx = 10.1.1.584.2867 | s2cid = 13830052 }}</ref> When the dimorphism produces that large of a variation between the sexes and between the members of the sexes, multiple evolutionary effects can take place. This timing could even lead to a speciation phenomenon if the variation becomes strongly drastic and favorable towards two different outcomes. Sexual dimorphism is maintained by the counteracting pressures of natural selection and sexual selection. For example, sexual dimorphism in coloration increases the vulnerability of bird species to predation by European sparrowhawks in Denmark.<ref name="Møller-2006">{{cite journal | vauthors = Møller AP, Nielsen JT |year=2006 |title=Prey vulnerability in relation to sexual coloration of prey | journal=[[Behavioral Ecology and Sociobiology]] |volume=60 |issue=2 |pages=227–233 |doi=10.1007/s00265-006-0160-x|bibcode=2006BEcoS..60..227M |s2cid=36836956 }}</ref> Presumably, increased sexual dimorphism means males are brighter and more conspicuous, leading to increased predation.<ref name="Møller-2006"/> Moreover, the production of more exaggerated ornaments in males may come at the cost of suppressed immune function.<ref name="Lindsay-2009"/> So long as the reproductive benefits of the trait due to sexual selection are greater than the costs imposed by natural selection, then the trait will propagate throughout the population. Reproductive benefits arise in the form of a larger number of offspring, while natural selection imposes costs in the form of reduced survival. This means that even if the trait causes males to die earlier, the trait is still beneficial so long as males with the trait produce more offspring than males lacking the trait. This balance keeps dimorphism alive in these species and ensures that the next generation of successful males will also display these traits that are attractive to females.

Such differences in form and reproductive roles often cause differences in behavior. As previously stated, males and females often have different roles in reproduction. The courtship and mating behavior of males and females are regulated largely by hormones throughout a bird's lifetime.<ref name="Adkins-Regan-2007">{{cite journal | vauthors = Adkins-Regan E |year=2007 |title=Hormones and the development of sex differences in behavior |journal=[[Journal of Ornithology]] |volume=148 |issue=Supplement 1 |pages=S17–S26 |doi=10.1007/s10336-007-0188-3|bibcode=  |s2cid=13868097 }}</ref> Activational hormones occur during puberty and adulthood and serve to 'activate' certain behaviors when appropriate, such as territoriality during breeding season.<ref name="Adkins-Regan-2007"/> Organizational hormones occur only during a critical period early in development, either just before or just after hatching in most birds, and determine patterns of behavior for the rest of the bird's life.<ref name="Adkins-Regan-2007"/> Such behavioral differences can cause disproportionate sensitivities to anthropogenic pressures.<ref name="Martin-2008">{{cite journal | vauthors = Martin U, Grüebler HS, Müller M, Spaar R, Horch P, Naef-Daenzer B |year=2008 |title=Female biased mortality caused by anthropogenic nest loss contributes to population decline and adult sex ratio of a meadow bird |journal=[[Biological Conservation (journal)|Biological Conservation]] | volume = 141 | issue = 12| pages = 3040–3049 |doi=10.1016/j.biocon.2008.09.008|bibcode=2008BCons.141.3040G }}</ref> Females of the whinchat in Switzerland breed in intensely managed grasslands.<ref name="Martin-2008"/> Earlier harvesting of the grasses during the breeding season lead to more female deaths.<ref name="Martin-2008"/> Populations of many birds are often male-skewed and when sexual differences in behavior increase this ratio, populations decline at a more rapid rate.<ref name="Martin-2008"/> Also not all male dimorphic traits are due to hormones like testosterone, instead they are a naturally occurring part of development, for example plumage.<ref>Owens, I. P. F., Short, R.V.,. (1995). Hormonal basis of sexual dimorphism in birds: Implications for new theories of sexual selection. Trends in Ecology & Evolution., 10(REF), 44.</ref> In addition, the strong hormonal influence on phenotypic differences suggests that the genetic mechanism and genetic basis of these sexually dimorphic traits may involve transcription factors or cofactors rather than regulatory sequences.<ref name="Coyne-2008" />

[[File:Male and female black casqued hornbill skeletons.jpg|thumb|upright=1.1|Skeletons of female (left) and Male (right) black-casqued hornbills (''[[Black-casqued hornbill|Ceratogymna atrata]]''). The difference between the sexes is apparent in the casque on the top of their bill. This pair is on display at the [[Museum of Osteology]]. ]]

Sexual dimorphism may also influence differences in parental investment during times of food scarcity. For example, in the [[blue-footed booby]], the female chicks grow faster than the males, resulting in booby parents producing the smaller sex, the males, during times of food shortage. This then results in the maximization of parental lifetime reproductive success.<ref>{{cite journal| vauthors = Velando A |title=Experimental Manipulation of Maternal Effort Produces Differential Effects in Sons and Daughters: Implications for Adaptive Sex Ratios in the Blue-footed Booby|journal=Behavioral Ecology|year=2002|volume=13|issue=4|pages=443–449|doi=10.1093/beheco/13.4.443 |doi-access=free}}</ref> In [[Black-tailed godwit|Black-tailed Godwits]] ''Limosa limosa limosa'' females are also the larger sex, and the growth rates of female chicks are more susceptible to limited environmental conditions.<ref>{{cite journal | vauthors = Loonstra AJ, Verhoeven MA, Piersma T | year=2018| title= Sex-specific growth in chicks of the sexually dimorphic Black-tailed Godwit. | journal=Ibis| volume=160| pages=89–100| doi= 10.1111/ibi.12541 | issue=1| s2cid=90880117| url=https://pure.rug.nl/ws/files/53667395/Sex_specific_growth_in_chicks_of_the_sexually_dimorphic.pdf}}</ref>

Sexual dimorphism may also only appear during mating season; some species of birds only show dimorphic traits in seasonal variation. The males of these species will molt into a less bright or less exaggerated color during the off-breeding season.<ref name="Coyne-2008">{{cite journal | vauthors = Coyne JA, Kay EH, Pruett-Jones S | title = The genetic basis of sexual dimorphism in birds | journal = Evolution; International Journal of Organic Evolution | volume = 62 | issue = 1 | pages = 214–9 | date = January 2008 | pmid = 18005159 | doi = 10.1111/j.1558-5646.2007.00254.x | s2cid = 11490688 | doi-access = free }}</ref> This occurs because the species is more focused on survival than on reproduction, causing a shift into a less ornate state. {{Dubious|date=October 2016}}

Consequently, sexual dimorphism has important ramifications for conservation. However, sexual dimorphism is not only found in birds and is thus important to the conservation of many animals. Such differences in form and behavior can lead to [[sexual segregation (biology)|sexual segregation]], defined as sex differences in space and resource use.<ref name="Main-2008">{{cite journal | vauthors = Main MB | title = Reconciling competing ecological explanations for sexual segregation in ungulates | journal = Ecology | volume = 89 | issue = 3 | pages = 693–704 | date = March 2008 | pmid = 18459333 | doi = 10.1890/07-0645.1 | bibcode = 2008Ecol...89..693M }}</ref> Most sexual segregation research has been done on ungulates,<ref name="Main-2008"/> but such research extends to [[bat]]s,<ref name="Safi-2007">{{cite journal |vauthors=Safi K, König B, Kerth G |year=2007 |title=Sex differences in population genetics, home range size and habitat use of the parti-colored bat (''Vespertilio murinus'', Linnaeus 1758) in Switzerland and their consequences for conservation |journal=[[Biological Conservation (journal)|Biological Conservation]] |volume=137 |issue=1 |pages=28–36 |doi=10.1016/j.biocon.2007.01.011 |bibcode=2007BCons.137...28S |url=http://www.zora.uzh.ch/id/eprint/394/1/Safi_2007.pdf |access-date=2 February 2019 |archive-date=25 September 2017 |archive-url=https://web.archive.org/web/20170925231845/http://www.zora.uzh.ch/id/eprint/394/1/Safi_2007.pdf |url-status=dead }}</ref> [[kangaroo]]s,<ref>{{cite journal | vauthors = Coulson G, MacFarlane AM, Parsons SE, Cutter J  |year=2006 |title=Evolution of sexual segregation in mammalian herbivores: kangaroos as marsupial models |journal=[[Australian Journal of Zoology]] |volume=54 |issue=3 |pages=217–224 |doi=10.1071/ZO05062}}</ref> and birds.<ref>{{cite journal | vauthors = González-Solís J, Croxall JP, Wood AG |year=2000 | title=Sexual dimorphism and sexual segregation in foraging strategies of northern giant petrels, ''Macronectes halli'', during incubation |journal=[[Oikos (journal)|Oikos]] |volume=90 |issue=2 |pages=390–398 |doi=10.1034/j.1600-0706.2000.900220.x|bibcode=2000Oikos..90..390G }}</ref> Sex-specific conservation plans have even been suggested for species with pronounced sexual segregation.<ref name="Safi-2007"/>

The term sesquimorphism (the Latin [[numeral prefix]] ''sesqui''- means one-and-one-half, so halfway between ''mono''- (one) and ''di''- (two)) has been proposed for bird species in which "both sexes have basically the same plumage pattern, though the female is clearly distinguishable by reason of her paler or washed-out {{nowrap|colour".<ref name="Summers-Smith-1988">{{cite book |title=The Sparrows | vauthors = Summers-Smith JD |author-link=J. Denis Summers-Smith |publisher=[[T. & A. D. Poyser]] |location=Calton, Staffordshire, UK |date=1988 |isbn=978-0-85661-048-6 |url=https://archive.org/details/sparrowsstudyofg0000summ }}</ref>{{rp|14}} }} Examples include [[Cape sparrow]] (''Passer melanurus''),<ref name="Summers-Smith-1988" />{{rp|67}} [[Great sparrow|rufous sparrow]] (subspecies ''P.&nbsp;motinensis motinensis''),<ref name="Summers-Smith-1988" />{{rp|80}} and [[saxaul sparrow]] (''P.&nbsp;ammodendri'').<ref name="Summers-Smith-1988" />{{rp|245}}

== Non-avian dinosaurs ==
{{Main|Sexual dimorphism in dinosaurs}}
Examining fossils of [[Dinosaur|non-avian dinosaurs]] in search of sexually dimorphic characteristics requires the supply of complete and articulated skeletal and tissue remains. As terrestrial organisms, dinosaur carcasses are subject to ecological and geographical influence that inevitably constitutes the degree of preservation. The availability of well-preserved remains is not a probable outcome as a consequence of [[decomposition]] and [[fossil]]ization. Some [[paleontologists]] have looked for sexual dimorphism among dinosaurs using statistics and comparison to ecologically or [[Phylogenetics|phylogenetically]] related modern animals.

'''Apatosaurus and Diplodocus'''

Female ''[[Apatosaurus]]'' and ''[[Diplodocus]]'' had interconnected [[caudal vertebrae]] that allowed them to keep their tails elevated to aid in copulation. Discovering that this fusion occurred in only 50% of ''Apatosaurus'' and ''Diplodocus'' skeletons and 25% of ''[[Camarasaurus]]'' skeletons indicated that this is a sexually dimorphic trait.

'''Theropoda'''

It has been hypothesized that male [[Theropoda|theropods]] possessed a retractable penis, a feature similar to modern day [[crocodilia]]ns. Crocodilian skeletons were examined to determine whether there is a skeletal component that is distinctive between both sexes, to help provide an insight on the physical disparities between male and female theropods. Findings revealed the caudal [[Haemal arch|chevrons]] of male crocodiles, used to anchor the penis muscles, were significantly larger than those of females. There have been criticisms of these findings, but it remains a subject of debate among advocates and adversaries.{{Citation needed|date=August 2018}}

'''Ornithopoda'''

Studies of sexual dimorphism in [[Hadrosaurid|hadrosaurs]] have generally centered on the distinctive [[Cranial neural crest|cranial crests]], which likely provided a function in sexual display. A biometric study of 36 skulls found sexual dimorphism was exhibited in the crest of 3 species of hadrosaurids. The crests could be categorized as full (male) or narrow (female) and may have given some advantage in intrasexual mating-competition.

'''Ceratopsians'''

According to Scott D. Sampson, if ceratopsids were to exhibit sexual dimorphism, modern ecological analogues suggest it would be found in display structures, such as horns and frills. No convincing evidence for sexual dimorphism in body size or mating signals is known in ceratopsids, although there is evidence that the more primitive ceratopsian ''[[Protoceratops andrewsi]]'' possessed sexes that were distinguishable based on frill and nasal prominence size. This is consistent with other known [[tetrapod]] groups where midsized animals tend to exhibit markedly more sexual dimorphism than larger ones. However, it has been proposed that these differences can be better explained by intraspecific and ontogenic variation rather than sexual dimorphism.<ref name="Maiorino-2015">{{cite journal |last1=Maiorino |first1=Leonardo |last2=Farke |first2=Andrew A. |last3=Kotsakis |first3=Tassos |last4=Piras |first4=Paolo |date=7 May 2015 |title=Males Resemble Females: Re-Evaluating Sexual Dimorphism in Protoceratops andrewsi (Neoceratopsia, Protoceratopsidae) |journal=PLOS ONE |language=en |volume=10 |issue=5 |page=e0126464 |doi=10.1371/journal.pone.0126464 |issn=1932-6203 |pmc=4423778 |pmid=25951329 |doi-access=free}}</ref> In addition, many sexually dimorphic traits that may have existed in ceratopsians include soft tissue variations such as coloration or [[dewlaps]], which would be unlikely to have been preserved in the fossil record.

'''Stegosaurians'''

A 2015 study on specimens of [[Hesperosaurus|''Hesperosaurus'' ''mjosi'']] found evidence of sexual dimorphism in the shape of the dermal plates. Two plate morphs were described: one was short, wide, and oval-shaped, the other taller and narrower.<ref>{{Cite journal |last=Saitta |first=Evan Thomas |date=2015-04-22 |title=Evidence for Sexual Dimorphism in the Plated Dinosaur Stegosaurus mjosi (Ornithischia, Stegosauria) from the Morrison Formation (Upper Jurassic) of Western USA |journal=PLOS ONE |language=en |volume=10 |issue=4 |pages=e0123503 |bibcode=2015PLoSO..1023503S |doi=10.1371/journal.pone.0123503 |issn=1932-6203 |pmc=4406738 |pmid=25901727 |doi-access=free}}</ref><ref>{{Cite web |title=Stegosaurus plates may have differed between male, female |url=https://www.sciencedaily.com/releases/2015/04/150422142403.htm |access-date=2024-03-12 |website=ScienceDaily |language=en}}</ref>

==Mammals==
In 45% of mammal species, males are larger than females, in 39%, males and females are the same size and in 16% of mammals, females are larger than males.<ref>{{Cite web |title=Most male mammals aren't bigger than females |url=https://www.nhm.ac.uk/discover/news/2024/march/most-male-mammals-arent-bigger-than-females.html |access-date=2024-12-23 |website=www.nhm.ac.uk |language=en}}</ref> Both [[genes]] and [[hormone]]s affect the formation of many animal brains before "[[birth]]" (or [[egg (biology)|hatching]]), and also behaviour of adult individuals. Hormones significantly affect human brain formation, and also brain development at puberty. A 2004 review in ''[[Nature Reviews Neuroscience]]'' observed that "because it is easier to manipulate hormone levels than the expression of sex chromosome genes, the effects of hormones have been studied much more extensively, and are much better understood, than the direct actions in the brain of sex chromosome genes." It concluded that while "the differentiating effects of gonadal secretions seem to be dominant," the existing body of research "support the idea that sex differences in neural expression of X and Y genes significantly contribute to sex differences in brain functions and disease."<ref>{{cite journal | vauthors = Arnold AP | title = Sex chromosomes and brain gender | journal = Nature Reviews. Neuroscience | volume = 5 | issue = 9 | pages = 701–8 | date = September 2004 | pmid = 15322528 | doi = 10.1038/nrn1494 | s2cid = 7419814 }}</ref>

===Pinnipeds===
[[File:Male and female Mirounga angustirostris 2.jpg|thumb|right|260 px|Male and female [[northern elephant seal]], the male being larger with a big [[proboscis]]]]
[[Marine mammals]] show some of the greatest sexual size differences of mammals, because of sexual selection and environmental factors like breeding location.<ref>{{Cite journal| vauthors = Cassini MH |date=January 2020|title=A mixed model of the evolution of polygyny and sexual size dimorphism in mammals |journal=Mammal Review|language=en|volume=50|issue=1|pages=112–120|doi=10.1111/mam.12171|bibcode=2020MamRv..50..112C |s2cid=208557639|issn=0305-1838}}</ref> The mating system of [[pinnipeds]] varies from polygamy to [[serial monogamy]]. Pinnipeds are known for early differential growth and maternal investment since the only nutrients for newborn pups is the milk provided by the mother.<ref>{{cite journal | vauthors = Cappozzo HL, Campagna C, Monserrat J | year = 1991 | title = Sexual Dimorphism in Newborn Southern Sea Lions | journal = Marine Mammal Science | volume = 7 | issue = 4| pages = 385–394 | doi=10.1111/j.1748-7692.1991.tb00113.x| bibcode = 1991MMamS...7..385C }}</ref> For example, the males are significantly larger (about 10% heavier and 2% longer) than the females at birth in sea lion pups.<ref>{{Cite journal| vauthors = Salogni E, Galimberti F, Sanvito S, Miller EH |date=March 2019|title=Male and female pups of the highly sexually dimorphic northern elephant seal (Mirounga angustirostris) differ slightly in body size |journal=Canadian Journal of Zoology|volume=97|issue=3|pages=241–250|doi=10.1139/cjz-2018-0220|bibcode=2019CaJZ...97..241S |s2cid=91796880|issn=0008-4301}}</ref> The pattern of differential investment can be varied principally prenatally and post-natally.<ref>{{cite journal | vauthors = Ono KA, Boness DJ | title = Sexual dimorphism in sea lion pups: differential maternal investment, or sex-specific differences in energy allocation?. | journal = Behavioral Ecology and Sociobiology | date = January 1996 | volume = 38 | issue = 1 | pages = 31–41 | doi = 10.1007/s002650050214 | bibcode = 1996BEcoS..38...31O | s2cid = 25307359 }}</ref> ''Mirounga leonina'', the [[southern elephant seal]], is one of the most dimorphic mammals.<ref>{{cite journal | vauthors = Tarnawski BA, Cassini GH, Flores DA | year = 2014 | title = Skull allometry and sexual dimorphism in the ontogeny of the southern elephant seal (Mirounga leonina) | journal = Canadian Journal of Zoology | volume = 31 | issue = 1 | pages = 19–31 | doi=10.1139/cjz-2013-0106| bibcode = 2014CaJZ...92...19T | hdl = 11336/29893 | hdl-access = free }}</ref>

===Primates===
{{Main|Sexual dimorphism in non-human primates}}

Most [[Simian|anthropoid primates]] are sexually dimorphic for different biological characteristics, such as body size, canine tooth size, craniofacial structure, skeletal dimensions, pelage color and markings, and vocalization. But [[strepsirrhine]] primates and [[tarsiers]] are mostly monomorphic.

====Humans====
{{Main|Sex differences in humans|Sex differences in psychology}}

{| class="wikitable"  style="width:200px; float:right;"
|-
| colspan="2" | [[File:Human.svg|center|200px|Pioneer plaque]]
|-
| [[File:Gray241.png|100px|right|Male pelvis]]
| [[File:Gray242.png|100px|right|Female pelvis]]
|-
| colspan="2" |
'''Top:''' Stylised illustration of humans on the [[Pioneer plaque]], showing both male (left) and female (right).
<br />'''Bottom:''' Comparison between male (left) and female (right) [[pelvis]]es.
|}

According to Clark Spencer Larsen, modern day ''[[Homo sapiens]]'' show a range of sexual dimorphism, with average body mass between the sexes differing by roughly 15%.<ref>{{cite journal | vauthors = Larsen CS | title = Equality for the sexes in human evolution? Early hominid sexual dimorphism and implications for mating systems and social behavior | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 100 | issue = 16 | pages = 9103–4 | date = August 2003 | pmid = 12886010 | pmc = 170877 | doi = 10.1073/pnas.1633678100 | bibcode = 2003PNAS..100.9103L | doi-access = free }}</ref> Substantial discussion in academic literature considers potential evolutionary advantages associated with sexual competition (both intrasexual and intersexual), as well as short- and long-term sexual strategies.<ref>{{cite journal |author-link=David M. Buss |vauthors=Buss DM |year=2007 |title=The evolution of human mating |url=http://homepage.psy.utexas.edu/homepage/Group/BussLAB/pdffiles/evolution_of_human_mating_2007.pdf |journal=[[Acta Psychologica Sinica]] |volume=39 |issue=3 |pages=502–512 |access-date=14 May 2011 |archive-date=12 August 2011 |archive-url=https://web.archive.org/web/20110812161208/http://www.homepage.psy.utexas.edu/homepage/group/busslab/pdffiles/evolution_of_human_mating_2007.pdf |url-status=dead }}</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>{{cite book |url=https://archive.org/details/sexpowerconflict00buss |title=Sex, Power, Conflict: Evolutionary and Feminist Perspectives |vauthors=Daly M, Wilson M |publisher=[[Oxford University Press]] |year=1996 |isbn=978-0-19-510357-1 |editor-link1=David M. Buss |veditors=Buss DM, Malamuth NM |page=[https://archive.org/details/sexpowerconflict00buss/page/n19 13] |chapter=Evolutionary psychology and marital conflict |url-access=limited}}</ref>

Pubertal changes in males lead to a ten times increase in [[testosterone]] compared to females. It is because of the effects of testosterone that males develop stronger and denser bones, as well as increased muscle mass and strength during puberty. On average, adult males have 10-20 times more testosterone than adult females, with male testosterone levels ranging from 300 to 1,000 nanograms per deciliter (ng/dL), while adult females have testosterone levels between 15 and 70&nbsp;ng/dL.<ref>{{Cite web |title=Content - Health Encyclopedia - University of Rochester Medical Center |url=https://www.urmc.rochester.edu/encyclopedia/content?contentid=testosterone_total&contenttypeid=167 |access-date=2025-04-22 |website=www.urmc.rochester.edu}}</ref><ref>{{Cite web |date=2025-04-05 |title=Testosterone: What It Is, Function & Levels |url=https://my.clevelandclinic.org/health/articles/24101-testosterone |access-date= |archive-url=https://web.archive.org/web/20250405015804/https://my.clevelandclinic.org/health/articles/24101-testosterone |archive-date=5 April 2025 }}</ref> Studies show that men have a higher [[Bone density|Bone Mineral Density]] (BMD) than women. It shows BMD values of approximately 3.88 g/cm² for men and 2.90 g/cm² for women, showing that male bones are about 30% denser than female bones.<ref>{{Cite journal |last1=Daly |first1=Robin M. |last2=Rosengren |first2=Bjorn E. |last3=Alwis |first3=Gayani |last4=Ahlborg |first4=Henrik G. |last5=Sernbo |first5=Ingemar |last6=Karlsson |first6=Magnus K. |date=2013-07-06 |title=Gender specific age-related changes in bone density, muscle strength and functional performance in the elderly: a-10 year prospective population-based study |journal=BMC Geriatrics |volume=13 |issue=1 |pages=71 |doi=10.1186/1471-2318-13-71 |doi-access=free |issn=1471-2318 |pmc=3716823 |pmid=23829776}}</ref><ref>{{Cite journal |last1=Chen |first1=Kexun Kenneth |last2=Wee |first2=Shiou-Liang |last3=Pang |first3=Benedict Wei Jun |last4=Lau |first4=Lay Khoon |last5=Jabbar |first5=Khalid Abdul |last6=Seah |first6=Wei Ting |last7=Srinivasan |first7=Sivasubramanian |last8=Jagadish |first8=Mallya Ullal |last9=Ng |first9=Tze Pin |date=2020-09-25 |title=Bone mineral density reference values in Singaporean adults and comparisons for osteoporosis establishment – The Yishun Study |journal=BMC Musculoskeletal Disorders |volume=21 |issue=1 |pages=633 |doi=10.1186/s12891-020-03646-y |doi-access=free |issn=1471-2474 |pmc=7519574 |pmid=32977780}}</ref>

The average [[basal metabolic rate]] is about 6 percent higher in adolescent males than females and increases to about 10 percent higher after puberty. Females tend to convert more food into [[fat]], while males convert more into [[muscle]] and expendable circulating energy reserves. According to Tim Hewett, director of research in the department of sports medicine at [[Ohio State University Wexner Medical Center]], women have 60% of the upper body strength of men and 80-90% of the lower body strength of men.<ref>{{Cite web |author1=Rachael Rettner |date=2014-01-03 |title=Why Pull-Ups Are Harder for Women |url=https://www.livescience.com/42318-women-pullups.html |access-date=2024-12-23 |website=livescience.com |language=en}}</ref>

The difference in strength relative to body mass is less pronounced in trained individuals. In Olympic weightlifting, male records vary from 5.5× body mass in the lowest weight category to 4.2× in the highest weight category, while female records vary from 4.4× to 3.8×, a weight-adjusted difference of only 10–20%, and an absolute difference of about 30% (i.e., 492&nbsp;kg vs 348&nbsp;kg for unlimited weight classes; see [[List of world records in Olympic weightlifting|Olympic weightlifting records]]).

A study, carried out by analyzing annual world rankings from 1980 to 1996, found that males' running times were, on average, 10% faster than females', due to wider hips in females being a biomechanical disadvantage for running.<ref>{{cite journal | vauthors = Sparling PB, O'Donnell EM, Snow TK | title = The gender difference in distance running performance has plateaued: an analysis of world rankings from 1980 to 1996 | journal = Medicine and Science in Sports and Exercise | volume = 30 | issue = 12 | pages = 1725–9 | date = December 1998 | pmid = 9861606 | doi = 10.1097/00005768-199812000-00011 | doi-access = free }}</ref> This is because wider hips lead to a larger [[Q-angle]] (the angle between the hip and knee), which alters the alignment of the lower limbs in females. This would affect the efficiency of force transmission through the legs during running, and also less efficient running [[biomechanics]] compared to males with narrower hips and longer [[femur]]s.<ref>{{Cite journal |last1=Noehren |first1=Brian |last2=Pohl |first2=Michael B. |last3=Sanchez |first3=Zack |last4=Cunningham |first4=Tom |last5=Lattermann |first5=Christian |date=May 2012 |title=Proximal and distal kinematics in female runners with patellofemoral pain |journal=Clinical Biomechanics (Bristol, Avon) |volume=27 |issue=4 |pages=366–371 |doi=10.1016/j.clinbiomech.2011.10.005 |issn=1879-1271 |pmc=3984468 |pmid=22071426}}</ref><ref>{{Cite journal |last1=Gant |first1=Hannah |last2=Ghimire |first2=Nabin |last3=Min |first3=Kisuk |last4=Musa |first4=Ibrahim |last5=Ashraf |first5=Maryam |last6=Lawan |first6=Ahmed |date=2024-11-22 |title=Impact of the Quadriceps Angle on Health and Injury Risk in Female Athletes |journal=International Journal of Environmental Research and Public Health |language=en |volume=21 |issue=12 |pages=1547 |doi=10.3390/ijerph21121547 |doi-access=free |pmid=39767389 |pmc=11675324 |issn=1660-4601 }}</ref> However, females have higher [[endurance]] than males.<ref>{{Cite news |last=Rettner |first=Rachel |date=3 January 2014 |title=Why Pull-Ups Are Harder for Women |url=https://www.livescience.com/42318-women-pullups.html |work=LiveScience}}</ref><ref>{{Cite journal |last1=Le Mat |first1=Franck |last2=Géry |first2=Mathias |last3=Besson |first3=Thibault |last4=Ferdynus |first4=Cyril |last5=Bouscaren |first5=Nicolas |last6=Millet |first6=Guillaume Y. |date=April 2023 |title=Running Endurance in Women Compared to Men: Retrospective Analysis of Matched Real-World Big Data |url=https://pubmed.ncbi.nlm.nih.gov/36802328/#article-details |journal=Sports Medicine (Auckland, N.Z.) |volume=53 |issue=4 |pages=917–926 |doi=10.1007/s40279-023-01813-4 |issn=1179-2035 |pmid=36802328}}</ref>

In early adolescence, females are on average taller than males (as females tend to [[Puberty#Differences between male and female puberty|go through puberty earlier]]), but males, on average, surpass them in height in later adolescence and adulthood. In the United States, adult males are on average 9% taller<ref>{{cite journal|title=National Health Statistics Reports|journal=National Health Statistics Reports|date=22 October 2008|volume=10|url=https://www.cdc.gov/nchs/data/nhsr/nhsr010.pdf|access-date=21 April 2012}}</ref> and 16.5% heavier<ref>{{cite web|url=https://www.cdc.gov/nchs/data/ad/ad347.pdf |title=United States National Health and Nutrition Examination Survey, 1999–2002 |access-date=1 May 2014}}</ref> than adult females.

Males typically have larger [[Vertebrate trachea|tracheae]] and branching [[Bronchus|bronchi]], with about 30 percent greater [[Lung volumes|lung volume]] per [[body mass]]. On average, males have larger [[heart]]s, slower [[heart rate]]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 lower sensitivity to nerve [[pain]] after injury.<ref name="Glucksman-1981">{{cite book | vauthors = Glucksman A |year=1981 |title=Sexual Dimorphism in Human and Mammalian Biology and Pathology |publisher=[[Academic Press]] |isbn=978-0-12-286960-0 |pages=66–75 |oclc=7831448 }}</ref> In males, pain-causing injury to the [[Peripheral nervous system|peripheral nerve]] occurs through the [[microglia]], while in females it occurs through the [[T cell]]s (except in pregnant women, who follow a male pattern).<ref name="Dance-2019">{{Cite journal | vauthors = Dance A |date=27 March 2019 |title=Why the sexes don't feel pain the same way |journal=Nature |language=en |volume=567 |issue=7749 |pages=448–450 |doi=10.1038/d41586-019-00895-3|pmid=30918396 |bibcode=2019Natur.567..448D |s2cid=85527866 |doi-access=free }}</ref>

Females typically have more [[white blood cell]]s (stored and circulating), as well as more [[granulocyte]]s and B and T [[lymphocyte]]s. Additionally, they produce more [[Antibody|antibodies]] at a faster rate than males, hence they develop fewer [[Infection|infectious]] diseases and succumb for shorter periods.<ref name="Glucksman-1981" /> [[Ethology|Ethologists]] argue that females, interacting with other females and multiple offspring in social groups, have experienced such traits as a [[Natural selection|selective]] advantage.<ref>{{cite book | vauthors = Durden-Smith J, deSimone D | year=1983 |title=Sex and the Brain |location=New York |publisher=[[Arbor House]] |isbn=978-0-87795-484-2 |url= https://archive.org/details/sexbrain00durd }}</ref><ref>{{cite journal | vauthors = Gersh ES, Gersh I |author-link1=Eileen S. Gersh | name-list-style = vanc |author-link2=Isidore Gersh |year=1981 |title=Biology of Women |journal=Nature |volume=306 |issue=5942 |pages=511 |location=Baltimore |publisher=University Park Press (original from the University of Michigan) |isbn=978-0-8391-1622-6 |bibcode=1983Natur.306..511. |doi=10.1038/306511b0 |s2cid=28060318 |url=https://archive.org/details/biologyofwomen00eile |url-access=limited }}</ref><ref>{{cite book | vauthors = Stein JH |year=1987 |title=Internal Medicine |publisher=[[Little, Brown]] |location=Boston |isbn=978-0-316-81236-8 |edition=2nd}}</ref><ref>{{cite journal | vauthors = McLaughlin M, Shryer T | title = Men vs women: the new debate over sex differences |journal=[[U.S. News & World Report]] |date=8 August 1988 |pages=50–58}}</ref><ref>{{cite journal | vauthors = McEwen BS | title = Neural gonadal steroid actions | journal = Science | volume = 211 | issue = 4488 | pages = 1303–11 | date = March 1981 | pmid = 6259728 | doi = 10.1126/science.6259728 | bibcode = 1981Sci...211.1303M }}</ref> Females have a higher sensitivity to pain due to aforementioned nerve differences that increase the sensation, and females thus require higher levels of pain medication after injury.<ref name="Dance-2019"/> Hormonal changes in females affect pain sensitivity, and pregnant women have the same sensitivity as males. Acute pain tolerance is also more consistent over a lifetime in females than males, despite these hormonal changes.<ref>{{Cite web |title=Acute Pain Tolerance Is More Consistent Over Time in Women Than Men, According to New Research |url=https://www.nccih.nih.gov/research/research-results/acute-pain-tolerance-is-more-consistent-over-time-in-women-than-men-according-to-new-research |access-date=11 May 2022 |website=NCCIH |language=en}}</ref> Despite differences in physical feeling, both sexes have similar psychological tolerance to (or ability to [[Coping|cope]] with and ignore) pain.<ref>{{Cite web | vauthors = Woznicki K | title=Pain Tolerance and Sensitivity in Men, Women, Redheads, and More |url=https://www.webmd.com/pain-management/features/whats-your-pain-tolerance |access-date=11 May 2022 |website=WebMD |language=en}}</ref>

In the [[human brain]], a difference between sexes was observed in the [[gene transcription|transcription]] of the [[PCDH11X]]/Y gene pair unique to ''Homo sapiens''.<ref>{{cite journal | vauthors = Lopes AM, Ross N, Close J, Dagnall A, Amorim A, Crow TJ | title = Inactivation status of PCDH11X: sexual dimorphisms in gene expression levels in brain | journal = Human Genetics | volume = 119 | issue = 3 | pages = 267–75 | date = April 2006 | pmid = 16425037 | doi = 10.1007/s00439-006-0134-0 | s2cid = 19323646 }}</ref> Sexual differentiation in the human brain from the undifferentiated state is triggered by testosterone from the fetal testis. Testosterone is converted to estrogen in the brain through the action of the enzyme aromatase. Testosterone acts on many brain areas, including the [[INAH 3|SDN-POA]], to create the masculinized brain pattern.<ref>{{cite journal | vauthors = Lombardo MV, Ashwin E, Auyeung B, Chakrabarti B, Taylor K, Hackett G, Bullmore ET, Baron-Cohen S | display-authors = 6 | title = Fetal testosterone influences sexually dimorphic gray matter in the human brain | journal = The Journal of Neuroscience | volume = 32 | issue = 2 | pages = 674–80 | date = January 2012 | pmid = 22238103 | pmc = 3306238 | doi = 10.1523/JNEUROSCI.4389-11.2012 }}</ref> The brains of pregnant females carrying male fetuses may be shielded from the masculinizing effects of androgen through the action of [[sex hormone-binding globulin]].<ref>{{cite web|url=http://www.biolreprod.org/content/85/3/431.full?sid=1d62e556-e3a3-48f8-8394-62821a22a6f3|archive-url=https://web.archive.org/web/20150923203726/http://www.biolreprod.org/content/85/3/431.full?sid=1d62e556-e3a3-48f8-8394-62821a22a6f3|url-status=dead|archive-date=23 September 2015|title=Diverse Roles for Sex Hormone-Binding Globulin in Reproduction|work=biolreprod.org}}</ref>

The relationship between sex differences in the brain and human behavior is a subject of controversy in psychology and society at large.<ref name="Fine-2010">{{Cite book | vauthors = Fine C |title=Delusions of Gender: How Our Minds, Society, and Neurosexism Create Difference |publisher=[[W. W. Norton & Company]] |date=August 2010 |edition=1st |isbn=978-0-393-06838-2 |url=https://archive.org/details/delusionsofgende00cord |author-link=Cordelia Fine }}</ref><ref name="Jordan-Young-2010">{{Cite book | vauthors = Jordan-Young R |date=September 2010 |title=Brain Storm: The Flaws in the Science of Sex Differences |publisher=[[Harvard University Press]] |isbn=978-0-674-05730-2 |url=https://archive.org/details/isbn_9780674057302 }}</ref> Many females tend to have a higher ratio of [[gray matter]] in the left hemisphere of the brain in comparison to males.<ref name="Marner-2003">{{cite journal | vauthors = Marner L, Nyengaard JR, Tang Y, Pakkenberg B | title = Marked loss of myelinated nerve fibers in the human brain with age | journal = The Journal of Comparative Neurology | volume = 462 | issue = 2 | pages = 144–52 | date = July 2003 | pmid = 12794739 | doi = 10.1002/cne.10714 | s2cid = 35293796 }}</ref><ref name="Gur-1999">{{cite journal | vauthors = Gur RC, Turetsky BI, Matsui M, Yan M, Bilker W, Hughett P, Gur RE | title = Sex differences in brain gray and white matter in healthy young adults: correlations with cognitive performance | journal = The Journal of Neuroscience | volume = 19 | issue = 10 | pages = 4065–72 | date = May 1999 | pmid = 10234034 | pmc = 6782697 | doi = 10.1523/JNEUROSCI.19-10-04065.1999 }}</ref> Males on average have larger brains than females; however, when adjusted for total brain volume, the gray matter differences between sexes are almost nonexistent. Thus, the percentage of gray matter appears to be more related to brain size than it is to sex.<ref>{{cite journal | vauthors = Leonard CM, Towler S, Welcome S, Halderman LK, Otto R, Eckert MA, Chiarello C | title = Size matters: cerebral volume influences sex differences in neuroanatomy | journal = Cerebral Cortex | volume = 18 | issue = 12 | pages = 2920–31 | date = December 2008 | pmid = 18440950 | pmc = 2583156 | doi = 10.1093/cercor/bhn052 }}</ref><ref>{{cite journal | vauthors = Lüders E, Steinmetz H, Jäncke L | title = Brain size and grey matter volume in the healthy human brain | journal = NeuroReport | volume = 13 | issue = 17 | pages = 2371–4 | date = December 2002 | pmid = 12488829 | doi = 10.1097/00001756-200212030-00040  }}</ref> Differences in brain physiology between sexes do not necessarily relate to differences in intellect. Haier ''et al.'' found in a 2004 study that "men and women apparently achieve similar IQ results with different brain regions, suggesting that there is no singular underlying neuroanatomical structure to general intelligence and that different types of brain designs may manifest equivalent intellectual performance".<ref name="Haier-2005">{{cite journal | vauthors = Haier RJ, Jung RE, Yeo RA, Head K, Alkire MT | title = The neuroanatomy of general intelligence: sex matters | journal = NeuroImage | volume = 25 | issue = 1 | pages = 320–7 | date = March 2005 | pmid = 15734366 | doi = 10.1016/j.neuroimage.2004.11.019 | url = http://www.themindinstitute.org/pubs/Haier(2005)NeuroanatomyofGeneralIntelligence.pdf | s2cid = 4127512 | archive-url = https://web.archive.org/web/20100524194658/http://www.themindinstitute.org/pubs/Haier%282005%29NeuroanatomyofGeneralIntelligence.pdf | archive-date = 24 May 2010 }}</ref> (See the [[sex and intelligence]] article for more on this subject.) Strict graph-theoretical analysis of the human brain connections revealed<ref name="Szalkai-2015">{{cite journal | vauthors = Szalkai B, Varga B, Grolmusz V | title = Graph Theoretical Analysis Reveals: Women's Brains Are Better Connected than Men's | journal = PLOS ONE| volume = 10 | issue = 7 | pages = e0130045 | date = 2015 | pmid = 26132764 | pmc = 4488527 | doi = 10.1371/journal.pone.0130045 | arxiv = 1501.00727 | bibcode = 2015PLoSO..1030045S | doi-access = free }}</ref> that in numerous graph-theoretical parameters (e.g., minimum bipartition width, edge number, the [[expander graph]] property, minimum [[vertex cover]]), the structural connectome of women are significantly "better" connected than the connectome of men. It was shown<ref name="Szalkai-2018">{{cite journal | vauthors = Szalkai B, Varga B, Grolmusz V | title = Brain size bias compensated graph-theoretical parameters are also better in women's structural connectomes | journal = Brain Imaging and Behavior | volume = 12 | issue = 3 | pages = 663–673 | date = June 2018 | pmid = 28447246 | doi = 10.1007/s11682-017-9720-0 | s2cid = 4028467 }}</ref> that the graph-theoretical differences are due to the sex and not to the differences in the cerebral volume, by analyzing the data of 36 females and 36 males, where the brain volume of each man in the group was smaller than the brain volume of each woman in the group.

Sexual dimorphism was also described in the gene level and shown to extend from the sex chromosomes. Overall, about 6500 genes have been found to have sex-differential expression in at least one tissue. Many of these genes are not directly associated with reproduction, but rather linked to more general biological features. In addition, it has been shown that genes with sex-specific expression undergo reduced selection efficiency, which leads to higher population frequencies of deleterious mutations and contributes to the prevalence of several human diseases.<ref>{{cite journal | vauthors = Gershoni M, Pietrokovski S | title = The landscape of sex-differential transcriptome and its consequent selection in human adults | journal = BMC Biology | volume = 15 | issue = 1 | pages = 7 | date = February 2017 | pmid = 28173793 | pmc = 5297171 | doi = 10.1186/s12915-017-0352-z | doi-access = free }}</ref><ref>{{cite journal | vauthors = Gershoni M, Pietrokovski S | title = Reduced selection and accumulation of deleterious mutations in genes exclusively expressed in men | journal = Nature Communications | volume = 5 | pages = 4438 | date = July 2014 | pmid = 25014762 | doi = 10.1038/ncomms5438 | doi-access = free | bibcode = 2014NatCo...5.4438G }}</ref>

== Immune function ==
Sexual dimorphism in immune function is a common pattern in vertebrates and also in a number of invertebrates. Most often, females are more 'immunocompetent' than males. This trait is not consistent among all animals, but differs depending on taxonomy, with the most female-biased immune systems being found in insects.<ref>{{cite journal | vauthors = Kelly CD, Stoehr AM, Nunn C, Smyth KN, Prokop ZM | title = Sexual dimorphism in immunity across animals: a meta-analysis | journal = Ecology Letters | volume = 21 | issue = 12 | pages = 1885–1894 | date = December 2018 | pmid = 30288910 | doi = 10.1111/ele.13164 | doi-access = free | bibcode = 2018EcolL..21.1885K }}</ref> In mammals this results in more frequent and severe infections in males and higher rates of autoimmune disorders in females. One potential cause may be differences in gene expression of immune cells between the sexes.<ref>{{cite journal | vauthors = Gal-Oz ST, Maier B, Yoshida H, Seddu K, Elbaz N, Czysz C, Zuk O, Stranger BE, Ner-Gaon H, Shay T | display-authors = 6 | title = ImmGen report: sexual dimorphism in the immune system transcriptome | journal = Nature Communications | volume = 10 | issue = 1 | pages = 4295 | date = September 2019 | pmid = 31541153 | pmc = 6754408 | doi = 10.1038/s41467-019-12348-6 | bibcode = 2019NatCo..10.4295G }}</ref> Another explanation is that endocrinological differences between the sexes impact the immune system – for example, testosterone acts as an immunosuppressive agent.<ref>{{cite journal | vauthors = Grossman C | title = Possible underlying mechanisms of sexual dimorphism in the immune response, fact and hypothesis | journal = Journal of Steroid Biochemistry | volume = 34 | issue = 1–6 | pages = 241–251 | year = 1989 | pmid = 2696846 | doi = 10.1016/0022-4731(89)90088-5 }}</ref>

==Cells==
Phenotypic differences between sexes are evident even in [[cell culture|cultured cells]] from tissues.<ref name="Pollitzer-2013">{{cite journal | vauthors = Pollitzer E | title = Biology: Cell sex matters | journal = Nature | volume = 500 | issue = 7460 | pages = 23–4 | date = August 2013 | pmid = 23903733 | doi = 10.1038/500023a | s2cid = 4318641 | bibcode = 2013Natur.500...23P | doi-access = free }}</ref> For example, female muscle-derived [[stem cells]] have a better muscle regeneration efficiency than male ones.<ref name="Deasy-2007">{{cite journal | vauthors = Deasy BM, Lu A, Tebbets JC, Feduska JM, Schugar RC, Pollett JB, Sun B, Urish KL, Gharaibeh BM, Cao B, Rubin RT, Huard J | display-authors = 6 | title = A role for cell sex in stem cell-mediated skeletal muscle regeneration: female cells have higher muscle regeneration efficiency | journal = The Journal of Cell Biology | volume = 177 | issue = 1 | pages = 73–86 | date = April 2007 | pmid = 17420291 | pmc = 2064113 | doi = 10.1083/jcb.200612094 }}</ref> There are reports of several metabolic differences between male and female cells<ref name="Mittelstrass-2011">{{cite journal | vauthors = Mittelstrass K, Ried JS, Yu Z, Krumsiek J, Gieger C, Prehn C, Roemisch-Margl W, Polonikov A, Peters A, Theis FJ, Meitinger T, Kronenberg F, Weidinger S, Wichmann HE, Suhre K, Wang-Sattler R, Adamski J, Illig T | display-authors = 6 | title = Discovery of sexual dimorphisms in metabolic and genetic biomarkers | journal = PLOS Genetics | volume = 7 | issue = 8 | pages = e1002215 | date = August 2011 | pmid = 21852955 | pmc = 3154959 | doi = 10.1371/journal.pgen.1002215 | veditors = McCarthy MI | doi-access = free }}</ref> and they also respond to [[stress (biology)|stress]] differently.<ref name="Penaloza-2009">{{cite journal | vauthors = Penaloza C, Estevez B, Orlanski S, Sikorska M, Walker R, Smith C, Smith B, Lockshin RA, Zakeri Z | display-authors = 6 | title = Sex of the cell dictates its response: differential gene expression and sensitivity to cell death inducing stress in male and female cells | journal = FASEB Journal | volume = 23 | issue = 6 | pages = 1869–79 | date = June 2009 | pmid = 19190082 | pmc = 2698656 | doi = 10.1096/fj.08-119388 | doi-access = free }}</ref> These differences align with concepts of [[cell autonomous sex identity]], where cells maintain intrinsic sex-based traits regardless of systemic influences.

==Reproductively advantageous==
In theory, larger females are favored by competition for mates, especially in polygamous species. Larger females offer an advantage in fertility, since the physiological demands of reproduction are limiting in females. Hence there is a theoretical expectation that females tend to be larger in species that are monogamous.
Females are larger in many species of [[insect]]s, many [[spider]]s, many [[fish]], many reptiles, [[owl]]s, birds of prey and certain mammals such as the [[spotted hyena]], and baleen whales such as [[blue whale]]. As an example, in some species, females are sedentary, and so males must search for them. Fritz Vollrath and [[Geoff Parker]] argue that this difference in behaviour leads to radically different selection pressures on the two sexes, evidently favouring smaller males.<ref name="Vollrath-1992" /> Cases where the male is larger than the female have been studied as well,<ref name="Vollrath-1992">{{cite journal | vauthors = Vollrath F, Parker GA | year=1992 |title=Sexual dimorphism and distorted sex ratios in spiders |journal=[[Nature (journal)|Nature]] |volume=360 |issue=6400 |pages=156–159 |doi=10.1038/360156a0 |bibcode=1992Natur.360..156V|s2cid=4320130 }}</ref> and require alternative explanations.

One example of this type of sexual size dimorphism is the bat ''[[Myotis nigricans]]'', (black myotis bat) where females are substantially larger than males in terms of body weight, skull measurement, and forearm length.<ref>{{cite journal | vauthors = Bornholdt R, Oliveira LR, Fabián ME | title = Sexual size dimorphism in Myotis nigricans (Schinz, 1821) (Chiroptera: Vespertilionidae) from south Brazil | journal = Brazilian Journal of Biology | volume = 68 | issue = 4 | pages = 897–904 | date = November 2008 | pmid = 19197511 | doi = 10.1590/S1519-69842008000400028 | url = http://www.scielo.br/pdf/bjb/v68n4/28.pdf | doi-access = free }}</ref> The interaction between the sexes and the energy needed to produce viable offspring makes it favorable for females to be larger in this species. Females bear the energetic cost of producing eggs, which is much greater than the cost of making sperm by the males. The fecundity advantage hypothesis states that a larger female is able to produce more offspring and give them more favorable conditions to ensure their survival; this is true for most ectotherms. A larger female can provide parental care for a longer time while the offspring matures. The gestation and lactation periods are fairly long in ''M. nigricans'', the females suckling their offspring until they reach nearly adult size.<ref>{{cite journal | vauthors = Hayssen V, Kunz TH | year=1996 |title=Allometry of litter mass in bats: comparisons with maternal size, wing morphology, and phylogeny |journal=[[Journal of Mammalogy]] |volume=77 |issue=2 |pages=476–490 |jstor=1382823 |doi=10.2307/1382823 |doi-access=free }}</ref> They would not be able to fly and catch prey if they did not compensate for the additional mass of the offspring during this time. Smaller male size may be an adaptation to increase maneuverability and agility, allowing males to compete better with females for food and other resources.

[[File:Cryptopsaras couesii (triplewart seadevil).png|thumb|300px|Female [[triplewart seadevil]], an anglerfish, with male attached near vent (arrow)]]

Some species of [[anglerfish]] also display extreme sexual dimorphism. Females are more typical in appearance to other fish, whereas males are tiny rudimentary creatures with stunted digestive systems. A male must find a female and fuse with her: he then lives parasitically, becoming little more than a sperm-producing body in what amounts to an effectively hermaphrodite composite organism. 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, which clings to her (although males can survive away from females, they generally are not free-living).<ref>{{cite journal | vauthors = Arnqvist G, Jones TM, Elgar MA | title = Insect behaviour: reversal of sex roles in nuptial feeding | journal = Nature | volume = 424 | issue = 6947 | pages = 387 | date = July 2003 | pmid = 12879056 | doi = 10.1038/424387a | url = http://www.ebc.uu.se/zooeko/GoranA/nature2003.pdf | bibcode = 2003Natur.424..387A | s2cid = 4382038 | archive-url = https://web.archive.org/web/20040915020554/http://www.ebc.uu.se/zooeko/GoranA/nature2003.pdf | archive-date = 15 September 2004 }}</ref> This is taken to the logical extreme in the [[Rhizocephala]] crustaceans, like the [[Sacculina]], where the male injects itself into the female's body and becomes nothing more than sperm producing cells, to the point that the superorder used to be mistaken for hermaphroditic.<ref>''[https://books.google.com/books?id=fVjxCAAAQBAJ&pg=PA702 Mechanism of Fertilization: Plants to Humans]'',
edited by Brian Dale</ref>

Some plant species also exhibit dimorphism in which the females are significantly larger than the males, such as in the moss ''[[Dicranum]]''<ref>{{cite book | vauthors = Shaw AJ |year=2000 |chapter=Population ecology, population genetics, and microevolution |pages=379–380 | veditors = Shaw AJ, Goffinet B |title=Bryophyte Biology |location=Cambridge |publisher=Cambridge University Press |isbn=978-0-521-66097-6}}</ref> and the liverwort ''[[Sphaerocarpos]]''.<ref name="Schuster-1984">{{cite book | vauthors = Schuster RM | year=1984 | chapter=Comparative Anatomy and Morphology of the Hepaticae | title=New Manual of Bryology | location=Nichinan, Miyazaki, Japan | publisher=The Hattori botanical Laboratory | volume=2 | page=891 }}</ref> There is some evidence that, in these genera, the dimorphism may be tied to a sex chromosome,<ref name="Schuster-1984" /><ref name="Crum-1980">{{cite book | vauthors = Crum HA, Anderson LE | year=1980 |title=Mosses of Eastern North America |volume=1 |page=196 |location=New York |publisher=Columbia University Press |isbn=978-0-231-04516-2}}</ref> or to chemical signalling from females.<ref>{{cite journal | vauthors = Briggs D |year=1965 |title=Experimental taxonomy of some British species of genus ''Dicranum'' |journal=[[New Phytologist]] |volume=64 |issue=3 |pages=366–386 |doi=10.1111/j.1469-8137.1965.tb07546.x |jstor=2430169|doi-access=free |bibcode=1965NewPh..64..366B }}</ref>

Another complicated example of sexual dimorphism is in ''[[Vespula squamosa]]'', the southern yellowjacket.  In this wasp species, the female workers are the smallest, the male workers are slightly larger, and the female queens are significantly larger than her female workers and male counterparts.{{citation needed|date=February 2018}}

==Evolution==
{{Main|Evolution of sexual reproduction}}
{{See also|Sexual selection|Mate choice}}
[[File:Eccaparadoxides mediterraneus - Murero, Zaragoza - Museo Ciencias Naturales Universidad Zaragoza.jpg|thumb|260 px|Sexual dimorphism in [[Cambrian]] [[trilobite]]s<ref>{{cite journal | vauthors = Dies Alvarez ME, Rushton AW, Gozalo R, Pillola GL, Linan E, Ahlberg P | year = 2010 | title = ''Paradoxides brachyrhachis'' Linnarsson, 1883 versus ''Paradoxides mediterraneus'' Pompeckj, 1901: a problematic determination | url = https://www.researchgate.net/publication/232865261 | journal = GFF | volume = 132 | issue = 2| pages = 95–104 | doi = 10.1080/11035897.2010.481363 | bibcode = 2010GFF...132...95D | s2cid = 129620469 }}</ref>]]
In 1871, [[Charles Darwin]] advanced the [[theory of sexual selection]], which related sexual dimorphism to [[sexual selection]].<ref>{{Cite journal | vauthors = Padian K, Horner JR |date=2014-11-01 |title=Darwin's sexual selection: Understanding his ideas in context |url=https://www.sciencedirect.com/science/article/pii/S1631068314001493 |journal=Comptes Rendus Palevol |language=en |volume=13 |issue=8 |pages=709–715 |doi=10.1016/j.crpv.2014.09.001 |bibcode=2014CRPal..13..709P |issn=1631-0683|url-access=subscription }}</ref>

The first step towards sexual dimorphism is the size differentiation of sperm and eggs ([[anisogamy]]).<ref>{{cite journal | vauthors = Togashi T, Bartelt JL, Yoshimura J, Tainaka K, Cox PA | title = Evolutionary trajectories explain the diversified evolution of isogamy and anisogamy in marine green algae | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 109 | issue = 34 | pages = 13692–7 | date = August 2012 | pmid = 22869736 | pmc = 3427103 | doi = 10.1073/pnas.1203495109 | bibcode = 2012PNAS..10913692T | author-link5 = Paul Alan Cox | doi-access = free }}</ref><ref>{{Cite journal |last1=Hanschen |first1=Erik R. |last2=Herron |first2=Matthew D. |last3=Wiens |first3=John J. |last4=Nozaki |first4=Hisayoshi |last5=Michod |first5=Richard E. |date=September 2018 |title=Multicellularity Drives the Evolution of Sexual Traits |journal=The American Naturalist |language=en |volume=192 |issue=3 |pages=E93–E105 |doi=10.1086/698301 |issn=0003-0147 |pmc=6685534 |pmid=30125231|bibcode=2018ANat..192E..93H }}</ref><ref>{{Cite journal |date=2024 |title=Anisogamy Does Not Always Promote the Evolution of Mating Competition Traits in Males |doi=10.1086/727968 |url=https://www.journals.uchicago.edu/doi/abs/10.1086/727968?journalCode=an |last1=Siljestam |first1=Mattias |last2=Martinossi-Allibert |first2=Ivain |journal=The American Naturalist |volume=203 |issue=2 |pages=230–253 |pmid=38306281 |bibcode=2024ANat..203..230S }}</ref><ref>{{Cite book |last1=Resh |first1=Vincent H. |author-link=Vincent H. Resh |url=https://books.google.com/books?id=Jk0Hym1yF0cC&dq=sexual+dimorphism+encyclopedia+gamete+size&pg=PA917 |title=Encyclopedia of Insects |last2=Cardé |first2=Ring T. |date=2009-07-22 |publisher=Academic Press |isbn=978-0-08-092090-0 |language=en}}</ref>{{Rp|page=917}} [[Anisogamy]] and the usually large number of small male gametes relative to the larger female gametes usually lies in the development of strong [[sperm competition]],<ref>{{cite journal | vauthors = Parker GA | title = Why are there so many tiny sperm? Sperm competition and the maintenance of two sexes | journal = Journal of Theoretical Biology | volume = 96 | issue = 2 | pages = 281–94 | date = May 1982 | pmid = 7121030 | doi = 10.1016/0022-5193(82)90225-9 | bibcode = 1982JThBi..96..281P | s2cid = 29879237 }}</ref><ref>{{cite journal | vauthors = Yang JN | title = Cooperation and the evolution of anisogamy | journal = Journal of Theoretical Biology | volume = 264 | issue = 1 | pages = 24–36 | date = May 2010 | pmid = 20097207 | doi = 10.1016/j.jtbi.2010.01.019 | bibcode = 2010JThBi.264...24Y }}</ref> because small sperm enable organisms to produce a large number of sperm, and make males (or male function of hermaphrodites<ref>{{cite journal | vauthors = Bell G |year=1985 |title=On the function of flowers |journal=[[Proceedings of the Royal Society B: Biological Sciences]] |volume=224 |issue= 1235|pages=223–266 |jstor=36033 |bibcode=1985RSPSB.224..223B |doi=10.1098/rspb.1985.0031|s2cid=84275261 }}</ref>) more redundant.

Volvocine algae have been useful in understanding the evolution of sexual dimorphism<ref>{{cite journal | vauthors = Geng S, De Hoff P, Umen JG | title = Evolution of sexes from an ancestral mating-type specification pathway | journal = PLOS Biology | volume = 12 | issue = 7 | pages = e1001904 | date = July 2014 | pmid = 25003332 | pmc = 4086717 | doi = 10.1371/journal.pbio.1001904 | doi-access = free }}</ref> and species like the beetle ''[[Callosobruchus maculatus|C. maculatus]]'', where the females are larger than the males, are used to study its underlying genetic mechanisms.<ref>{{cite journal | vauthors = Kaufmann P, Wolak ME, Husby A, Immonen E | title = Rapid evolution of sexual size dimorphism facilitated by Y-linked genetic variance | journal = Nature Ecology & Evolution | volume = 5 | issue = 10 | pages = 1394–1402 | date = October 2021 | pmid = 34413504 | doi = 10.1038/s41559-021-01530-z | bibcode = 2021NatEE...5.1394K | s2cid = 237242736 | url = http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-450941 }}</ref>

In many non-monogamous species, the benefit to a male's reproductive fitness of mating with multiple females is large, whereas the benefit to a female's reproductive fitness of mating with multiple males is small or nonexistent.<ref name="Futuyma 2005">{{harvnb|Futuyma|2005|p=330}}</ref> In these species, there is a [[selection pressure]] for whatever traits enable a male to have more matings. The male may therefore come to have different traits from the female.

[[File:Sumatran orangutan family in Toronto Zoo.JPG|thumb|Male (left), offspring (center), and female (right) [[Sumatran orangutan]]s]]
These traits could be ones that allow him to fight off other males for control of territory or a [[Harem (zoology)|harem]], such as large size or weapons;<ref name="Futuyma 2005-2">{{harvnb|Futuyma|2005|p=331}}</ref> or they could be traits that females, for whatever reason, prefer in mates.<ref name="Futuyma 2005-3">{{harvnb|Futuyma|2005|p=332}}</ref> [[Male–male competition]] poses no deep theoretical questions<ref name="Ridley 2004">{{harvnb|Ridley|2004|p=328}}</ref> but [[mate choice]] does.

Females may choose males that appear strong and healthy, thus likely to possess "good [[allele]]s" and give rise to healthy offspring.<ref name="Futuyma 2005-4">{{harvnb|Futuyma|2005|p=335}}</ref> In some species, however, females seem to choose males with traits that do not improve offspring survival rates, and even traits that reduce it (potentially leading to traits like the peacock's tail).<ref name="Ridley 2004" /> Two hypotheses for explaining this fact are the [[sexy son hypothesis]] and the [[handicap principle]].

The sexy son hypothesis states that females may initially choose a trait because it improves the survival of their young, but once this preference has become widespread, females must continue to choose the trait, even if it becomes harmful. Those that do not will have sons that are unattractive to most females (since the preference is widespread) and so receive few matings.<ref name="Ridley 2004-2">{{harvnb|Ridley|2004|p=330}}</ref>

The handicap principle states that a male who survives despite possessing some sort of handicap thus proves that the rest of his genes are "good alleles". If males with "bad alleles" could not survive the handicap, females may evolve to choose males with this sort of handicap; the trait is acting as a hard-to-fake signal of fitness.<ref name="Ridley 2004-3">{{harvnb|Ridley|2004|p=332}}</ref>

== See also ==

{{Div col}}
* [[Bateman's principle]]
* [[List of homologues of the human reproductive system]]
* [[Sex differences in humans]]
* [[Sex differences in human psychology]]
* [[Sexual differentiation]]
* [[Sexual dimorphism measures]]
* [[Sexually dimorphic nucleus]]
* [[Gynandromorphism]]

{{Div col end}}

== References ==
{{Reflist}}

=== Sources ===
{{refbegin}}
* {{cite book | vauthors = Andersson MB |title = Sexual Selection |publisher = [[Princeton University Press]] |year = 1994 |url = https://books.google.com/books?id=lNnHdvzBlTYC |isbn = 978-0-691-00057-2 }}
* {{cite book |isbn=978-0-87893-187-3 | vauthors = Futuyma D |year=2005 |title=Evolution |edition=1st |location=Sunderland, Massachusetts |publisher=Sinauer Associates |url=https://archive.org/details/evolution0000futu }}
* {{cite book | isbn=978-1-4051-0345-9 | vauthors = Ridley M | author-link=Mark Ridley (zoologist) | year=2004 | title=Evolution |edition=3rd | publisher = Blackwell Publishing | location = Malden, Massachusetts }}
{{refend}}

== Further reading ==
{{refbegin}}
* {{cite journal | vauthors = Bonduriansky R | title = The evolution of condition-dependent sexual dimorphism | journal = The American Naturalist | volume = 169 | issue = 1 | pages = 9–19 | date = January 2007 | pmid = 17206580 | doi = 10.1086/510214 | bibcode = 2007ANat..169....9B | s2cid = 17439073 }}
* {{cite journal | vauthors = Figuerola J | title = A comparative study on the evolution of reversed size dimorphism in monogamous waders | journal = [[Biological Journal of the Linnean Society]] | year=1999 | volume= 67 |issue=1 |pages=1–18 | doi = 10.1111/j.1095-8312.1999.tb01926.x | hdl = 10261/44557 | s2cid = 85330510 | hdl-access = free }}
* {{cite book | vauthors = Székely T, Lislevand T, Figuerola J, Fairbairn D, Blanckenhorn W | title = Sex, Size, and Gender Roles: Evolutionary Studies of Sexual Size Dimorphism. | date = 2007 | url = http://www.oupcanada.com/catalog/9780199545582.html | pages = 16–26}} {{refend}}

== External links ==
{{Spoken Wikipedia|Sexual_dimorphism.ogg|date=30 August 2019}}
{{Wiktionary}}
{{Commons category}}
* {{MeshName|Sex+dimorphism}}

{{Clear}}
{{Sex (biology)}}
{{Reproductive physiology|state=collapsed}}
{{Evolution}}
{{Evolutionary psychology}}
{{Authority control}}

{{DEFAULTSORT:Sexual Dimorphism}}
[[Category:Sexual dimorphism| ]]
[[Category:Animal anatomy]]
[[Category:Sexual selection]]
[[Category:Polymorphism (biology)]]
[[Category:Asymmetry]]