Sexual dimorphism

Sexual dimorphismis the condition wheresexesof the samespeciesexhibit differentmorphologicalcharacteristics, particularly characteristics not directly involved inreproduction.^[1]The condition occurs in mostdioeciousspecies, which consist of most animals and some plants. Differences may includesecondary sex characteristics,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.^[2]These differences may be subtle or exaggerated and may be subjected tosexual selectionandnatural selection.The opposite of dimorphism ismonomorphism,when both biological sexes arephenotypicallyindistinguishable from each other.^[3]

Overview[edit]

Ornamentation and coloration[edit]

Common and easily identified types of dimorphism consist ofornamentationand 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.^[4]Sexual selectionleads to exaggerated dimorphic traits that are used predominantly in competition over mates.^[5]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.^[6]

Thepeafowlconstitute conspicuous illustrations of the principle. The ornateplumageof peacocks, as used in the courting display, attractspeahens.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.^[7]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.^[7]Similar examples are manifold, such as inbirds of paradise^[8]andargus pheasants.^{[citation needed]}

Another example of sexual dichromatism is that of nestlingblue tits.Males are chromatically more yellow than females. It is believed that this is obtained by the ingestion of greenLepidopteranlarvae, which contain large amounts of thecarotenoids luteinandzeaxanthin.^[9]This diet also affects the sexually dimorphic colours in the human-invisibleultravioletspectrum.^[10]^[11]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.^[12]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.^[13]Carotenoids play an important role inimmune functionfor many animals, so carotenoid dependent signals might indicate health.^[14]

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.Ranoidea lesueuriis an example of a dynamic frog with temporary color changes in males during the breeding season.^[15]Hyperolius ocellatusis 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.^[16]In contrast, the females are rusty red to silver with small spots. The bright coloration in the male population attracts females and is anaposematicsign to potential predators.

Females often show a preference for exaggerated malesecondary sexual characteristicsin mate selection.^[17]Thesexy son hypothesisexplains that females prefer more elaborate males and select against males that are dull in color, independent of the species' vision.^[18]

Similar sexual dimorphism and mating choice are also observed in many fish species. For example, maleguppieshave colorful spots and ornamentations, while females are generally grey. Female guppies prefer brightly colored males to duller males.^[19]^{[page needed]}

Inredlip 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.^[20]

InAustralian ambrosia beetles,males are seen to have smaller bodies than females. This is interesting, as this is a reversal in the trends seen in sexual dimorphism. Females are seen to have larger bodies since they guard the colonies they live in.

Plants[edit]

Mostflowering plantsarehermaphroditicbut approximately 6% of species have separate males and females (dioecy).^[21]Sexual dimorphism is common in dioecious plants^[22]^: 403anddioicousspecies.^[23]^: 71

Males and females ininsect-pollinatedspecies generally look similar to one another because plants provide rewards (e.g.nectar) that encouragepollinatorsto visit another similarflower,completingpollination.Catasetumorchids are one interesting exception to this rule. MaleCatasetumorchidsviolently attachpolliniatoeuglossinebee pollinators. The bees will then avoid other male flowers but may visit the female, which looks different from the males.^[24]

Various other dioecious exceptions, such asLoxostylis alatahave 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]}

Some plants, such as some species ofGeraniumhave what amounts to serial sexual dimorphism. The flowers of such species might, for example, present theirantherson opening, then shed the exhausted anthers after a day or two and perhaps change their colours as well while thepistilmatures; 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]}

Females of the aquatic plantVallisneria americanahave floating flowers attached by a longflower stalkthat are fertilized if they contact one of the thousands of free-floating flowers released by a male.^[25]^{[better source needed]}Sexual dimorphism is most often associated withwind-pollinationin plants due to selection for efficientpollendispersal in males vs pollen capture in females, e.g.Leucadendron rubrum.^[26]

Sexual dimorphism in plants can also be dependent on reproductive development. This can be seen inCannabis 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.^[27]

Every sexually reproducing extant species of the vascular plant has an alternation of generations; the plants we see about us generally arediploid sporophytes,but their offspring are not the seeds that people commonly recognize as the new generation. The seed actually is the offspring of thehaploidgeneration ofmicrogametophytes(pollen) andmegagametophytes(theembryo sacsin theovules). 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]}

Insects[edit]

Insects display a wide variety of sexual dimorphism between taxa including size, ornamentation and coloration.^[28]The female-biased sexual size dimorphism observed in many taxa evolved despite intense male-male competition for mates.^[29]InOsmia rufa,for example, the female is larger/broader than males, with males being 8–10 mm in size and females being 10–12 mm in size.^[30]In thehackberry emperorfemales are similarly larger than males.^[31]The reason for the sexual dimorphism is due to provision size mass, in which females consume more pollen than males.^[32]

In some species, there is evidence of male dimorphism, but it appears to be for distinctions of roles. This is seen in the bee speciesMacrotera portalisin which there is a small-headed morph, capable of flight, and large-headed morph, incapable of flight, for males.^[33]Anthidium manicatumalso 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.^[34]Another example isLasioglossum hemichalceum,which is a species of sweat bee that shows drastic physical dimorphisms between male offspring.^[35]Not all dimorphism has to have a drastic difference between the sexes.Andrena agilissimais a mining bee where the females only have a slightly larger head than the males.^[36]

Weaponry leads to increased fitness by increasing success in male–male competition in many insect species.^[37]The beetle horns inOnthophagus taurusare enlarged growths of the head or thorax expressed only in the males.Copris ochusalso has distinct sexual and male dimorphism in head horns.^[38]Another beetle with a distinct horn-related sexual dimorphism isAllomyrina dichotoma,also known as theJapanese rhinoceros beetle.^[39]These structures are impressive because of the exaggerated sizes.^[40]There is a direct correlation between male horn lengths and body size and higher access to mates and fitness.^[40]In other beetle species, both males and females may have ornamentation such as horns.^[38] Generally, insect sexual size dimorphism (SSD) within species increases with body size.^[41]

Sexual dimorphism within insects is also displayed by dichromatism. In butterfly generaBicyclusandJunonia,dimorphic wing patterns evolved due to sex-limited expression, which mediates theintralocus sexual conflictand leads to increased fitness in males.^[42]The sexual dichromatic nature ofBicyclus anynanais reflected by female selection on the basis of dorsal UV-reflective eyespot pupils.^[43]Thecommon brimstonealso displays sexual dichromatism; males have yellow and iridescent wings, while female wings are white and non-iridescent.^[44]Naturally selected deviation in protective female coloration is displayed in mimetic butterflies.^[45]

Spiders and sexual cannibalism[edit]

Manyarachnidgroups exhibit sexual dimorphism,^[46]but it is most widely studied in the spiders. In the orb-weaving spiderZygiella x-notata,for example, adult females have a larger body size than adult males.^[47]Size dimorphism shows a correlation withsexual cannibalism,^[48]which is prominent in spiders (it is also found in insects such aspraying mantises). In the size dimorphicwolf spiderTigrosa helluo,food-limited females cannibalize more frequently.^[49]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: higherfecundityand lower rates of cannibalism.^[49]In addition, female fecundity is positively correlated with female body size and large female body size is selected for, which is seen in the familyAraneidae.AllArgiopespecies, includingArgiope bruennichi,use this method. Some males evolved ornamentation^[vague]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.^[49]Male body size is not under selection due to cannibalism in all spider species such asNephila pilipes,but is more prominently selected for in less dimorphic species of spiders, which often selects for larger male size.^[50]In the speciesMaratus volans,the males are known for their characteristic colorful fan which attracts the females during mating.^[51]

Fish[edit]

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."^[52]^{[page needed]}

There are cases where males are substantially larger than females. An example isLamprologus 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.^[53]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.^[54]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.^[55]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 thedragonet,in which males are considerably larger than females and possess longer fins.

Sexual dimorphism also occurs in hermaphroditic fish. These species are known assequential hermaphrodites.In fish,reproductive historiesoften 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.^[56]In protogynous mating systems where males dominate mating with many females, size plays a significant role in male reproductive success.^[57]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.^[58]Larger males are able to stifle the growth of females and control environmental resources.^{[citation needed]}

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, insockeye 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 ingonadsize, which accounts for about 25% of body mass.^[59]

Sexual selection was observed for female ornamentation inGobiusculus flavescens,known as two-spotted gobies.^[60]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.^[60]Since carotenoid-based ornamentation suggests mate quality, female two-spotted guppies that develop colorful orange bellies during breeding season are considered favorable to males.^[61]The males invest heavily in offspring during incubation, which leads to the sexual preference in colorful females due to higher egg quality.^[61]

Amphibians and non-avian reptiles[edit]

In amphibians and reptiles, the degree of sexual dimorphism varies widely amongtaxonomic 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 ofvipersandlizards;coloration as in manyamphibians,snakes,and lizards, as well as in someturtles;an ornament as in manynewtsand lizards; the presence of specific sex-related behaviour is common to many lizards; and vocal qualities which are frequently observed infrogs.^{[citation needed]}

Anolelizards show prominent size dimorphism with males typically being significantly larger than females. For instance, the average maleAnolis sagreiwas 53.4 mm vs. 40 mm in females.^[62]Different sizes of the heads in anoles have been explained by differences in the estrogen pathway.^[63]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.^[64]The development of color dimorphism in lizards is induced by hormonal changes at the onset of sexual maturity, as seen inPsamodromus algirus,Sceloporus gadoviae,andS. undulates erythrocheilus.^[64]Sexual dimorphism in size is also seen in frog species likeP. bibronii.

Male painted dragon lizards,Ctenophorus pictus.are brightly conspicuous in their breeding coloration, but male colour declines withaging.Male coloration appears to reflect innate anti-oxidation capacity that protects againstoxidative DNA damage.^[65]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.^[65]

Birds[edit]

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.^[66]^[67]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,hummingbirds,and some species of flightless birds.^[68]^[69]Plumage dimorphism, in the form of ornamentation or coloration, also varies, though males are typically the more ornamented or brightly colored sex.^[70]Such differences have been attributed to the unequal reproductive contributions of the sexes.^[71]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^[72]or survival.^[73]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 ofsexual polymorphismdetermined by environmental conditions exists in thered-backed fairywren.Red-backed fairywren males can be classified into three categories duringbreeding season:black breeders, brown breeders, and brown auxiliaries.^[72]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.^[72]Thereproductive successof 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.

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.^[74]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.^[75]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.^[76]Presumably, increased sexual dimorphism means males are brighter and more conspicuous, leading to increased predation.^[76]Moreover, the production of more exaggerated ornaments in males may come at the cost of suppressed immune function.^[72]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.^[77]Activational hormones occur during puberty and adulthood and serve to 'activate' certain behaviors when appropriate, such as territoriality during breeding season.^[77]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.^[77]Such behavioral differences can cause disproportionate sensitivities to anthropogenic pressures.^[78]Females of the whinchat in Switzerland breed in intensely managed grasslands.^[78]Earlier harvesting of the grasses during the breeding season lead to more female deaths.^[78]Populations of many birds are often male-skewed and when sexual differences in behavior increase this ratio, populations decline at a more rapid rate.^[78]Also not all male dimorphic traits are due to hormones like testosterone, instead they are a naturally occurring part of development, for example plumage.^[79]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.^[80]

Sexual dimorphism may also influence differences in parental investment during times of food scarcity. For example, in theblue-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.^[81]InBlack-tailed GodwitsLimosa limosa limosafemales are also the larger sex, and the growth rates of female chicks are more susceptible to limited environmental conditions.^[82]

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.^[80]This occurs because the species is more focused on survival than on reproduction, causing a shift into a less ornate state.^{[dubious–discuss]}

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 tosexual segregation,defined as sex differences in space and resource use.^[83]Most sexual segregation research has been done on ungulates,^[83]but such research extends tobats,^[84]kangaroos,^[85]and birds.^[86]Sex-specific conservation plans have even been suggested for species with pronounced sexual segregation.^[84]

The term sesquimorphism (the Latinnumeral prefixsesqui- means one-and-one-half, so halfway betweenmono- (one) anddi- (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-outcolour ".^[87]^: 14Examples includeCape sparrow(Passer melanurus),^[87]^: 67rufous sparrow(subspeciesP. motinensis motinensis),^[87]^: 80andsaxaul sparrow(P. ammodendri).^[87]^: 245

Non-avian dinosaurs[edit]

Examining fossils ofnon-avian dinosaursin 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 ofdecompositionandfossilization.Somepaleontologistshave looked for sexual dimorphism among dinosaurs using statistics and comparison to ecologically orphylogeneticallyrelated modern animals.

Apatosaurus and Diplodocus

FemaleApatosaurusandDiplodocushad interconnectedcaudal vertebraethat allowed them to keep their tails elevated to aid in copulation. Discovering that this fusion occurred in only 50% ofApatosaurusandDiplodocusskeletons and 25% ofCamarasaurusskeletons indicated that this is a sexually dimorphic trait.

Theropoda

It has been hypothesized that maletheropodspossessed a retractable penis, a feature similar to modern daycrocodilians.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 caudalchevronsof 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]}

Ornithopoda

Studies of sexual dimorphism inhadrosaurshave generally centered on the distinctivecranial 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 ceratopsianProtoceratops andrewsipossessed sexes that were distinguishable based on frill and nasal prominence size. This is consistent with other knowntetrapodgroups 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.^[88]In addition, many sexually dimorphic traits that may have existed in ceratopsians include soft tissue variations such as coloration ordewlaps,which would be unlikely to have been preserved in the fossil record.

Stegosaurians

A 2015 study on specimens ofHesperosaurusmjosifound 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.^[89]^[90]

Mammals[edit]

In a large proportion of mammal species, males are larger than females. Bothgenesandhormonesaffect the formation of many animal brains before "birth"(orhatching), and also behaviour of adult individuals. Hormones significantly affect human brain formation, and also brain development at puberty. A 2004 review inNature Reviews Neuroscienceobserved 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."^[91]

Pinnipeds[edit]

Marine mammalsshow some of the greatest sexual size differences of mammals, because of sexual selection and environmental factors like breeding location.^[92]The mating system ofpinnipedsvaries from polygamy toserial 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.^[93]For example, the males are significantly larger (about 10% heavier and 2% longer) than the females at birth in sea lion pups.^[94]The pattern of differential investment can be varied principally prenatally and post-natally.^[95]Mirounga leonina,thesouthern elephant seal,is one of the most dimorphic mammals.^[96]

Primates[edit]

Humans[edit]

Pioneer plaque
Male pelvis	Female pelvis
Top:Stylised illustration of humans on thePioneer plaque,showing both male (left) and female (right). Bottom:Comparison between male (left) and female (right)pelvises.

According to Clark Spencer Larsen, modern dayHomo sapiensshow a range of sexual dimorphism, with average body mass between the sexes differing by roughly 15%.^[97]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.^[98]According to Daly and Wilson, "The sexes differ more in human beings than in monogamous mammals, but much less than in extremely polygamous mammals."^[99]

The averagebasal metabolic rateis about 6 percent higher in adolescent males than females and increases to about 10 percent higher after puberty. Females tend to convert more food intofat,while males convert more intomuscleand expendable circulating energy reserves. According to Tim Hewett, director of research in the department of sports medicine atOhio State University Wexner Medical Center,females have, on average, 50–60% of the upper body strength of males, and 80-90% of the lower body strength of males, relative to body size, but females have higherendurancethan males.^[100]

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 kg vs 348 kg for unlimited weight classes; seeOlympic 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'.^[101]

In early adolescence, females are on average taller than males (as females tend togo 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^[102]and 16.5% heavier^[103]than adult females.

Males typically have largertracheaeand branchingbronchi,with about 30 percent greaterlung volumeperbody mass.On average, males have largerhearts,10 percent higherred blood cellcount, higherhemoglobin,hence greater oxygen-carrying capacity. They also have higher circulatingclotting factors(vitamin K,prothrombinandplatelets). These differences lead to faster healing ofwoundsand lower sensitivity to nervepainafter injury.^[104]In males, pain-causing injury to theperipheral nerveoccurs through themicroglia,while in females it occurs through theT cells(except in pregnant women, who follow a male pattern).^[105]

Females typically have morewhite blood cells(stored and circulating), as well as moregranulocytesand B and Tlymphocytes.Additionally, they produce moreantibodiesat a faster rate than males, hence they develop fewerinfectiousdiseases and succumb for shorter periods.^[104]Ethologistsargue that females, interacting with other females and multiple offspring in social groups, have experienced such traits as aselectiveadvantage.^[106]^[107]^[108]^[109]^[110]^{[excessive citations]}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.^[105]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.^[111]Despite differences in physical feeling, both sexes have similar psychological tolerance to (or ability tocopewith and ignore) pain.^[112]

In thehuman brain,a difference between sexes was observed in thetranscriptionof thePCDH11X/Y gene pair unique toHomo sapiens.^[113]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 theSDN-POA,to create the masculinized brain pattern.^[114]The brains of pregnant females carrying male fetuses may be shielded from the masculinizing effects of androgen through the action ofsex hormone-binding globulin.^[115]

The relationship between sex differences in the brain and human behavior is a subject of controversy in psychology and society at large.^[116]^[117]Many females tend to have a higher ratio ofgray matterin the left hemisphere of the brain in comparison to males.^[118]^[119]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.^[120]^[121]Differences in brain physiology between sexes do not necessarily relate to differences in intellect. Haieret 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".^[122](See thesex and intelligencearticle for more on this subject.) Strict graph-theoretical analysis of the human brain connections revealed^[123]that in numerous graph-theoretical parameters (e.g., minimum bipartition width, edge number, theexpander graphproperty, minimumvertex cover), the structural connectome of women are significantly "better" connected than the connectome of men. It was shown^[124]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.^[125]^[126]

Immune function[edit]

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.^[127]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.^[128]Another explanation is that endocrinological differences between the sexes impact the immune system – for example, testosterone acts as an immunosuppressive agent.^[129]

Cells[edit]

Phenotypic differences between sexes are evident even incultured cellsfrom tissues.^[130]For example, female muscle-derivedstem cellshave a better muscle regeneration efficiency than male ones.^[131]There are reports of several metabolic differences between male and female cells^[132]and they also respond tostressdifferently.^[133]

Reproductively advantageous[edit]

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 ofinsects,manyspiders,manyfish,many reptiles,owls,birds of prey and certain mammals such as thespotted hyena,and baleen whales such asblue whale.As an example, in some species, females are sedentary, and so males must search for them. Fritz Vollrath andGeoff Parkerargue that this difference in behaviour leads to radically different selection pressures on the two sexes, evidently favouring smaller males.^[134]Cases where the male is larger than the female have been studied as well,^[134]and require alternative explanations.

One example of this type of sexual size dimorphism is the batMyotis nigricans,(black myotis bat) where females are substantially larger than males in terms of body weight, skull measurement, and forearm length.^[135]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 inM. nigricans,the females suckling their offspring until they reach nearly adult size.^[136]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.

Some species ofanglerfishalso 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 bugPhoreticovelia disparatawhere 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).^[137]This is taken to the logical extreme in theRhizocephalacrustaceans, like theSacculina,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.^[138]

Some plant species also exhibit dimorphism in which the females are significantly larger than the males, such as in the mossDicranum^[139]and the liverwortSphaerocarpos.^[140]There is some evidence that, in these genera, the dimorphism may be tied to a sex chromosome,^[140]^[141]or to chemical signalling from females.^[142]

Another complicated example of sexual dimorphism is inVespula 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]}

Evolution[edit]

In 1871,Charles Darwinadvanced thetheory of sexual selection,which related sexual dimorphism tosexual selection.^[144]

The first step towards sexual dimorphism is the size differentiation of sperm and eggs (anisogamy).^[145]^[146]^[147]^[148]^: 917Anisogamyand the usually large number of small male gametes relative to the larger female gametes usually lies in the development of strongsperm competition,^[149]^[150]because small sperm enable organisms to produce a large number of sperm, and make males (or male function of hermaphrodites^[151]) more redundant.

Volvocine algae have been useful in understanding the evolution of sexual dimorphism^[152]and species like the beetleC. maculatus,where the females are larger than the males, are used to study its underlying genetic mechanisms.^[153]

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.^[154]In these species, there is aselection pressurefor whatever traits enable a male to have more matings. The male may therefore come to have different traits from the female.

These traits could be ones that allow him to fight off other males for control of territory or aharem,such as large size or weapons;^[155]or they could be traits that females, for whatever reason, prefer in mates.^[156]Male–male competitionposes no deep theoretical questions^[157]butmate choicedoes.

Females may choose males that appear strong and healthy, thus likely to possess "goodalleles"and give rise to healthy offspring.^[158]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).^[157]Two hypotheses for explaining this fact are thesexy son hypothesisand thehandicap 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.^[159]

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.^[160]

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v t e Sex
Biological terms	Sexual dimorphism Male Female Sexual differentiation Feminization Virilization Sex-determination system XY ZW XO ZO Temperature-dependent Haplodiploidy Heterogametic sex / Homogametic sex Sex chromosome X chromosome Y chromosome Testis-determining factor Hermaphrodite Sequential hermaphroditism Simultaneous hermaphroditism Intersex parasexuality Sex as a biological variable
Sexual reproduction	Evolution of sexual reproduction Anisogamy Isogamy Germ cell Reproductive system Sex organ Mating Meiosis Gametogenesis Spermatogenesis Oogenesis Gamete spermatozoon ovum Fertilization External Internal Sexual selection Plant reproduction Fungal reproduction Sexual reproduction in animals Sexual intercourse Penile-vaginal intercourse Copulation Hormonal motivation Human reproduction Lordosis behavior Pelvic thrust
Sexuality	Plant sexuality Animal sexuality Human sexuality Mechanics Differentiation Activity
Category

v t e Human physiologyofsexual reproduction
Menstrual cycle	Menarche Menstruation Follicular phase Ovulation Luteal phase
Gametogenesis	Spermatogenesis spermatogonium spermatocyte spermatid sperm Oogenesis oogonium oocyte ootid ovum Germ cell gonocyte gamete
Human sexual activity	Sexual arousal Sexual intercourse Masturbation Erection Penile Clitoral Orgasm Ejaculation Male Female Insemination Fertilization/Fertility Implantation Pregnancy Postpartum period Mechanics of sex Vaginal lubrication
Development of the reproductive system	Sexual differentiation Sexual dimorphism Feminization Virilization Puberty Gonadarche Tanner scale Pubarche Menarche Spermarche Adrenarche Maternal age/Paternal age Menopause
Egg	Ovum Vivipary
Reproductive endocrinology and infertility	Hypothalamic–pituitary–gonadal axis Hypothalamic–pituitary–prolactin axis Andrology Hormone
Breast	Thelarche Development Lactation Breastfeeding
Human reproductive system	Male Female
Diseases and disorders	Anorgasmia Dysorgasmia Dyspareunia Erectile dysfunction Hard flaccid syndrome Premature ejaculation Vaginismus

v t e Evolutionary biology
Introduction Outline Timeline of evolution History of life Index
Evolution	Abiogenesis Adaptation Adaptive radiation Altruism Cheating Reciprocal Baldwin effect Cladistics Coevolution Mutualism Common descent Convergence Divergence Earliest known life forms Evidence of evolution Evolutionary arms race Evolutionary pressure Exaptation Extinction Event Homology Last universal common ancestor Macroevolution Microevolution Mismatch Non-adaptive radiation Origin of life Panspermia Parallel evolution Signalling theory Handicap principle Speciation Species Species complex Taxonomy Unit of selection Gene-centered view of evolution
Population genetics	Artificial selection Biodiversity Evolutionarily stable strategy Fisher's principle Fitness Inclusive Gene flow Genetic drift Kin selection Parental investment Parent–offspring conflict Mutation Population Natural selection Sexual dimorphism Sexual selection Mate choice Social selection Trivers–Willard hypothesis Variation
Development	Canalisation Evolutionary developmental biology Genetic assimilation Inversion Modularity Phenotypic plasticity
Oftaxa	Bacteria Birds origin Brachiopods Molluscs Cephalopods Dinosaurs Fish Fungi Insects butterflies Life Mammals cats canids wolves dogs hyenas dolphins and whales horses Kangaroos primates humans lemurs sea cows Plants pollinator-mediated Reptiles Spiders Tetrapods Viruses
Oforgans	Cell DNA Flagella Eukaryotes symbiogenesis chromosome endomembrane system mitochondria nucleus plastids In animals eye hair auditory ossicle nervous system brain
Ofprocesses	Aging Death Programmed cell death Avian flight Biological complexity Cooperation Color vision in primates Emotion Empathy Ethics Eusociality Immune system Metabolism Monogamy Morality Mosaic evolution Multicellularity Sexual reproduction Gamete differentiation/sexes Life cycles/nuclear phases Mating types Meiosis Sex-determination Snake venom
Tempo and modes	Gradualism/Punctuated equilibrium/Saltationism Micromutation/Macromutation Uniformitarianism/Catastrophism
Speciation	Allopatric Anagenesis Catagenesis Cladogenesis Cospeciation Ecological Hybrid Non-ecological Parapatric Peripatric Reinforcement Sympatric
History	Renaissance and Enlightenment Transmutation of species David Hume Dialogues Concerning Natural Religion Charles Darwin On the Origin of Species History of paleontology Transitional fossil Blending inheritance Mendelian inheritance The eclipse of Darwinism Neo-Darwinism Modern synthesis History of molecular evolution Extended evolutionary synthesis
Philosophy	Darwinism Alternatives Catastrophism Lamarckism Orthogenesis Mutationism Saltationism Structuralism Spandrel Theistic Vitalism Teleology in biology
Related	Biogeography Ecological genetics Evolutionary medicine Group selection Cultural evolution Cultural group selection Dual inheritance theory Hologenome theory of evolution Missing heritability problem Molecular evolution Astrobiology Phylogenetics Tree Polymorphism Protocell Systematics Transgenerational epigenetic inheritance
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