Sexual Dimorphism

Sexual dimorphism is the condition where two sexes of the same species exhibit different characteristics beyond differences in their sex organs. [1] [2] This condition occurs in most animals and some plants. Differences may include secondary sex characteristics , size, weight, color, markings, and may also include behavioral and cognitive differences. These differences may be subtle or exaggerated, and may be subject to sexual selection and natural selection . The opposite of dimorphism is monomorphism , which occurs when both biological sexes are phenotypically indistinguishable from each other.


embellishment and dyeing

Mandarin ducks , male (left) and female (right), show dramatic differences between sexes

Common and easily recognized types of dimorphism include ornamentation and coloration, although not always obvious. The difference in coloration of the sexes within a given species is called sexual dimorphism, commonly seen in many species of birds and reptiles. [4] Sexual selection produces hyperbolic dimorphic traits that are primarily used in competition over mates. The increased fitness resulting from ornamentation offsets its cost to suggest or maintain complex evolutionary effects, but costs and evolutionary effects differ from species to species. [5] [6] The cost and implications vary depending on the nature of the ornamentation (such as the color mechanism involved).

Peacocks make distinctive drawings of the principle. The ornate feathers of the peacock , as used in courtship displays , attract peacocks . At first glance one might mistake the peacock and the peahen for completely different species because of the vibrant colors and feathery shape of the male; Peacock is light brown in colour. [7] The peacock’s plumage increases its sensitivity to predators as a barrier to flight, and makes the bird generally distinctive. [7] Similar examples abound, such as in the birds of paradise and the Argus pheasant .

Another example of sexual dimorphism is the nesting of blue breasts . Males are characteristically more yellow than females. It is believed to be derived from ingestion of green lepidopteran larvae, which contain large amounts of the carotenoids lutein and zeaxanthin . [8] This diet also affects sexually dimorphic colors in the human-invisible ultraviolet spectrum. [9] [10] Therefore, male birds, although appearing yellow to humans, actually have a purple plumage that is seen by females. This feather is considered an indicator of the capabilities of the male parent. [11]Perhaps this is a good indicator for women because it shows that they are good at getting a food supply from which to obtain carotenoids. There is a positive correlation between the chromosomes of the tail and breast feathers and the position of the body. [12] Carotenoids play an important role in immune function for many animals, so carotenoid-dependent signals may indicate health.

Frogs make a more typical illustration of the principle. There are two types of dichotomy for frog species: ontogenetic and motile. Ontogenetic frogs are more common and have permanent color changes in either the male or the female. Ranoidea lesueuri is an example of a motile frog in which the males have temporary color changes during the breeding season. [14] Hyperolius ocellatus is an ontogenetic frog with dramatic differences in both color and pattern between the sexes. At sexual maturity, males exhibit a bright green color with white dorsal lines. [15] In contrast, females are rusty red with small silver spots. The bright coloration in the male population attracts females and serves as an apostrophic signal to potential predators.

Females often show a preference for exaggerated male secondary sex characteristics in mate selection. [16] The sexy son hypothesis suggests that females prefer more elaborate males and select against males that are duller in colour, independent of species. [17]

Similar sexual dimorphism and mating preference are also observed in many fish species. For example, male guppies have colored spots and ornamentation while females are usually brown. Female guppies prefer brightly colored males over dull males. [18]

In redlip blennies, only the male fish develops an organ at the anus-urogenital region that produces antimicrobial substances. During parental care, males rub their anus-urogenital areas on the inner surfaces of their nests, thereby protecting their eggs from microbial infection, which is one of the most common causes of death in young fish. [19]


Most flowering plants are bisexual but about 6% of the species have separate males and females (dioecy). [20] In insect-pollinated species, males and females generally look very similar to each other because the plant provides rewards (for example, nectar) that allow pollinators to move on to another similar flower while completing pollination. encourage. The Catasetum orchid is an interesting exception to this rule. Male Catasetum orchids violently associate pollination with Euglocene bee pollinators. The bees then avoid other male flowers, but may move to the female, which looks different from the male. [21]

Various other dichotomous exceptions, such as Loxostylis elata , have clearly distinct sexes with the effect of obtaining the most efficient behavior from pollinators , which then use the most efficient strategy to visit each sex of the flower rather than searching. As for the pollen in the pollen- holding the female flower.

some plants, such as geraniumsWhat is the degree of serial sexual dimorphism in some species of For example, the flowers of such species may present their anthers when they open, then release the exhausted anthers after a day or two, and possibly change their color as the pistil matures; Expert pollinators are very inclined to focus on the precise appearance of the flowers they serve, which saves their time and effort and serves the plant’s interests accordingly. Some such plants go even further and change their appearance again once fertilized, Thereby discouraging further visits from pollinators. This is beneficial to both parties as it protects the developing fruit from damage and saves the pollinator’s efforts from wasting on useless trips. Basically the strategy ensures that pollinators can expect a reward every time they visit the appropriate advertising flower.

Females of the aquatic plant Vallisneria americana have floating flowers attached to a long flower stalk that are fertilized if they approach one of the thousands of free-floating flowers left by a male. [22] Sexual dimorphism is most often associated with wind-pollination in plants, due to selection for efficient pollen dispersal in males versus pollen capture in females, for example Leucadendron rubrum . Sexual dimorphism in plants may also depend on reproductive development. This can be seen in Cannabis sativa , a type of hemp, which has a high photosynthesis rate in males while growing but a higher rate in females once the plant is sexually mature.

Each sexually reproducing species of vascular plant actually has an alternation of generations; The plants we see about ourselves are usually diploid sporophytes, but their progeny are not actually the seeds that people usually recognize as the new generation. The seed is actually the progeny of the haploid generation of microgametophytes (pollen) and megagametophytes (spores in embryo sacs). Accordingly each pollen grain can be seen as a male plant in itself; It produces a sperm cell and differs dramatically from the female plant, the megagametophyte which produces female gametes.


Insects display a wide variety of sexual dimorphism between taxa, including size, ornamentation and colour. [25] The female-biased sexual size dimorphism observed in many taxa evolved despite intense male–male competition for mates. [26] In Osmia rufa , for example, females are larger/wider than males, with males being 8–10 mm in size and females being 10–12 mm in size. [27] In HACKBERRY monarch females are similarly larger than males. [28] The reason for sexual dimorphism is due to the provision size mass, in which females consume more pollen than males. [29]

In some species, there is evidence of male dimorphism, but this appears to be aimed at the distinction of roles. It is seen in the bee species Macrotera portalis with males having a smaller head size, capable of flight, and a larger head form, incapable of flight. [30] Anthidium manicatum also exhibits male-biased sexual dimorphism. The larger size selection in males rather than females in this species may be due to their aggressive territorial behavior and subsequent differential mating success. [31] Another example is Lasioglossum hemichalcium , a species of sweat bee that shows rigid physical dimorphism between male offspring. [32]Not all dimorphisms have significant differences between the sexes. Andrena agillissima is a mining bee where the heads of females are only slightly larger than those of males.

Weapons increase fitness by increasing success in male–male competition in many insect species. [34] Beetle horns in Onthophagus taurus are enlarged growths of the head or thorax that are expressed only in males. There is also distinctive sexual and male dimorphism in the horns of the head in Copris ochus . [35] These structures are impressive because of their exaggerated size. [36] There is a direct correlation between male horn length and body size and higher accessibility to mates and fitness. [36] In other species of beetles, both males and females may have horn-like ornamentation. [35] Generally, insect sexual size dimorphism (SSD) within species increases with body size. [37]

Sexual dimorphism within insects is also exhibited by dichotomy. In the butterfly genera Bicyclus and Junonia , the dimorphic wing pattern evolved because of sex-limited expression, which mediates the intralocus sexual conflict and leads to increased fitness in males. [38] The sexually dimorphic nature of Bicyclus annana is reflected by female selection based on dorsal UV-reflecting eyespot pupae . [39] Common sulfur also displays sexual dichromatism; Males have yellow and iridescent plumage, while female plumage is white and non-iridescent. [40] Naturally selected aberrations in protective female coloration are exhibited in mimetic butterflies.

Spiders and Sexual Cannibalism

Many arachnid groups exhibit sexual dimorphism, [42] but this is widely studied in spiders. For example, in the orb-weaving spider Zygiella x-notata , adult females have larger body sizes than adult males. [43] Size dimorphism suggests an association with sexual cannibalism, [44] predominant in spiders (also found in insects such as the praying mantis). In the shape of the dimorphic wolf spider Tigrossa heluo, food-limited females cannibalize more frequently. [45] Therefore, there is a higher risk of reduced fitness for males due to pre-copulatory cannibalism, which led to male selection of larger females for two reasons: higher fertility and lower rates of cannibalism. [45]Furthermore, female fertility is positively correlated with female body size and the larger female body size is selected for, which is observed in the family Araneidae. All Argiope species, including Argiope bruennichi , use this method. Some males developed ornamentation vague ] that included tying the female to silk, having proportionally longer legs, modifying the female’s web, mating while the female was feeding, or providing a marriage gift in response to sexual cannibalism. Is. [45] Male body size is not subject to selection due to cannibalism in all spider species such as Nephila pilips , but is more prominently selected for less dimorphic species of spiders, which often opt for larger male sizes.[46] In the species Maratus volans , males are known for their distinctive colored fans that attract females during mating. [47]


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 warfare or male paternal care… [size range] from dwarf males to males compared to females. 12 times heavier.” [48]

There are cases where males are significantly larger than females. An example is Laprologus calypterus , a type of cichlid fish. In this fish, males are thought to be 60 times larger than females. The increased size of the male is considered advantageous because the males collect and defend empty snail shells in each of which the female breeds. [49]Males need to be larger and more powerful to collect the largest shells. The body size of the female should remain small as she must lay her eggs inside empty shells in order to breed. If she grows too large, she will not fit into the shell and will be unable to breed. The small body size of the female is also beneficial to her chances of finding an empty shell. Large shells, although preferred by females, are often limited in availability. [50] Therefore, the female is limited by the growth of shell size and can actually change her growth rate according to the availability of shell size. [51]In other words, the male’s ability to collect large shells depends on his size. The bigger the male, the bigger the balls he is able to collect. This then allows the females to grow larger in their brooding nest thereby reducing the size difference between the sexes. Male–male competition in this fish species also selects for larger size in males. There is aggressive competition by males over territory and access to larger shells. The older men win the battle and steal the shells from the competitors. Another example is dragonets, in which males are much larger than females and have longer wings.

There is also sexual dimorphism in bisexual fish. These species are known as sequential hermaphrodites. In fish, reproductive history often includes a sex change from female to male, where there is a strong link between evolution, an individual’s sex, and the mating system that operates within it. [52] In protogynous mating systems where males mate with multiple females, size plays an important role in male reproductive success. [53] Males tend to be larger than females of a comparable age, but it is not clear whether the increase in size is due to an increase in growth at the time of sexual transition or a history of rapid growth in individuals who change sex. is due to. [54]Older males are able to inhibit the growth of females and control environmental resources.

Social organization plays a big role in sex change by fish. It is often observed that fish change its sex when there is a lack of dominant male within the social hierarchy. Women who change sex are often the ones who achieve and preserve the size advantage early in life. In any case, females who change gender to males tend to be larger and often prove to be a good example of dimorphism.

In other cases with fish, males will undergo noticeable changes in body shape, and females will undergo morphological changes that can only be observed inside the body. For example, in sockeye salmon, males develop larger body sizes at maturity, including increased body depth, hump height and snout length. Females experience a slight change in snout length, but the most noticeable difference is a drastic increase in gonad size, which accounts for about 25% of body mass. [55]

Sexual selection for female ornamentation was observed in Gobiusculus flavescens , known as the two-spotted goby. [56] Traditional hypotheses suggest that male–male competition drives selection. However, selection for ornamentation within this species suggests that showy female traits may be selected for through female–female competition or male mate choice. [56] Since carotenoid-based ornamentation suggests mate quality, female two-spotted guppies that develop colorful orange bellies during the breeding season are considered to be friendly to males. [57] Males invest heavily in offspring during incubation, leading to sexual preference for colored females due to the higher egg quality. [57]

amphibians and non-avian reptiles

In amphibians and reptiles, the degree of sexual dimorphism varies widely between taxonomic groups. Sexual dimorphism in amphibians and reptiles may be reflected in any of the following: anatomy; relative length of the tail; relative size of the head; Overall size similar to that of many species of vipers and lizards; coloration in many amphibians, snakes and lizards as well as some turtles; An ornament in the form of many newts and lizards ; The presence of sex-related behavior is common to many lizards; and vocal qualities that are often observed in frogs.

Anole lizards show major size dimorphism with males generally being significantly larger than females. For example, the average male Anolis sagrei was 53.4 mm versus 40 mm in females. [58] The different head sizes in the enol are explained by differences in the estrogen pathway. [59] Sexual dimorphism in lizards is usually attributed to the effects of sexual selection, but other mechanisms, including ecological divergence and fertility selection, provide alternative explanations. [60] The development of color dimorphism in lizards is driven by hormonal changes early in sexual maturation, as in Samodromus algirus , Sceloporus gadovia and S. undulates erythrochilushas been seen in [60]

Male painted dragon lizard, Ctenophorus pictus . Their breeding is clearly evident in colour, but the male color diminishes with age. Male coloration appears to reflect an innate anti-oxidation ability that protects against oxidative DNA damage. [61] Male reproductive color is an indicator for females of the underlying level of oxidative DNA damage (an important component of ageing) in a potential mate. [61]


Sexual dimorphism in birds may be manifested in differences in the size or plumage of the sexes. Sexual size dimorphism varies among taxa with males usually being larger, although this is not always the case, for example birds of prey, hummingbirds and some species of flightless birds. [62] [63] Plumage dimorphism also varies in terms of ornamentation or coloration, although males are usually the more ornate or brightly colored sex. [64] Such differences have been attributed to unequal reproductive contributions of the sexes. [65]This difference creates a stronger female choice as they have a higher risk of producing offspring. In some species, the male’s contribution to reproduction ends at copulation, while in other species the male becomes the main caretaker. Plumage polymorphisms have evolved to reflect these differences and other measures of reproductive fitness, such as body condition [66] or survival. [67] The male phenotype sends signals to females who then select the ‘most fit’ male available.

Female (left) and male (right) common pheasant, indicating that the male is much larger and more colorful than the femaleSome bird species, such as this mute swan, do not exhibit sexual dimorphism through their plumage, and can instead be distinguished by other physical or behavioral characteristics. In general, the male mute swan, or cob, is taller and larger than the female, or pen, and has a thicker neck and a more pronounced ‘knob’ above their bill.

Sexual dimorphism is a product of both genetics and environmental factors. An example of sexual polymorphism determined by environmental conditions exists in red-backed fairies. During the breeding season, red-backed fairies can be classified into three categories: black breeders, brown breeders and brown helpers. [66] These differences arise in response to the state of the bird’s body: if they are healthy they will produce more androgens and thus become black breeders, while less healthy birds produce less androgens and helpers of brown. become. [66] The male’s reproductive success is thus determined by his success during each year’s non-breeding season, with reproductive success varying with each year’s environmental conditions.

Migratory patterns and behavior also influence sexual dimorphism. This aspect also stems from size dimorphism across species. It has been shown that larger males are better able to cope with migration difficulties and thus are more successful in breeding when they reach the breeding site. [68]Many theories and explanations come to mind when looking at it from an evolutionary perspective. If these are the result of every migration and breeding season, then the expected result should be a shift towards a larger male population through sexual selection. Sexual selection is stronger when a factor of environmental selection is also introduced. Environmental selection may favor a smaller chick size if those chicks are born in an area that allows them to grow to a larger size, even though under normal conditions they would not reach this optimum size for migration. . When the environment gives such advantages and disadvantages, the forces of selection are weakened and more morphological weights are given to the environmental forces. Sexual dimorphism can also lead to changes in timing of migration leading to differences in mating success within bird populations. [69]When dimorphism produces such a large variation between sexes and between members of the sexes there can be many evolutionary effects. This timing can also give rise to a speculative event if the variation becomes overly rigid and favorable towards two different outcomes.

Skeletons of female (left) and male (right) black-cased hornbills ( Ceratogymna atrta ) . The difference between the sexes is evident in the casks on the top of their bill. The pair is on display at the Museum of Osteology.

Sexual dimorphism is maintained by natural selection and by the unfavorable pressures of sexual selection. For example, sexual dimorphism in color increases vulnerability to bird species predation by the European sparrow in Denmark. [70] Presumably, the increased sexual dimorphism means that males are brighter and more conspicuous, leading to an increase in prey. [70] In addition, the production of more exaggerated jewelry in men may come at the cost of weakened immune function. [66]As long as the reproductive benefits of the trait due to sexual selection outweigh the costs imposed by natural selection, the trait will spread throughout the population. Reproductive advantages arise in the form of a large number of offspring, while natural selection imposes costs in the form of reduced survival. This means that even though the trait causes males to die earlier, the trait is beneficial as 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 traits that are attractive to females.

Such differences in form and reproductive roles often lead to differences in behavior. As stated earlier, men and women often have different roles in reproduction. The courtship and mating behavior of males and females are largely controlled by hormones throughout a bird’s lifetime. [71] Activated hormones occur during puberty and adulthood and act to ‘activate’ certain behaviors when appropriate, such as territoriality during the breeding season. [71] Organizational hormones occur only during a critical period early in development, either immediately before or after hatching in most birds, and determine behavior patterns for the rest of the bird’s life. [71]Such behavioral differences can lead to disproportionate sensitivity to anthropogenic pressures. [72] In Switzerland, Whinchat females breed in intensively managed grasslands. [72] During the first breeding season, females were more likely to die from mowing. [72] Populations of many birds are often male-skewed and when sex differences in behavior increase this proportion, populations decline rapidly. [72] Furthermore, not all male dimorphic traits are caused by hormones such as testosterone, instead they are a naturally occurring part of growth, for example plumage. [73]Furthermore, the strong hormonal influence on phenotypic differences suggests that the genetic mechanisms and genetic basis of these sexually dimorphic traits may involve transcription factors or cofactors, rather than regulatory sequences. [74]

Sexual dimorphism may also influence differences in parental investment in times of food scarcity. For example, in the blue-footed booby, the female chicks grow faster than the male, resulting in the booby parent producing smaller sexes, males in times of lack of food. This then maximizes reproductive success over the lifetime of the parents. [75] In 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. [76]

Sexual dimorphism may also appear only during the mating season, with some species of birds showing dimorphic traits only in seasonal variation. During the off-breeding season, males of these species become less brightly colored or less exaggerated. [74] This is because the species is more focused on survival than reproduction, leading to a transition to a less ornate state. questionable _

Consequently, there are important implications for the conservation of sexual dimorphism. However, sexual dimorphism is not only found in birds and is thus important for conservation in many animals. Such differences in appearance and behavior can lead to sexual segregation, which is defined as gender differences in the use of space and resources. [77] Most sexual isolation research has been done on ungulates, [77] but such research extends to bats, [78] kangaroos, [79] and birds. [80] Gender-specific conservation plans have also been suggested for species with a clear sexual separation. [78]

The term sesquimorphism (the Latin numeral prefix cesci- means one and a half, hence halfway between mono- (one) and d- ( two)) has been proposed for bird species in which “both sexes have basically the same plumage pattern.” , although the female is clearly visible in her pale or washed out complexion. The cause is recognized separately “. [81] 14 examples include the Cape sparrow ( Passar melanurus ), [81] : 67 rufous sparrow (subspecies P. motinensis motinensis ), [81] : 80 and the saxaul sparrow ( P. ammodendri). ) [81] : 245


In a large proportion of mammal species, males are larger than females. [82] Both genes and hormones influence the formation of the brains of many animals before “birth” (or hatching) and the behavior of adult individuals. Hormones significantly affect the formation of the human brain and brain development at puberty. Nature Reviews NeuroscienceIn 2004 a review found that “because sex chromosomes are easier to manipulate hormone levels than gene expression, the effects of hormones have been studied more extensively, and are much more important than direct actions in the brain.” better understood. of sex chromosome genes.” It concluded that while “differential effects of gonadal secretion appear to be dominant,” the existing body of research “supports the idea that sex differences in neural expression of X and Y genes contribute significantly to sex differences in brain function and disease.” “


Marine mammals show some of the largest sexual size differences of mammals due to environmental factors such as sexual selection and breeding location. [84] [85] The mating system of pinnipeds varies from polygamy to serial monogamy. Pinnipeds are known for early differential growth and maternal investment as the only nutrient for newborn pups is milk provided by the mother. [86] For example, males are significantly larger (about 10% heavier and 2% longer) than females born in sea lion pups. [87] The pattern of differential investment may differ mainly prenatally and postnatally. [88] Mirounga leonina , the southern elephant seal, is one of the most dimorphic mammals. [89]

Sexual dimorphism in elephant seals is associated with the ability of a single male to defend territories and control large groups of females, which is related to polygamous behavior. [90] The large sexual size dimorphism is partly due to sexual selection, but also because females reach reproductive age much earlier than males. In addition, males do not provide parental care for the children and allocate more energy for development. [91] This is supported by secondary growth in males during adolescence. [91]



Sexual dimorphism among humans includes differentiation between the gonads, internal genitalia, external genitalia, breasts, muscles, height, endocrine (hormonal) systems and their physiological and behavioral effects. Human sexual differentiation is primarily influenced, at the gene level, by the presence or absence of the Y-chromosome, which encodes biochemical modifiers for sexual development in males. [92] According to Clark Spencer Larsen, modern-day Homo sapiens show a range of sexual dimorphism, with the average body mass difference between the sexes as high as about 15%. [93]

The average basal metabolic rate is about 6 percent higher in adolescent males than in females and rises to about 10 percent higher after puberty. Women convert more food into fat, while men convert more muscle and expendable circulating energy stores. Aggregate statistics of absolute strength indicate that females have on average 40–60% lower upper body strength and lower body strength 70–75% less than males. [94]Differences in strength relative to body mass are less pronounced in trained individuals. In Olympic weightlifting, men’s records vary from 5.5× body mass in the lowest weight class to 4.2× in the highest weight class, while women’s records vary from 4.4× to 3.8×, a weight-adjusted difference of only 10–20%. , and an absolute difference of about 30% (472 kg versus 333 kg for the unweighted weight classes) (see Olympic Weightlifting Records). A study by analyzing annual world rankings from 1980 to 1996 found that men’s running times were, on average, 11% faster than women’s. [95]

Females are, on average, taller than males in early adolescence, but males on average exceed them in later adolescence and adulthood. In the United States, adult males are on average 9% taller [ 96 ] and 16.5% heavier [97] than adult females . There is no comparative evidence of different levels of sexual selection producing sexual size dimorphism among human populations. [98]

Males typically have larger trachea and branched bronchi, with about 30 percent more lung volume per body mass. On average, men have larger hearts, 10 percent higher red blood cell counts, higher hemoglobin, hence greater oxygen-carrying capacity. They also have high circulating clotting factors (vitamin K, pro thrombin and platelets). These differences lead to faster wound healing and higher peripheral pain tolerance. [99]

Women usually have more white blood cells (stored and circulating), more granulocytes, and B and T lymphocytes. Additionally, they produce more antibodies at a faster rate than men. So they develop less infectious disease and succumb to the short duration. [99] Ethologists argue that females interacting with other females and multiple offspring in social groups may have experienced such traits as a selective advantage. [100] [101] [102] [103] [104]

Notable discussion in the academic literature concerns the potential evolutionary advantages associated with sexual competition (both intrasexual and intersexual) and short- and long-term sexual strategies. [105] According to Daly and Wilson, “the sexes in humans differ more than in monogamous mammals, but much less than in extremely polygamous mammals.” [106]

In the human brain, a difference between the sexes was observed in the transcription of the PCDH11X/Y gene pair unique to Homo sapiens . [107] Sexual differentiation from the undifferentiated stage in the human brain is initiated by testosterone from the fetal testis. Testosterone is converted into estrogen in the brain through the action of the enzyme aromatase. Testosterone acts on several brain regions, including the SDN-POA, to produce masculine brain patterns. [108] The brain of pregnant women carrying male fetuses may be protected from the masculinizing effects of androgens through the action of sex hormone-binding globulin. [109]

The relationship between gender differences in the brain and human behavior is the subject of controversy in psychology and society at large. [110] [111] Many women have a greater proportion of gray matter in the left hemisphere of the brain than men. [112] [113] Men on average have larger brains than women; However, when adjusted for total brain volume, gray matter differences between the sexes are almost non-existent. Thus, the percentage of gray matter appears to be more related to sex than to brain size. [114] [115] Differences in brain physiology between the sexes are not necessarily related to differences in intelligence. Haier et al.A 2004 study found that “men and women with markedly different brain regions achieve similar IQ results, suggesting that there is no singular underlying neuroanatomical structure for general intelligence and different brain types.” Designs can reveal similar intellectual performance”. [116] (See the article Sex and Intelligence for more on this topic.) Strict graph-theoretic analysis of human brain connections has shown [117] that several graph-theoretic parameters (eg, minimum bifurcation width, edge number, extensor ) in Graph Property, Minimum Vertex Cover), women’s structural connectomes are significantly “better” connected than men’s connectomes. It was shown [118]that the graph-theoretic differences were due to gender, not differences in brain volume, where each male in the group had a smaller brain volume, by analyzing the data from 36 females and 36 males. compared the brain volume of each woman in the group.

Sexual dimorphism was also described at the gene level and was shown to extend from the sex chromosomes. In total, approximately 6500 genes have been found to have sex-differential expression in at least one tissue. Many of these genes are not directly linked to reproduction, but to more general biological characteristics. Furthermore, it has been shown that genes with gender specific expression undergo low selection efficiency, leading to a high population frequency of lethal mutations and contributing to the prevalence of many human diseases. 119] [120]

immune function

Sexual dimorphism in immune function is also a common pattern in vertebrates and in many invertebrates. Often, women are more ‘immune’ than men. The underlying causes are explained either by the role of immunosuppressive substances, such as testosterone, or by fundamental differences in male and female life histories. It has been shown that female mammals have higher white blood cell count (WBC), with further association between cell count and longevity in females. There is also a positive covariance between sexual dimorphism in immunity, as measured by a subset of WBCs, and dimorphism in duration of effective reproduction. This is in line with the application of ‘Bateman’s principle’ to immunity, in which women maximize fitness by increasing lifespan through greater investment in immune defenses. [121]


Phenotypic differences between the sexes are evident even in cells cultured from tissues. [122] For example, female muscle-derived stem cells have better muscle regeneration capacity than males. [123] There are reports of several metabolic differences between male and female cells [124] and they also respond differently to stress. [125]

beneficial to reproduction

In theory, larger females are favored by competition for mates, especially in polygamous species. Large females offer an advantage in fertility, as the physiological demands of reproduction are becoming more limited in females. Therefore there is a theoretical expectation that females are larger in species that are monogamous. Females are larger in many species of insects, many spiders, many fish, many reptiles, owls, birds of prey and some mammals such as the spotted hyena, and baleen whales such as the blue whale. As an example, in some species, females are motionless, and therefore males must search for them. Fritz Vollrath and Geoff Parker argue that this difference in behavior leads to radically different selection pressures on the two sexes, apparently in favor of younger males. [126] suchCases where the male is larger than the female have also been studied, [126] and require an alternative explanation.

An example of this type of sexual size dimorphism is the bat Myotis nigricans , (black Myotis bat), where females are significantly larger than males in terms of body weight, skull measurements, and arm length. [127] The interaction between the sexes and the energy required to produce viable offspring enables females in this species to become larger. Females bear the energetic cost of egg production, which is much higher than the cost of producing sperm by males. The fertility benefit hypothesis states that an older female is able to produce more offspring and gives them more favorable conditions to ensure their survival; This is true for most ectotherms. An older female can take care of the parents for a long time until the offspring mature. M. nigricansThe period of gestation and lactation in the U.S. is quite long, with females suckling their offspring until they reach almost adult size . [128] They would not have been able to fly and capture prey if they did not compensate for the extra mass of the offspring during this time. The smaller size of males may have been an adaptation to increase mobility and agility, allowing males to better compete with females for food and other resources.

Some species of anglerfish also exhibit extreme sexual dimorphism. Females are more specialized than other fish, while males are small rudimentary creatures whose digestive system is blocked. A male must find a female and mate with her: she then lives parasitic, with little more than a sperm-producing body, which amounts to an effectively hermaphrodite organism. A similar situation is found in the Zeus water bug Phorticovellia disperata where the female has a glandular area on her back that may serve to feed on a male, which clings to her (note that although males stay away from females). can, they are generally not free-living). [129]This is taken to the logical extreme in crustaceans like Rhizocephala, Sacculina, where the male injects itself into the female body and becomes nothing more than sperm-producing cells, to the point that the superior class used to be mistaken for hermaphrodite For. [130]

Some plant species also exhibit dimorphism in which females are significantly larger than males, such as Moss dicranum [131] and the liverwort Sphaerocarpos . [132] There is some evidence that, in these species, the dimorphism may be tied to a sex chromosome, [132] [133] or to a chemical signal from females. [134]

A more complex example of sexual dimorphism is in Vespula squamosa , the southern yellowjacket. In this wasp species, female workers are the smallest, male workers are slightly larger, and female queens are significantly larger than their female workers and male counterparts.


Size-wise sexual dimorphism is evident in some extinct species such as Velociraptor. Sexual size dimorphism in the case of Velociraptor may be due to two factors: male competition for hunting grounds to attract mates, and/or female competition for nest sites and mates, with males being a scarce reproductive resource. [136]

In 1871, Charles Darwin put forward the theory of sexual selection, which related sexual dimorphism with sexual selection.

It has been proposed that the earliest sexual dimorphism is a difference in the size of the sperm and egg (anisogamy), but the evolutionary significance of sexual dimorphism is more complex than one would suggest. [137] Anisogamy and generally a larger number of smaller male gametes relative to larger female gametes are implicated in the development of stronger sperm competition, [138] [139] because smaller spermatozoa enable organisms to produce larger numbers of spermatozoa, and make the male (or the male act of hermaphrodite [140]) more redundant. It intensifies male competition for mates and promotes the development of other sexual dimorphisms in many species, particularly in vertebrates, including mammals. However, in some species, females may be larger than males, regardless of gametes, and in some species females (usually species in which males invest heavily in parenting and thus are no longer so redundant). not considered) compete for mates. More commonly associated with men.

In many non-monogamous species, the reproductive fitness advantage of a male to mate with multiple females is large, while the reproductive fitness advantage of a female to mate with multiple males is small or nonexistent. [141] In these species, there is selection pressure for whatever traits enable a male to mate more. Therefore males may have different traits from females.

These traits may be those that allow him to fight other males for control of territory or harems, such as larger sizes or weapons; [142] Or they may be traits that women, for whatever reason, prefer in a mate. [143] There are no deep theoretical questions in male-to-male competition [144] but Mate Choice does.

Females can choose males who look strong and healthy, thus are more likely to have “good alleles” and give birth to healthy offspring. [145] In some species, however, females choose males with traits that do not improve the offspring’s survival rate, and even traits that reduce it (potentially peacocks). tail-like symptoms). [144] Two hypotheses to explain this fact are the erotic son hypothesis and the barrier theory.

The sexy son hypothesis states that females may initially choose a trait because it improves the survival of their young, but once this preference becomes widespread, females should continue to choose the trait, even if it be harmful. Those who will not have sons are unattractive to most women (since the preference is widespread) and therefore achieve few orgasms. [146]

Disability theory states that a male who has some kind of handicap survives, thus proving that the rest of his genes are “good alleles”. If males with the “bad allele” cannot avoid the handicap, females may evolve to choose males with this type of disability; The trait is serving as a hard-to-fake sign of fitness.