Sexual dimorphism

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For sex differences in humans, see Sex differences in humans. Sexual dimorphism_sentence_0

Sexual dimorphism is the condition where the two sexes of the same species exhibit different characteristics beyond the differences in their sexual organs. Sexual dimorphism_sentence_1

The condition occurs in many animals and some plants. Sexual dimorphism_sentence_2

Differences may include secondary sex characteristics, size, weight, colour, markings, and may also include behavioral and cognitive differences. Sexual dimorphism_sentence_3

These differences may be subtle or exaggerated, and may be subjected to sexual selection and natural selection. Sexual dimorphism_sentence_4

The opposite of dimorphism is monomorphism. Sexual dimorphism_sentence_5

Overview Sexual dimorphism_section_0

Ornamentation and coloration Sexual dimorphism_section_1

Common and easily identified types of dimorphism consist of ornamentation and coloration, though not always apparent. Sexual dimorphism_sentence_6

A difference in coloration of sexes within a given species is called sexual dichromatism, which is commonly seen in many species of birds and reptiles. Sexual dimorphism_sentence_7

Sexual selection leads to the exaggerated dimorphic traits that are used predominantly in competition over mates. Sexual dimorphism_sentence_8

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. Sexual dimorphism_sentence_9

The costs and implications differ depending on the nature of the ornamentation (such as the colour mechanism involved). Sexual dimorphism_sentence_10

The peafowl constitute conspicuous illustrations of the principle. Sexual dimorphism_sentence_11

The ornate plumage of peacocks, as used in the courting display, attracts peahens. Sexual dimorphism_sentence_12

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 being of a subdued brown coloration. Sexual dimorphism_sentence_13

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. Sexual dimorphism_sentence_14

Similar examples are manifold, such as in birds of paradise and argus pheasants. Sexual dimorphism_sentence_15

Another example of sexual dichromatism is that of the nestling blue tits. Sexual dimorphism_sentence_16

Males are chromatically more yellow than females. Sexual dimorphism_sentence_17

It is believed that this is obtained by the ingestion of green Lepidopteran larvae, which contain large amounts of the carotenoids lutein and zeaxanthin. Sexual dimorphism_sentence_18

This diet also affects the sexually dimorphic colours in the human-invisible ultraviolet spectrum. Sexual dimorphism_sentence_19

Hence, the male birds, although appearing yellow to humans actually have a violet-tinted plumage that is seen by females. Sexual dimorphism_sentence_20

This plumage is thought to be an indicator of male parental abilities. Sexual dimorphism_sentence_21

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. Sexual dimorphism_sentence_22

There is a positive correlation between the chromas of the tail and breast feathers and body condition. Sexual dimorphism_sentence_23

Carotenoids play an important role in immune function for many animals, so carotenoid dependent signals might indicate health. Sexual dimorphism_sentence_24

Frogs constitute another conspicuous illustration of the principle. Sexual dimorphism_sentence_25

There are two types of dichromatism for frog species: ontogenetic and dynamic. Sexual dimorphism_sentence_26

Ontogenetic frogs are more common and have permanent color changes in males or females. Sexual dimorphism_sentence_27

Ranoidea lesueuri is an example of a dynamic frog that has temporary color changes in males during breeding season. Sexual dimorphism_sentence_28

Hyperolius ocellatus is an ontogenetic frog with dramatic differences in both color and pattern between the sexes. Sexual dimorphism_sentence_29

At sexual maturity, the males display a bright green with white dorsolateral lines. Sexual dimorphism_sentence_30

In contrast, the females are rusty red to silver with small spots. Sexual dimorphism_sentence_31

The bright coloration in the male population serves to attract females and as an aposematic sign to potential predators. Sexual dimorphism_sentence_32

Females often show a preference for exaggerated male secondary sexual characteristics in mate selection. Sexual dimorphism_sentence_33

The sexy son hypothesis explains that females prefer more elaborate males and select against males that are dull in color, independent of the species' vision. Sexual dimorphism_sentence_34

Similar sexual dimorphism and mating choice are also observed in many fish species. Sexual dimorphism_sentence_35

For example, male guppies have colorful spots and ornamentations while females are generally grey in color. Sexual dimorphism_sentence_36

Female guppies prefer brightly colored males to duller males. Sexual dimorphism_sentence_37

Physiological differentiation Sexual dimorphism_section_2

In redlip blennies, only the male fish develops an organ at the anal-urogenital region that produces antimicrobial substances. Sexual dimorphism_sentence_38

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. Sexual dimorphism_sentence_39

Plants Sexual dimorphism_section_3

Most flowering plants are hermaphroditic but approximately 6% of species have separate males and females (dioecy). Sexual dimorphism_sentence_40

Males and females in insect-pollinated species generally look similar to one another because plants provide rewards (e.g. nectar) that encourage pollinators to visit another similar flower, completing pollination. Sexual dimorphism_sentence_41

Catasetum orchids are one interesting exception to this rule. Sexual dimorphism_sentence_42

Male Catasetum orchids violently attach pollinia to euglossine bee pollinators. Sexual dimorphism_sentence_43

The bees will then avoid other male flowers but may visit the female, which look different from the males. Sexual dimorphism_sentence_44

Various other dioecious exceptions, such as Loxostylis alata have visibly different genders, with the effect of eliciting the most efficient behaviour 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. Sexual dimorphism_sentence_45

Some plants, such as some species of Geranium have what amounts to serial sexual dimorphism. Sexual dimorphism_sentence_46

The flowers of such species might for example present their anthers on opening, then shed the exhausted anthers after a day or two and perhaps change their colours as well while the pistil matures; specialist pollinators are very much inclined to concentrate on the exact appearance of the flowers they serve, which saves their time and effort and serves the interests of the plant accordingly. Sexual dimorphism_sentence_47

Some such plants go even further and change their appearance again once they have been fertilised, thereby discouraging further visits from pollinators. Sexual dimorphism_sentence_48

This is advantageous to both parties because it avoids damage to the developing fruit and avoids wasting the pollinator's effort on unrewarding visits. Sexual dimorphism_sentence_49

In effect the strategy ensures that the pollinators can expect a reward every time they visit an appropriately advertising flower. Sexual dimorphism_sentence_50

Females of the aquatic plant Vallisneria americana have floating flowers attached by a long flower stalk that are fertilized if they contact one of the thousands of free floating flowers released by a male. Sexual dimorphism_sentence_51

Sexual dimorphism is most often associated with wind-pollination in plants due to selection for efficient pollen dispersal in males vs pollen capture in females, e.g. Leucadendron rubrum. Sexual dimorphism_sentence_52

Sexual dimorphism in plants can also be dependent on reproductive development. Sexual dimorphism_sentence_53

This can be seen in Cannabis sativa, a type of hemp, which have higher photosynthesis rates in males while growing but higher rates in females once the plants become sexually mature. Sexual dimorphism_sentence_54

Every sexually reproducing extant species of vascular plant actually has an alternation of generations; the plants we see about us generally are diploid sporophytes, but their offspring really are not the seeds that people commonly recognise as the new generation. Sexual dimorphism_sentence_55

The seed actually is the offspring of the haploid generation of microgametophytes (pollen) and megagametophytes (the embryo sacs in the ovules). Sexual dimorphism_sentence_56

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. Sexual dimorphism_sentence_57

Insects Sexual dimorphism_section_4

Insects display a wide variety of sexual dimorphism between taxa including size, ornamentation and coloration. Sexual dimorphism_sentence_58

The female-biased sexual size dimorphism observed in many taxa evolved despite intense male-male competition for mates. Sexual dimorphism_sentence_59

In Osmia 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. Sexual dimorphism_sentence_60

In the hackberry emperor females are similarly larger than males. Sexual dimorphism_sentence_61

The reason for the sexual dimorphism is due to provision size mass, in which females consume more pollen than males. Sexual dimorphism_sentence_62

In some species, there is evidence of male dimorphism, but it appears to be for the purpose of distinctions of roles. Sexual dimorphism_sentence_63

This is seen in the bee species Macrotera portalis in which there is a small-headed morph, capable of flight, and large-headed morph, incapable of flight, for males. Sexual dimorphism_sentence_64

Anthidium manicatum also displays male-biased sexual dimorphism. Sexual dimorphism_sentence_65

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. Sexual dimorphism_sentence_66

Another example is Lasioglossum hemichalceum, which is a species of sweat bee that shows drastic physical dimorphisms between male offspring. Sexual dimorphism_sentence_67

Not all dimorphism has to have a drastic difference between the sexes. Sexual dimorphism_sentence_68

Andrena agilissima is a mining bee where the females only have a slightly larger head than the males. Sexual dimorphism_sentence_69

Weaponry leads to increased fitness by increasing success in male-male competition in many insect species. Sexual dimorphism_sentence_70

The beetle horns in Onthophagus taurus are enlarged growths of the head or thorax expressed only in the males. Sexual dimorphism_sentence_71

Copris ochus also has distinct sexual and male dimorphism in head horns. Sexual dimorphism_sentence_72

These structures are impressive because of the exaggerated sizes. Sexual dimorphism_sentence_73

There is a direct correlation between male horn lengths and body size and higher access to mates and fitness. Sexual dimorphism_sentence_74

In other beetle species, both males and females may have ornamentation such as horns. Sexual dimorphism_sentence_75

Generally, insect sexual size dimorphism (SSD) within species increases with body size. Sexual dimorphism_sentence_76

Sexual dimorphism within insects is also displayed by dichromatism. Sexual dimorphism_sentence_77

In butterfly genera Bicyclus and Junonia, dimorphic wing patterns evolved due to sex-limited expression, which mediates the intralocus sexual conflict and leads to increased fitness in males. Sexual dimorphism_sentence_78

The sexual dichromatic nature of Bicyclus anynana is reflected by female selection on the basis of dorsal UV-reflective eyespot pupils. Sexual dimorphism_sentence_79

The common brimstone also displays sexual dichromatism; males have yellow and iridescent wings, while female wings are white and non-iridescent. Sexual dimorphism_sentence_80

Naturally selected deviation in protective female coloration is displayed in mimetic butterflies. Sexual dimorphism_sentence_81

Spiders and sexual cannibalism Sexual dimorphism_section_5

Many arachnid groups exhibit sexual dimorphism, but it is most widely studied in the spiders. Sexual dimorphism_sentence_82

In the orb-weaving spider Zygiella x-notata, for example, adult females have a larger body size than adult males. Sexual dimorphism_sentence_83

Size dimorphism shows a correlation with sexual cannibalism, which is prominent in spiders (it is also found in insects such as praying mantises). Sexual dimorphism_sentence_84

In the size dimorphic wolf spider Tigrosa Helluo, food-limited females cannibalize more frequently. Sexual dimorphism_sentence_85

Therefore, there is a high risk of low fitness for males due to pre-copulatory cannibalism, which led to male selection of larger females for two reasons: higher fecundity and lower rates of cannibalism. Sexual dimorphism_sentence_86

In addition, female fecundity is positively correlated with female body size and large female body size is selected for, which is seen in the family Araneidae. Sexual dimorphism_sentence_87

All Argiope species, including Argiope bruennichi, use this method. Sexual dimorphism_sentence_88

Some males evolved ornamentation 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. Sexual dimorphism_sentence_89

Male body size is not under selection due to cannibalism in all spider species such as Nephila pilipes, but is more prominently selected for in less dimorphic species of spiders, which often selects for larger male size. Sexual dimorphism_sentence_90

Fish Sexual dimorphism_section_6

Ray finned fish are an ancient and diverse class, with the widest degree of sexual dimorphism of any animal class. Sexual dimorphism_sentence_91

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." Sexual dimorphism_sentence_92

There are cases where males are substantially larger than females. Sexual dimorphism_sentence_93

An example is Lamprologus callipterus, a type of cichlid fish. Sexual dimorphism_sentence_94

In this fish, the males are characterized as being up to 60 times larger than the females. Sexual dimorphism_sentence_95

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. Sexual dimorphism_sentence_96

Males must be larger and more powerful in order to collect the largest shells. Sexual dimorphism_sentence_97

The female's body size must remain small because in order for her to breed, she must lay her eggs inside the empty shells. Sexual dimorphism_sentence_98

If she grows too large, she will not fit in the shells and will be unable to breed. Sexual dimorphism_sentence_99

The female's small body size is also likely beneficial to her chances of finding an unoccupied shell. Sexual dimorphism_sentence_100

Larger shells, although preferred by females, are often limited in availability. Sexual dimorphism_sentence_101

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. Sexual dimorphism_sentence_102

In other words, the male's ability to collect large shells depends on his size. Sexual dimorphism_sentence_103

The larger the male, the larger the shells he is able to collect. Sexual dimorphism_sentence_104

This then allows for females to be larger in his brooding nest which makes the difference between the sizes of the sexes less substantial. Sexual dimorphism_sentence_105

Male-male competition in this fish species also selects for large size in males. Sexual dimorphism_sentence_106

There is aggressive competition by males over territory and access to larger shells. Sexual dimorphism_sentence_107

Large males win fights and steal shells from competitors. Sexual dimorphism_sentence_108

Another example is the dragonet, in which males are considerably larger than females and possess longer fins. Sexual dimorphism_sentence_109

Sexual dimorphism also occurs in hermaphroditic fish. Sexual dimorphism_sentence_110

These species are known as sequential hermaphrodites. Sexual dimorphism_sentence_111

In fish, reproductive histories often include the sex-change from female to male where there is a strong connection between growth, the sex of an individual, and the mating system it operates within. Sexual dimorphism_sentence_112

In protogynous mating systems where males dominate mating with many females, size plays a significant role in male reproductive success. Sexual dimorphism_sentence_113

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. Sexual dimorphism_sentence_114

Larger males are able to stifle the growth of females and control environmental resources. Sexual dimorphism_sentence_115

Social organization plays a large role in the changing of sex by the fish. Sexual dimorphism_sentence_116

It is often seen that a fish will change its sex when there is a lack of dominant male within the social hierarchy. Sexual dimorphism_sentence_117

The females that change sex are often those who attain and preserve an initial size advantage early in life. Sexual dimorphism_sentence_118

In either case, females which change sex to males are larger and often prove to be a good example of dimorphism. Sexual dimorphism_sentence_119

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. Sexual dimorphism_sentence_120

For example, in sockeye salmon, males develop larger body size at maturity, including an increase in body depth, hump height, and snout length. Sexual dimorphism_sentence_121

Females experience minor changes in snout length, but the most noticeable difference is the huge increase in gonad size, which accounts for about 25% of body mass. Sexual dimorphism_sentence_122

Sexual selection was observed for female ornamentation in Gobiusculus flavescens, known as two-spotted gobies. Sexual dimorphism_sentence_123

Traditional hypotheses suggest that male-male competition drives selection. Sexual dimorphism_sentence_124

However, selection for ornamentation within this species suggests that showy female traits can be selected through either female-female competition or male mate choice. Sexual dimorphism_sentence_125

Since carotenoid-based ornamentation suggests mate quality, female two-spotted guppies that develop colorful orange bellies during the breeding season are considered favorable to males. Sexual dimorphism_sentence_126

The males invest heavily in offspring during the incubation, which leads to the sexual preference in colorful females due to higher egg quality. Sexual dimorphism_sentence_127

Amphibians and non-avian reptiles Sexual dimorphism_section_7

In amphibians and reptiles, the degree of sexual dimorphism varies widely among taxonomic groups. Sexual dimorphism_sentence_128

The sexual dimorphism in amphibians and reptiles may be reflected in any of the following: anatomy; relative length of tail; relative size of head; overall size as in many species of vipers and lizards; coloration as in many amphibians, snakes, and lizards, as well as in some turtles; an ornament as in many newts and lizards; the presence of specific sex-related behaviour is common to many lizards; and vocal qualities which are frequently observed in frogs. Sexual dimorphism_sentence_129

Anole lizards show prominent size dimorphism with males typically being significantly larger than females. Sexual dimorphism_sentence_130

For instance, the average male Anolis sagrei was 53.4 mm vs. 40 mm in females. Sexual dimorphism_sentence_131

Different sizes of the heads in anoles have been explained by differences in the estrogen pathway. Sexual dimorphism_sentence_132

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. Sexual dimorphism_sentence_133

The development of color dimorphism in lizards is induced by hormonal changes at the onset of sexual maturity, as seen in Psamodromus algirus, Sceloporus gadoviae, and S. undulates erythrocheilus. Sexual dimorphism_sentence_134

Male painted dragon lizards, Ctenophorus pictus. Sexual dimorphism_sentence_135

are brightly conspicuous in their breeding coloration, but male colour declines with aging. Sexual dimorphism_sentence_136

Male coloration appears to reflect innate anti-oxidation capacity that protects against oxidative DNA damage. Sexual dimorphism_sentence_137

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. Sexual dimorphism_sentence_138

Birds Sexual dimorphism_section_8

Sexual dimorphism in birds can be manifested in size or plumage differences between the sexes. Sexual dimorphism_sentence_139

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. Sexual dimorphism_sentence_140

Plumage dimorphism, in the form of ornamentation or coloration, also varies, though males are typically the more ornamented or brightly colored sex. Sexual dimorphism_sentence_141

Such differences have been attributed to the unequal reproductive contributions of the sexes. Sexual dimorphism_sentence_142

This difference produces a stronger female choice since they have more risk in producing offspring. Sexual dimorphism_sentence_143

In some species, the male's contribution to reproduction ends at copulation, while in other species the male becomes the main caregiver. Sexual dimorphism_sentence_144

Plumage polymorphisms have evolved to reflect these differences and other measures of reproductive fitness, such as body condition or survival. Sexual dimorphism_sentence_145

The male phenotype sends signals to females who then choose the 'fittest' available male. Sexual dimorphism_sentence_146

Sexual dimorphism is a product of both genetics and environmental factors. Sexual dimorphism_sentence_147

An example of sexual polymorphism determined by environmental conditions exists in the red-backed fairywren. Sexual dimorphism_sentence_148

Red-backed fairywren males can be classified into three categories during breeding season: black breeders, brown breeders, and brown auxiliaries. Sexual dimorphism_sentence_149

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. Sexual dimorphism_sentence_150

The reproductive success of the male is thus determined by his success during each year's non-breeding season, causing reproductive success to vary with each year's environmental conditions. Sexual dimorphism_sentence_151

Migratory patterns and behaviors also influence sexual dimorphisms. Sexual dimorphism_sentence_152

This aspect also stems back to the size dimorphism in species. Sexual dimorphism_sentence_153

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. Sexual dimorphism_sentence_154

When viewing this in an evolutionary standpoint many theories and explanations come to consideration. Sexual dimorphism_sentence_155

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 dimorphism_sentence_156

Sexual selection is strong when the factor of environmental selection is also introduced. Sexual dimorphism_sentence_157

The 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. Sexual dimorphism_sentence_158

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. Sexual dimorphism_sentence_159

The sexual dimorphism could also produce a change in timing of migration leading to differences in mating success within the bird population. Sexual dimorphism_sentence_160

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. Sexual dimorphism_sentence_161

This timing could even lead to a speciation phenomenon if the variation becomes strongly drastic and favorable towards two different outcomes. Sexual dimorphism_sentence_162

Sexual dimorphism is maintained by the counteracting pressures of natural selection and sexual selection. Sexual dimorphism_sentence_163

For example, sexual dimorphism in coloration increases the vulnerability of bird species to predation by European sparrowhawks in Denmark. Sexual dimorphism_sentence_164

Presumably, increased sexual dimorphism means males are brighter and more conspicuous, leading to increased predation. Sexual dimorphism_sentence_165

Moreover, the production of more exaggerated ornaments in males may come at the cost of suppressed immune function. Sexual dimorphism_sentence_166

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. Sexual dimorphism_sentence_167

Reproductive benefits arise in the form of a larger number of offspring, while natural selection imposes costs in the form of reduced survival. Sexual dimorphism_sentence_168

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. Sexual dimorphism_sentence_169

This balance keeps the dimorphism alive in these species and ensures that the next generation of successful males will also display these traits that are attractive to the females. Sexual dimorphism_sentence_170

Such differences in form and reproductive roles often cause differences in behavior. Sexual dimorphism_sentence_171

As previously stated, males and females often have different roles in reproduction. Sexual dimorphism_sentence_172

The courtship and mating behavior of males and females are regulated largely by hormones throughout a bird's lifetime. Sexual dimorphism_sentence_173

Activational hormones occur during puberty and adulthood and serve to 'activate' certain behaviors when appropriate, such as territoriality during breeding season. Sexual dimorphism_sentence_174

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. Sexual dimorphism_sentence_175

Such behavioral differences can cause disproportionate sensitivities to anthropogenic pressures. Sexual dimorphism_sentence_176

Females of the whinchat in Switzerland breed in intensely managed grasslands. Sexual dimorphism_sentence_177

Earlier harvesting of the grasses during the breeding season lead to more female deaths. Sexual dimorphism_sentence_178

Populations of many birds are often male-skewed and when sexual differences in behavior increase this ratio, populations decline at a more rapid rate. Sexual dimorphism_sentence_179

Also not all male dimorphic traits are due to hormones like testosterone, instead they are a naturally occurring part of development, for example plumage. Sexual dimorphism_sentence_180

Sexual dimorphism may also influence differences in parental investment during times of food scarcity. Sexual dimorphism_sentence_181

For example, in the blue-footed booby, the female chicks grow faster than the males, resulting in booby parents producing the smaller sex, the males, during times of food shortage. Sexual dimorphism_sentence_182

This then results in the maximization of parental lifetime reproductive success. Sexual dimorphism_sentence_183

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. Sexual dimorphism_sentence_184

Sexual dimorphism may also only appear during mating season, some species of birds only show dimorphic traits in seasonal variation. Sexual dimorphism_sentence_185

The males of these species will molt into a less bright or less exaggerated color during the off breeding season. Sexual dimorphism_sentence_186

This occurs because the species is more focused on survival than reproduction, causing a shift into a less ornate state. Sexual dimorphism_sentence_187

Consequently, sexual dimorphism has important ramifications for conservation. Sexual dimorphism_sentence_188

However, sexual dimorphism is not only found in birds and is thus important to the conservation of many animals. Sexual dimorphism_sentence_189

Such differences in form and behavior can lead to sexual segregation, defined as sex differences in space and resource use. Sexual dimorphism_sentence_190

Most sexual segregation research has been done on ungulates, but such research extends to bats, kangaroos, and birds. Sexual dimorphism_sentence_191

Sex-specific conservation plans have even been suggested for species with pronounced sexual segregation. Sexual dimorphism_sentence_192

The term sesquimorphism (the Latin numeral prefix sesqui- means one-and-one-half, so halfway between mono- (one) and di- (two)) has been proposed for bird species in which "both sexes have basically the same plumage pattern, though the female is clearly distinguishable by reason of her paler or washed-out colour". Sexual dimorphism_sentence_193

Examples include Cape sparrow (Passer melanurus), rufous sparrow (subspecies P. motinensis motinensis), and saxaul sparrow (P. ammodendri). Sexual dimorphism_sentence_194

Mammals Sexual dimorphism_section_9

In a large proportion of mammal species, males are larger than females. Sexual dimorphism_sentence_195

Both genes and hormones affect the formation of many animal brains before "birth" (or hatching), and also behaviour of adult individuals. Sexual dimorphism_sentence_196

Hormones significantly affect human brain formation, and also brain development at puberty. Sexual dimorphism_sentence_197

A 2004 review in Nature Reviews Neuroscience observed that "because it is easier to manipulate hormone levels than the expression of sex chromosome genes, the effects of hormones have been studied much more extensively, and are much better understood, than the direct actions in the brain of sex chromosome genes." Sexual dimorphism_sentence_198

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." Sexual dimorphism_sentence_199

Pinnipeds Sexual dimorphism_section_10

Marine mammals show some of the greatest sexual size differences of mammals, because of sexual selection and environmental factors like breeding location. Sexual dimorphism_sentence_200

The mating system of pinnipeds varies from polygamy to serial monogamy. Sexual dimorphism_sentence_201

Pinnipeds are known for early differential growth and maternal investment since the only nutrients for newborn pups is the milk provided by the mother. Sexual dimorphism_sentence_202

For example, the males are significantly larger (about 10% heavier and 2% longer) than the females at birth in sea lion pups. Sexual dimorphism_sentence_203

The pattern of differential investment can be varied principally prenatally and post-natally. Sexual dimorphism_sentence_204

Mirounga leonina, the southern elephant seal, is one of the most dimorphic mammals. Sexual dimorphism_sentence_205

Sexual dimorphism in elephant seals is associated with the ability of a male to defend territories and control large groups of females, which correlates with polygynic behavior. Sexual dimorphism_sentence_206

The large sexual size dimorphism is partially due to sexual selection, but also because females reach reproductive age much earlier than males. Sexual dimorphism_sentence_207

In addition the males do not provide parental care for the young and allocate more energy to growth. Sexual dimorphism_sentence_208

This is supported by the secondary growth spurt in males during adolescent years. Sexual dimorphism_sentence_209

Primates Sexual dimorphism_section_11

Main article: Sexual dimorphism in non-human primates Sexual dimorphism_sentence_210

Humans Sexual dimorphism_section_12

Main articles: Sex differences in humans and Sex differences in human psychology Sexual dimorphism_sentence_211

Sexual dimorphism_table_general_0

Sexual dimorphism_cell_0_1_0 Sexual dimorphism_cell_0_1_1
Top: Stylised illustration of humans on the Pioneer plaque, showing both male (left) and female (right).

Bottom: Comparison between male (left) and female (right) pelvises.Sexual dimorphism_cell_0_2_0

In humans, sex is determined by five factors present at birth: the presence or absence of a Y chromosome, the type of gonads, the sex hormones, the internal reproductive anatomy (such as the uterus in females), and the external genitalia. Sexual dimorphism_sentence_212

Generally, the five factors are either all male or all female. Sexual dimorphism_sentence_213

Sexual ambiguity is rare in humans, but wherein such ambiguity does occur, the individual is biologically classified as intersex. Sexual dimorphism_sentence_214

Sexual dimorphism among humans includes differentiation among gonads, internal genitals, external genitals, breasts, muscle mass, height, the endocrine (hormonal) systems and their physiological and behavioral effects. Sexual dimorphism_sentence_215

Human sexual differentiation is effected primarily at the gene level, by the presence or absence of a Y-chromosome, which encodes biochemical modifiers for sexual development in males. Sexual dimorphism_sentence_216

According to Clark Spencer Larsen, modern day Homo sapiens show a range of sexual dimorphism, with average body mass difference between the sexes being roughly equal to 15%. Sexual dimorphism_sentence_217

The average basal metabolic rate is about 6 percent higher in adolescent males than females and increases to about 10 percent higher after puberty. Sexual dimorphism_sentence_218

Females tend to convert more food into fat, while males convert more into muscle and expendable circulating energy reserves. Sexual dimorphism_sentence_219

Aggregated data of absolute strength indicates that females have, on average, 40–60% the upper body strength of males, and 70–75% the lower body strength. Sexual dimorphism_sentence_220

The difference in strength relative to body mass is less pronounced in trained individuals. Sexual dimorphism_sentence_221

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. 472 kg vs 333 kg for unlimited weight classes)(see Olympic weightlifting records). Sexual dimorphism_sentence_222

A study, carried about by analyzing annual world rankings from 1980 to 1996, found that males' running times were, on average, 11% faster than females'. Sexual dimorphism_sentence_223

Females are taller, on average, than males in early adolescence, but males, on average, surpass them in height in later adolescence and adulthood. Sexual dimorphism_sentence_224

In the United States, adult males are, on average, 9% taller and 16.5% heavier than adult females. Sexual dimorphism_sentence_225

There is no comparative evidence of differing levels of sexual selection having produced sexual size dimorphism between human populations. Sexual dimorphism_sentence_226

Males typically have larger tracheae and branching bronchi, with about 30 percent greater lung volume per body mass. Sexual dimorphism_sentence_227

On average, males have larger hearts, 10 percent higher red blood cell count, higher hemoglobin, hence greater oxygen-carrying capacity. Sexual dimorphism_sentence_228

They also have higher circulating clotting factors (vitamin K, prothrombin and platelets). Sexual dimorphism_sentence_229

These differences lead to faster healing of wounds and higher peripheral pain tolerance. Sexual dimorphism_sentence_230

Females typically have more white blood cells (stored and circulating), more granulocytes and B and T lymphocytes. Sexual dimorphism_sentence_231

Additionally, they produce more antibodies at a faster rate than males. Sexual dimorphism_sentence_232

Hence they develop fewer infectious diseases and succumb for shorter periods. Sexual dimorphism_sentence_233

Ethologists argue that females, interacting with other females and multiple offspring in social groups, have experienced such traits as a selective advantage. Sexual dimorphism_sentence_234

Considerable discussion in academic literature concerns potential evolutionary advantages associated with sexual competition (both intrasexual and intersexual) and short- and long-term sexual strategies. Sexual dimorphism_sentence_235

According to Daly and Wilson, "The sexes differ more in human beings than in monogamous mammals, but much less than in extremely polygamous mammals." Sexual dimorphism_sentence_236

One proposed explanation is that human sexuality has developed more in common with its close relative the bonobo, who have similar sexual dimorphism and which are polygynandrous and use recreational sex to reinforce social bonds and reduce aggression. Sexual dimorphism_sentence_237

In the human brain, a difference between sexes was observed in the transcription of the PCDH11X/Y gene pair unique to Homo sapiens. Sexual dimorphism_sentence_238

Sexual differentiation in the human brain from the undifferentiated state is triggered by testosterone from the fetal testis. Sexual dimorphism_sentence_239

Testosterone is converted to estrogen in the brain through the action of the enzyme aromatase. Sexual dimorphism_sentence_240

Testosterone acts on many brain areas, including the SDN-POA, to create the masculinized brain pattern. Sexual dimorphism_sentence_241

Brains of pregnant females carrying male fetuses may be shielded from the masculinizing effects of androgen through the action of sex hormone-binding globulin. Sexual dimorphism_sentence_242

The relationship between sex differences in the brain and human behavior is a subject of controversy in psychology and society at large. Sexual dimorphism_sentence_243

Many females tend to have a higher ratio of gray matter in the left hemisphere of the brain in comparison to males. Sexual dimorphism_sentence_244

Males on average have larger brains than females; however, when adjusted for total brain volume the gray matter differences between sexes is almost nonexistent. Sexual dimorphism_sentence_245

Thus, the percentage of gray matter appears to be more related to brain size than it is to sex. Sexual dimorphism_sentence_246

Differences in brain physiology between sexes do not necessarily relate to differences in intellect. Sexual dimorphism_sentence_247

Haier et al. Sexual dimorphism_sentence_248

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". Sexual dimorphism_sentence_249

(See the sex and intelligence article for more on this subject.) Sexual dimorphism_sentence_250

Strict graph-theoretical analysis of the human brain connections revealed that in numerous graph-theoretical parameters (e.g., minimum bipartition width, edge number, the expander graph property, minimum vertex cover), the structural connectome of women are significantly "better" connected than the connectome of men. Sexual dimorphism_sentence_251

It was shown 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_sentence_252

Sexual dimorphism was also described in the gene level and shown to extend from the sex chromosomes. Sexual dimorphism_sentence_253

Overall, about 6500 genes have been found to have sex-differential expression in at least one tissue. Sexual dimorphism_sentence_254

Many of these genes are not directly associated with reproduction, but rather linked to more general biological features. Sexual dimorphism_sentence_255

In addition, it has been shown that genes with sex specific expression undergo reduced selection efficiency, which lead to higher population frequencies of deleterious mutations and contributing to the prevalence of several human diseases. Sexual dimorphism_sentence_256

Immune function Sexual dimorphism_section_13

Sexual dimorphism in immune function is a common pattern in vertebrates and also in a number of invertebrates. Sexual dimorphism_sentence_257

Most often, females are more ‘immunocompetent’ than males. Sexual dimorphism_sentence_258

The underlying causes are explained by either the role of immunosuppressive substances, such as testosterone, or by fundamental differences in male and female life histories. Sexual dimorphism_sentence_259

It has been shown that female mammals tend to have higher white blood cell counts (WBC), with further associations between cell counts and longevity in females. Sexual dimorphism_sentence_260

There is also a positive covariance between sexual dimorphism in immunity, as measured by a subset of WBC, and dimorphism in the duration of effective breeding. Sexual dimorphism_sentence_261

This is consistent with the application of ‘Bateman’s principle’ to immunity, with females maximizing fitness by lengthening lifespan through greater investment in immune defences. Sexual dimorphism_sentence_262

Cells Sexual dimorphism_section_14

Phenotypic differences between sexes are evident even in cultured cells from tissues. Sexual dimorphism_sentence_263

For example, female muscle-derived stem cells have a better muscle regeneration efficiency than male ones. Sexual dimorphism_sentence_264

There are reports of several metabolic differences between male and female cells and they also respond to stress differently. Sexual dimorphism_sentence_265

Reproductively advantageous Sexual dimorphism_section_15

In theory, larger females are favored by competition for mates, especially in polygamous species. Sexual dimorphism_sentence_266

Larger females offer an advantage in fertility, since the physiological demands of reproduction are limiting in females. Sexual dimorphism_sentence_267

Hence there is a theoretical expectation that females tend to be larger in species that are monogamous. Sexual dimorphism_sentence_268

Females are larger in many species of insects, many spiders, many fish, many reptiles, owls, birds of prey and certain mammals such as the spotted hyena, and baleen whales such as blue whale. Sexual dimorphism_sentence_269

As an example, in some species, females are sedentary, and so males must search for them. Sexual dimorphism_sentence_270

Fritz Vollrath and Geoff Parker argue that this difference in behaviour leads to radically different selection pressures on the two sexes, evidently favouring smaller males. Sexual dimorphism_sentence_271

Cases where the male is larger than the female have been studied as well, and require alternative explanations. Sexual dimorphism_sentence_272

One example of this type of sexual size dimorphism is the bat Myotis nigricans, (black myotis bat) where females are substantially larger than males in terms of body weight, skull measurement, and forearm length. Sexual dimorphism_sentence_273

The interaction between the sexes and the energy needed to produce viable offspring make it favorable for females to be larger in this species. Sexual dimorphism_sentence_274

Females bear the energetic cost of producing eggs, which is much greater than the cost of making sperm by the males. Sexual dimorphism_sentence_275

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. Sexual dimorphism_sentence_276

A larger female can provide parental care for a longer time while the offspring matures. Sexual dimorphism_sentence_277

The gestation and lactation periods are fairly long in M. nigricans, the females suckling their offspring until they reach nearly adult size. Sexual dimorphism_sentence_278

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. Sexual dimorphism_sentence_279

Smaller male size may be an adaptation to increase maneuverability and agility, allowing males to compete better with females for food and other resources. Sexual dimorphism_sentence_280

Some species of anglerfish also display extreme sexual dimorphism. Sexual dimorphism_sentence_281

Females are more typical in appearance to other fish, whereas the males are tiny rudimentary creatures with stunted digestive systems. Sexual dimorphism_sentence_282

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. Sexual dimorphism_sentence_283

A similar situation is found in the Zeus water bug Phoreticovelia disparata where the female has a glandular area on her back that can serve to feed a male, which clings to her (note that although males can survive away from females, they generally are not free-living). Sexual dimorphism_sentence_284

This is taken to the logical extreme in the Rhizocephala crustaceans, like the Sacculina, where the male injects itself into the female's body and becomes nothing more than sperm producing cells, to the point that the superorder used to be mistaken for hermaphroditic. Sexual dimorphism_sentence_285

Some plant species also exhibit dimorphism in which the females are significantly larger than the males, such as in the moss Dicranum and the liverwort Sphaerocarpos. Sexual dimorphism_sentence_286

There is some evidence that, in these genera, the dimorphism may be tied to a sex chromosome, or to chemical signalling from females. Sexual dimorphism_sentence_287

Another complicated example of sexual dimorphism is in Vespula squamosa, the southern yellowjacket. Sexual dimorphism_sentence_288

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 worker and male counterparts. Sexual dimorphism_sentence_289

Evolution Sexual dimorphism_section_16

See also: Sexual selection and Mate choice Sexual dimorphism_sentence_290

Sexual dimorphism by size is evident in some extinct species such as the velociraptor. Sexual dimorphism_sentence_291

In the case of velociraptors the sexual size dimorphism may have been caused by two factors: male competition for hunting ground to attract mates, and/or female competition for nesting locations and mates, males being a scarce breeding resource. Sexual dimorphism_sentence_292

In 1871, Charles Darwin advanced the theory of sexual selection, which related sexual dimorphism with sexual selection. Sexual dimorphism_sentence_293

It has been proposed that the earliest sexual dimorphism is the size differentiation of sperm and eggs (anisogamy), but the evolutionary significance of sexual dimorphism is more complex than that would suggest. Sexual dimorphism_sentence_294

Anisogamy and the usually large number of small male gametes relative to the larger female gametes usually lies in the development of strong sperm competition, because small sperm enable organisms to produce a large number of sperm, and make males (or male function of hermaphrodites) more redundant. Sexual dimorphism_sentence_295

This intensifies male competition for mates and promotes the evolution of other sexual dimorphism in many species, especially in vertebrates including mammals. Sexual dimorphism_sentence_296

However, in some species, the females can be larger than males, irrespective of gametes, and in some species females (usually of species in which males invest a lot in rearing offspring and thus no longer considered as so redundant) compete for mates in ways more usually associated with males. Sexual dimorphism_sentence_297

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. Sexual dimorphism_sentence_298

In these species, there is a selection pressure for whatever traits enable a male to have more matings. Sexual dimorphism_sentence_299

The male may therefore come to have different traits from the female. Sexual dimorphism_sentence_300

These traits could be ones that allow him to fight off other males for control of territory or a harem, such as large size or weapons; or they could be traits that females, for whatever reason, prefer in mates. Sexual dimorphism_sentence_301

Male-male competition poses no deep theoretical questions but mate choice does. Sexual dimorphism_sentence_302

Females may choose males that appear strong and healthy, thus likely to possess "good alleles" and give rise to healthy offspring. Sexual dimorphism_sentence_303

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). Sexual dimorphism_sentence_304

Two hypotheses for explaining this fact are the sexy son hypothesis and the handicap principle. Sexual dimorphism_sentence_305

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. Sexual dimorphism_sentence_306

Those that do not will have sons that are unattractive to most females (since the preference is widespread) and so receive few matings. Sexual dimorphism_sentence_307

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". Sexual dimorphism_sentence_308

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. Sexual dimorphism_sentence_309

See also Sexual dimorphism_section_17

Credits to the contents of this page go to the authors of the corresponding Wikipedia page: dimorphism.