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Sexual dimorphism is the condition where the
Common and easily identified types of dimorphism consist of
Sexual dimorphism is the condition where the sex
Sex is the trait that determines whether a sexually reproducing animal or plant produces male or female gametes. Male plants and animals produce smaller mobile gametes (spermatozoa, sperm, pollen), while females produce larger ones (ova, ...
es of the same animal and/or plant species
In biology, a species is the basic unit of Taxonomy (biology), classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of ...
exhibit different morphological characteristics, particularly characteristics not directly involved in reproduction
Reproduction (or procreation or breeding) is the biological process by which new individual organisms – "offspring" – are produced from their "parent" or parents. Reproduction is a fundamental feature of all known life; each individual or ...
. The condition occurs in most animals and some plants. Differences may include secondary sex characteristic
Secondary sex characteristics are features that appear during puberty in humans, and at sexual maturity in other animals. These characteristics are particularly evident in the sexually dimorphic phenotypic traits that distinguish the sexes of a sp ...
s, size, weight, colour, markings, or behavioural or cognitive traits. These differences may be subtle or exaggerated and may be subjected to sexual selection
Sexual selection is a mode of natural selection in which members of one biological sex choose mates of the other sex to mate with (intersexual selection), and compete with members of the same sex for access to members of the opposite sex (int ...
and natural selection
Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. It is a key mechanism of evolution, the change in the heritable traits characteristic of a population over generations. Charl ...
. The opposite of dimorphism is ''monomorphism'', which is when both biological sexes are phenotypically
In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology or physical form and structure, its developmental processes, its biochemical and physiological proper ...
indistinguishable from each other.
Overview
Ornamentation and coloration
Common and easily identified types of dimorphism consist of ornamentation
An ornament is something used for decoration.
Ornament may also refer to:
Decoration
* Ornament (art), any purely decorative element in architecture and the decorative arts
*Biological ornament, a characteristic of animals that appear to serve o ...
and coloration, though not always apparent. 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 selection
Sexual selection is a mode of natural selection in which members of one biological sex choose mates of the other sex to mate with (intersexual selection), and compete with members of the same sex for access to members of the opposite sex (int ...
leads to the exaggerated dimorphic traits that are used predominantly in competition over mates. 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. The costs and implications differ depending on the nature of the ornamentation (such as the colour mechanism involved).
The peafowl
Peafowl is a common name for three bird species in the genera '' Pavo'' and ''Afropavo'' within the tribe Pavonini of the family Phasianidae, the pheasants and their allies. Male peafowl are referred to as peacocks, and female peafowl are re ...
constitute conspicuous illustrations of the principle. The ornate plumage
Plumage ( "feather") is a layer of feathers that covers a bird and the pattern, colour, and arrangement of those feathers. The pattern and colours of plumage differ between species and subspecies and may vary with age classes. Within species, ...
of peacocks, as used in the courting display, attracts peahen
Peafowl is a common name for three bird species in the genera '' Pavo'' and ''Afropavo'' within the tribe Pavonini of the family Phasianidae, the pheasants and their allies. Male peafowl are referred to as peacocks, and female peafowl are re ...
s. At first sight one might mistake peacocks and peahens for completely different species because of the vibrant colours and the sheer size of the male's plumage; the peahen being of a subdued brown coloration. 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. Similar examples are manifold, such as in birds of paradise
The birds-of-paradise are members of the family Paradisaeidae of the order Passeriformes. The majority of species are found in eastern Indonesia, Papua New Guinea and eastern Australia. The family has 44 species in 17 genera. The members of this ...
and argus pheasants.
Another example of sexual dichromatism is that of the nestling blue tit
The Eurasian blue tit (''Cyanistes caeruleus'') is a small passerine bird in the tit family, Paridae. It is easily recognisable by its blue and yellow plumage and small size.
Eurasian blue tits, usually resident and non-migratory birds, a ...
s. Males are chromatically more yellow than females. It is believed that this is obtained by the ingestion of green Lepidoptera
Lepidoptera ( ) is an order of insects that includes butterflies and moths (both are called lepidopterans). About 180,000 species of the Lepidoptera are described, in 126 families and 46 superfamilies, 10 percent of the total described specie ...
n larvae, which contain large amounts of the carotenoid
Carotenoids (), also called tetraterpenoids, are yellow, orange, and red organic pigments that are produced by plants and algae, as well as several bacteria, and fungi. Carotenoids give the characteristic color to pumpkins, carrots, parsnips, ...
s lutein
Lutein (;"Lutein"
Insects display a wide variety of sexual dimorphism between taxa including size, ornamentation and coloration. The female-biased sexual size dimorphism observed in many taxa evolved despite intense male–male competition for mates. 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. In the hackberry emperor females are similarly larger than males. The reason for the sexual dimorphism is due to provision size mass, in which females consume more pollen than males.
In some species, there is evidence of male dimorphism, but it appears to be for the purpose of distinctions of roles. This is seen in the bee species ''
Many
Sexual dimorphism in birds can be manifested in size or plumage differences between the sexes. Sexual size dimorphism varies among taxa with males typically being larger, though this is not always the case, e.g.
Some species of
In 1871, 
These traits could be ones that allow him to fight off other males for control of territory or a
zeaxanthin
Zeaxanthin is one of the most common carotenoids in nature, and is used in the xanthophyll cycle. Synthesized in plants and some micro-organisms, it is the pigment that gives paprika (made from bell peppers), corn, saffron, goji ( wolfberries), ...
. This diet also affects the sexually dimorphic colours in the human-invisible ultraviolet
Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30 PHz) to 400 nm (750 THz), shorter than that of visible light, but longer than X-rays. UV radiatio ...
spectrum. Hence, the male birds, although appearing yellow to humans actually have a violet-tinted plumage that is seen by females. This plumage is thought to be an indicator of male parental abilities. Perhaps this is a good indicator for females because it shows that they are good at obtaining a food supply from which the carotenoid is obtained. There is a positive correlation between the chromas of the tail and breast feathers and body condition. Carotenoids play an important role in immune function
The immune system is a network of biological processes that protects an organism from diseases. It detects and responds to a wide variety of pathogens, from viruses to parasitic worms, as well as cancer cells and objects such as wood splinters ...
for many animals, so carotenoid dependent signals might indicate health.
Frogs constitute another conspicuous illustration of the principle. There are two types of dichromatism for frog species: ontogenetic and dynamic. Ontogenetic frogs are more common and have permanent color changes in males or females. ''Ranoidea lesueuri'' is an example of a dynamic frog that has temporary color changes in males during breeding season. ''Hyperolius ocellatus
''Hyperolius ocellatus'' is a species of tropical West African frog in the family Hyperoliidae, that is split into the subspecies ''H. o. ocellatus'' and ''H. o. purpurescens''. It is found in Angola, Cameroon, Central African Republic, Republic ...
'' is an ontogenetic frog with dramatic differences in both color and pattern between the sexes. At sexual maturity, the males display a bright green with white dorsolateral lines. In contrast, the females are rusty red to silver with small spots. The bright coloration in the male population serves to attract females and as an aposematic
Aposematism is the advertising by an animal to potential predators that it is not worth attacking or eating. This unprofitability may consist of any defences which make the prey difficult to kill and eat, such as toxicity, venom, foul taste or ...
sign to potential predators.
Females often show a preference for exaggerated male secondary sexual characteristics
Secondary sex characteristics are features that appear during puberty in humans, and at sexual maturity in other animals. These characteristics are particularly evident in the sexually dimorphic phenotypic traits that distinguish the sexes of a ...
in mate selection. 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.
Similar sexual dimorphism and mating choice are also observed in many fish species. For example, male guppies
The guppy (), also known as millionfish and rainbow fish, is one of the world's most widely distributed tropical fish and one of the most popular freshwater aquarium fish species. It is a member of the family Poeciliidae and, like almost all ...
have colorful spots and ornamentations while females are generally grey in color. Female guppies prefer brightly colored males to duller males.
In redlip blennies, only the male fish develops an organ at the anal-urogenital region that produces antimicrobial substances. During parental care, males rub their anal-urogenital regions over their nests' internal surfaces, thereby protecting their eggs from microbial infections, one of the most common causes for mortality in young fish.
Plants
Mostflowering plant
Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (), commonly called angiosperms. The term "angiosperm" is derived from the Greek words ('container, vessel') and ('seed'), and refers to those plants th ...
s are hermaphroditic
In reproductive biology, a hermaphrodite () is an organism that has both kinds of reproductive organs and can produce both gametes associated with male and female sexes.
Many taxonomic groups of animals (mostly invertebrates) do not have sepa ...
but approximately 6% of species have separate males and females (dioecy
Dioecy (; ; adj. dioecious , ) is a characteristic of a species, meaning that it has distinct individual organisms (unisexual) that produce male or female gametes, either directly (in animals) or indirectly (in seed plants). Dioecious reproducti ...
). Sexual dimorphism is common in dioecious plants and dioicous
Dioicy () is a sexual system where archegonia and antheridia are produced on separate gametophytes. It is one of the two main sexual systems in bryophytes. Both dioicous () and monoicous gametophytes produce gametes in gametangia by mitosis rathe ...
species.
Males and females in insect-pollinated
Entomophily or insect pollination is a form of pollination whereby pollen of plants, especially but not only of flowering plants, is distributed by insects. Flowers pollinated by insects typically advertise themselves with bright colours, some ...
species generally look similar to one another because plants provide rewards (e.g. nectar
Nectar is a sugar-rich liquid produced by plants in glands called nectaries or nectarines, either within the flowers with which it attracts pollinating animals, or by extrafloral nectaries, which provide a nutrient source to animal mutualis ...
) that encourage pollinators
A pollinator is an animal that moves pollen from the male anther of a flower to the female carpel, stigma of a flower. This helps to bring about fertilization of the ovules in the flower by the male gametes from the pollen grains.
Insects are ...
to visit another similar flower
A flower, sometimes known as a bloom or blossom, is the reproductive structure found in flowering plants (plants of the division Angiospermae). The biological function of a flower is to facilitate reproduction, usually by providing a mechanis ...
, completing pollination
Pollination is the transfer of pollen from an anther of a plant to the stigma of a plant, later enabling fertilisation and the production of seeds, most often by an animal or by wind. Pollinating agents can be animals such as insects, birds, a ...
. ''Catasetum
''Catasetum'', abbreviated as Ctsm. in horticultural trade, is a genus of showy epiphytic Orchids, family Orchidaceae, subfamily Epidendroideae, tribe Cymbidieae, subtribe Catasetinae, with 166 species, many of which are highly prized in hortic ...
'' orchids are one interesting exception to this rule. Male ''Catasetum'' orchids
Orchids are plants that belong to the family Orchidaceae (), a diverse and widespread group of flowering plants with blooms that are often colourful and fragrant.
Along with the Asteraceae, they are one of the two largest families of flowering ...
violently attach pollinia A pollinium (plural pollinia) is a coherent mass of pollen grains in a plant that are the product of only one anther, but are transferred, during pollination, as a single unit. This is regularly seen in plants such as orchids and many species of mil ...
to euglossine
The tribe Euglossini, in the subfamily Apinae, commonly known as orchid bees or euglossine bees, are the only group of corbiculate bees whose non-parasitic members do not all possess eusocial behavior.
Description
Most of the tribe's species ...
bee pollinators. The bees will then avoid other male flowers but may visit the female, which look different from the males.
Various other dioecious exceptions, such as '' Loxostylis alata'' have visibly different sexes, 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.
Some plants, such as some species of ''Geranium
''Geranium'' is a genus of 422 species of annual, biennial, and perennial plants that are commonly known as geraniums or cranesbills. They are found throughout the temperate regions of the world and the mountains of the tropics, but mostly in ...
'' have what amounts to serial sexual dimorphism. The flowers of such species might for example present their anthers
The stamen (plural ''stamina'' or ''stamens'') is the pollen-producing reproductive organ of a flower. Collectively the stamens form the androecium., p. 10
Morphology and terminology
A stamen typically consists of a stalk called the filam ...
on opening, then shed the exhausted anthers after a day or two and perhaps change their colours as well while the pistil
Gynoecium (; ) is most commonly used as a collective term for the parts of a flower that produce ovules and ultimately develop into the fruit and seeds. The gynoecium is the innermost whorl of a flower; it consists of (one or more) '' pistils ...
matures; specialist pollinators are very much inclined to concentrate on the exact appearance of the flowers they serve, which saves their time and effort and serves the interests of the plant accordingly. Some such plants go even further and change their appearance again once they have been fertilised, thereby discouraging further visits from pollinators. This is advantageous to both parties because it avoids damage to the developing fruit and avoids wasting the pollinator's effort on unrewarding visits. In effect the strategy ensures that the pollinators can expect a reward every time they visit an appropriately advertising flower.
Females of the aquatic plant ''Vallisneria americana
''Vallisneria americana'', commonly called wild celery, water-celery, tape grass, or eelgrass, is a plant in the family Hydrocharitaceae, the "tape-grasses". ''V. americana'' is a fresh water species that can tolerate salt, living in salinitie ...
'' 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 is most often associated with wind-pollination in plants due to selection for efficient pollen
Pollen is a powdery substance produced by seed plants. It consists of pollen grains (highly reduced microgametophytes), which produce male gametes (sperm cells). Pollen grains have a hard coat made of sporopollenin that protects the gametophy ...
dispersal in males vs pollen capture in females, e.g. ''Leucadendron rubrum''.
Sexual dimorphism in plants can also be dependent on reproductive development. This can be seen in ''Cannabis sativa
''Cannabis sativa'' is an annual herbaceous flowering plant indigenous to Eastern Asia, but now of cosmopolitan distribution due to widespread cultivation. It has been cultivated throughout recorded history, used as a source of industrial fiber ...
'', a type of hemp, which have higher photosynthesis rates in males while growing but higher rates in females once the plants become sexually mature. p. 206
Every sexually reproducing extant species of vascular plant actually has an alternation of generations; the plants we see about us generally are diploid
Ploidy () is the number of complete sets of chromosomes in a cell, and hence the number of possible alleles for autosomal and pseudoautosomal genes. Sets of chromosomes refer to the number of maternal and paternal chromosome copies, respectiv ...
sporophyte
A sporophyte () is the diploid multicellular stage in the life cycle of a plant or alga which produces asexual spores. This stage alternates with a multicellular haploid gametophyte phase.
Life cycle
The sporophyte develops from the zygote p ...
s, but their offspring really are not the seeds that people commonly recognise as the new generation. The seed actually is the offspring of the haploid
Ploidy () is the number of complete sets of chromosomes in a cell, and hence the number of possible alleles for autosomal and pseudoautosomal genes. Sets of chromosomes refer to the number of maternal and paternal chromosome copies, respecti ...
generation of microgametophytes (pollen
Pollen is a powdery substance produced by seed plants. It consists of pollen grains (highly reduced microgametophytes), which produce male gametes (sperm cells). Pollen grains have a hard coat made of sporopollenin that protects the gametophy ...
) and megagametophytes (the embryo sac
In seed plants, the ovule is the structure that gives rise to and contains the female reproductive cells. It consists of three parts: the ''integument'', forming its outer layer, the ''nucellus'' (or remnant of the megasporangium), and the fem ...
s in the ovules
In seed plants, the ovule is the structure that gives rise to and contains the female reproductive cells. It consists of three parts: the ''integument'', forming its outer layer, the ''nucellus'' (or remnant of the megasporangium), and the fem ...
). 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.
Insects
Insects display a wide variety of sexual dimorphism between taxa including size, ornamentation and coloration. The female-biased sexual size dimorphism observed in many taxa evolved despite intense male–male competition for mates. 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. In the hackberry emperor females are similarly larger than males. The reason for the sexual dimorphism is due to provision size mass, in which females consume more pollen than males.
In some species, there is evidence of male dimorphism, but it appears to be for the purpose of distinctions of roles. This is seen in the bee species ''Macrotera portalis
''Macrotera portalis'' is a species of communal, ground nesting, partially bivoltine bees found in arid grasslands and desert regions of North America. An oligolectic bee, ''M. portalis'' gathers pollen only from plants in the genus '' Sphaeral ...
'' in which there is a small-headed morph, capable of flight, and large-headed morph, incapable of flight, for males. ''Anthidium manicatum
''Anthidium manicatum'', commonly called the European wool carder bee is a species of bee in the family Megachilidae, the leaf-cutter bees or mason bees.
They get the name " carder" from their behaviour of scraping hair from leaves such as lamb' ...
'' also displays male-biased sexual dimorphism. The selection for larger size in males rather than females in this species may have resulted due to their aggressive territorial behavior and subsequent differential mating success. Another example is ''Lasioglossum hemichalceum
''Lasioglossum hemichalceum'', which has sometimes been confused with '' L. erythrurum,'' is a sweat bee endemic to Australia. Large numbers of unrelated females will typically share a single nest, a behavior referred to as "communal". Nests ar ...
'', which is a species of sweat bee that shows drastic physical dimorphisms between male offspring. Not all dimorphism has to have a drastic difference between the sexes. ''Andrena agilissima
''Andrena agilissima'' is a species of mining bee. They are present in most of Europe, the Near East and North Africa and can be found from April through July. ''Andrena agilissima'' is an ''oligolectic'' species, feeding only on the pollen of a ...
'' is a mining bee where the females only have a slightly larger head than the males.
Weaponry leads to increased fitness by increasing success in male–male competition in many insect species. The beetle horns in '' Onthophagus taurus'' are enlarged growths of the head or thorax expressed only in the males. ''Copris ochus'' also has distinct sexual and male dimorphism in head horns. These structures are impressive because of the exaggerated sizes. There is a direct correlation between male horn lengths and body size and higher access to mates and fitness. In other beetle species, both males and females may have ornamentation such as horns.
Generally, insect sexual size dimorphism (SSD) within species increases with body size.
Sexual dimorphism within insects is also displayed by dichromatism. In butterfly genera '' Bicyclus'' and '' Junonia'', dimorphic wing patterns evolved due to sex-limited expression, which mediates the intralocus sexual conflict and leads to increased fitness in males. The sexual dichromatic nature of ''Bicyclus anynana
''Bicyclus anynana'' (squinting bush brown) is a small brown butterfly in the family Nymphalidae, the most globally diverse family of butterflies. It is primarily found in eastern Africa from southern Sudan to Eswatini.
'' is reflected by female selection on the basis of dorsal UV-reflective eyespot pupils. The common brimstone also displays sexual dichromatism; males have yellow and iridescent wings, while female wings are white and non-iridescent. Naturally selected deviation in protective female coloration is displayed in mimetic butterflies.
Spiders and sexual cannibalism
Many arachnid
Arachnida () is a class of joint-legged invertebrate animals (arthropods), in the subphylum Chelicerata. Arachnida includes, among others, spiders, scorpions, ticks, mites, pseudoscorpions, harvestmen, camel spiders, whip spiders and vinegaro ...
groups exhibit sexual dimorphism, but it is most widely studied in the spiders. In the orb-weaving spider '' Zygiella x-notata'', for example, adult females have a larger body size than adult males. Size dimorphism shows a correlation with sexual cannibalism
Sexual cannibalism is when an animal, usually the female, cannibalizes its mate prior to, during, or after copulation.Polis, G.A. & Farley, R.D. Behavior and Ecology of Mating in the journal of Arachnology 33-46 (1979). It is a trait observed in ...
, which is prominent in spiders (it is also found in insects such as praying mantis
Mantises are an order (Mantodea) of insects that contains over 2,400 species in about 460 genera in 33 families. The largest family is the Mantidae ("mantids"). Mantises are distributed worldwide in temperate and tropical habitats. They ha ...
es). In the size dimorphic wolf spider
Wolf spiders are members of the family Lycosidae (). They are robust and agile hunters with excellent eyesight. They live mostly in solitude, hunt alone, and do not spin webs. Some are opportunistic hunters, pouncing upon prey as they find it or ...
''Tigrosa helluo
''Tigrosa helluo'' is a species of spider belonging to the family Lycosidae, also known as wolf spiders. ''T. helluo'' was formerly known as ''Hogna helluo'' before differences between dorsal color patterns, habitat preferences, body structu ...
'', food-limited females cannibalize more frequently. 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
Fecundity is defined in two ways; in human demography, it is the potential for reproduction of a recorded population as opposed to a sole organism, while in population biology, it is considered similar to fertility, the natural capability to p ...
and lower rates of cannibalism. 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
Orb-weaver spiders are members of the spider family Araneidae. They are the most common group of builders of spiral wheel-shaped webs often found in gardens, fields, and forests. The English word "orb" can mean "circular", hence the English name ...
. All ''Argiope'' species, including ''Argiope bruennichi
''Argiope bruennichi'' (wasp spider) is a species of orb-web spider distributed throughout central Europe, northern Europe, north Africa, parts of Asia, and the Azores archipelago. Like many other members of the genus '' Argiope'' (including St ...
'', use this method. 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. 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. In the species ''Maratus volans
''Maratus volans'' is a species in the jumping spider family (Salticidae), belonging to the genus ''Maratus'' (peacock spiders). These spiders are native to certain areas in Australia and occupy a wide distribution of habitats. They have a specia ...
'', the males are known for their characteristic colorful fan which attracts the females during mating.
Fish
Ray finned fish are an ancient and diverse class, with the widest degree of sexual dimorphism of any animal class. Fairbairn notes that "females are generally larger than males but males are often larger in species with male–male combat or male paternal care ... izes rangefrom dwarf males to males more than 12 times heavier than females." There are cases where males are substantially larger than females. An example is '' Lamprologus callipterus'', a type of cichlid fish. In this fish, the males are characterized as being up to 60 times larger than the females. The male's increased size is believed to be advantageous because males collect and defend empty snail shells in each of which a female breeds. Males must be larger and more powerful in order to collect the largest shells. The female's body size must remain small because in order for her to breed, she must lay her eggs inside the empty shells. If she grows too large, she will not fit in the shells and will be unable to breed. The female's small body size is also likely beneficial to her chances of finding an unoccupied shell. Larger shells, although preferred by females, are often limited in availability. 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. In other words, the male's ability to collect large shells depends on his size. The larger the male, the larger the shells he is able to collect. This then allows for females to be larger in his brooding nest which makes the difference between the sizes of the sexes less substantial. Male–male competition in this fish species also selects for large size in males. There is aggressive competition by males over territory and access to larger shells. Large males win fights and steal shells from competitors. Another example is thedragonet
Dragonets are small, percomorph, marine fish of the diverse family Callionymidae (from the Greek ''kallis'', "beautiful" and ', "name") found mainly in the tropical waters of the western Indo-Pacific. They are benthic organisms, spending most o ...
, in which males are considerably larger than females and possess longer fins.
Sexual dimorphism also occurs in hermaphroditic fish. These species are known as sequential hermaphrodites. In fish, 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. In protogynous mating systems where males dominate mating with many females, size plays a significant role in male reproductive success. 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. Larger males are able to stifle the growth of females and control environmental resources.
Social organization plays a large role in the changing of sex by the fish. It is often seen that a fish will change its sex when there is a lack of dominant male within the social hierarchy. The females that change sex are often those who attain and preserve an initial size advantage early in life. In either case, females which change sex to males are larger and often prove to be a good example of dimorphism.
In other cases with fish, males will go through noticeable changes in body size, and females will go through morphological changes that can only be seen inside of the body. For example, in sockeye salmon
The sockeye salmon (''Oncorhynchus nerka''), also called red salmon, kokanee salmon, blueback salmon, or simply sockeye, is an anadromous species of salmon found in the Northern Pacific Ocean and rivers discharging into it. This species is a P ...
, males develop larger body size at maturity, including an increase in body depth, hump height, and snout length. Females experience minor changes in snout length, but the most noticeable difference is the huge increase in gonad
A gonad, sex gland, or reproductive gland is a mixed gland that produces the gametes and sex hormones of an organism. Female reproductive cells are egg cells, and male reproductive cells are sperm. The male gonad, the testicle, produces sperm ...
size, which accounts for about 25% of body mass.
Sexual selection was observed for female ornamentation in '' Gobiusculus flavescens'', known as two-spotted gobies. Traditional hypotheses suggest that male–male competition drives selection. However, selection for ornamentation within this species suggests that showy female traits can be selected through either female–female competition or male mate choice. 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. The males invest heavily in offspring during the incubation, which leads to the sexual preference in colorful females due to higher egg quality.
Amphibians and non-avian reptiles
In amphibians and reptiles, the degree of sexual dimorphism varies widely among taxonomic groups. The sexual dimorphism in amphibians and reptiles may be reflected in any of the following: anatomy; relative length of tail; relative size of head; overall size as in many species ofvipers
The Viperidae (vipers) are a family of snakes found in most parts of the world, except for Antarctica, Australia, Hawaii, Madagascar, and various other isolated islands. They are venomous and have long (relative to non-vipers), hinged fangs t ...
and lizards
Lizards are a widespread group of squamate reptiles, with over 7,000 species, ranging across all continents except Antarctica, as well as most oceanic island chains. The group is paraphyletic since it excludes the snakes and Amphisbaenia altho ...
; coloration as in many amphibians
Amphibians are four-limbed and ectothermic vertebrates of the class Amphibia. All living amphibians belong to the group Lissamphibia. They inhabit a wide variety of habitats, with most species living within terrestrial, fossorial, arbore ...
, snakes
Snakes are elongated, limbless, carnivorous reptiles of the suborder Serpentes . Like all other squamates, snakes are ectothermic, amniote vertebrates covered in overlapping scales. Many species of snakes have skulls with several more joints ...
, and lizards, as well as in some turtles
Turtles are an order of reptiles known as Testudines, characterized by a special shell developed mainly from their ribs. Modern turtles are divided into two major groups, the Pleurodira (side necked turtles) and Cryptodira (hidden necked t ...
; an ornament as in many newts
A newt is a salamander in the subfamily Pleurodelinae. The terrestrial juvenile phase is called an eft. Unlike other members of the family Salamandridae, newts are semiaquatic, alternating between aquatic and terrestrial habitats. Not all aqua ...
and lizards; the presence of specific sex-related behaviour is common to many lizards; and vocal qualities which are frequently observed in frogs
A frog is any member of a diverse and largely carnivorous group of short-bodied, tailless amphibians composing the order Anura (ανοὐρά, literally ''without tail'' in Ancient Greek). The oldest fossil "proto-frog" '' Triadobatrachus'' i ...
.
Anole
Dactyloidae are a family of lizards commonly known as anoles () and native to warmer parts of the Americas, ranging from southeastern United States to Paraguay. Instead of treating it as a family, some authorities prefer to treat it as a subfami ...
lizards show prominent size dimorphism with males typically being significantly larger than females. For instance, the average male '' Anolis sagrei'' was 53.4 mm vs. 40 mm in females. Different sizes of the heads in anoles have been explained by differences in the estrogen pathway. 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.Pinto, A., Wiederhecker, H., & Colli, G. (2005). Sexual dimorphism in the Neotropical lizard, Tropidurus torquatus (Squamata, Tropiduridae). Amphibia-Reptilia. 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 in size is also seen in frog species like '' P. bibroniCtenophorus pictus
''Ctenophorus pictus'', commonly known as the painted ground-dragon or painted dragon,Waite ER (Editor). (1929). ''The Reptiles and Amphibians of South Australia''. Adelaide: Government Printer. 270 pp. is a species of lizard from the family Ag ...
''. are brightly conspicuous in their breeding coloration, but male colour declines with aging
Ageing ( BE) or aging ( AE) is the process of becoming older. The term refers mainly to humans, many other animals, and fungi, whereas for example, bacteria, perennial plants and some simple animals are potentially biologically immortal. I ...
. Male coloration appears to reflect innate anti-oxidation capacity that protects against oxidative DNA damage. 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.
Birds
Sexual dimorphism in birds can be manifested in size or plumage differences between the sexes. Sexual size dimorphism varies among taxa with males typically being larger, though this is not always the case, e.g. birds of prey
Birds of prey or predatory birds, also known as raptors, are hypercarnivorous bird species that actively hunt and feed on other vertebrates (mainly mammals, reptiles and other smaller birds). In addition to speed and strength, these predators h ...
, hummingbird
Hummingbirds are birds native to the Americas and comprise the biological family Trochilidae. With about 361 species and 113 genera, they occur from Alaska to Tierra del Fuego, but the vast majority of the species are found in the tropics aro ...
s, and some species of flightless birds. Plumage dimorphism, in the form of ornamentation or coloration, also varies, though males are typically the more ornamented or brightly colored sex. Such differences have been attributed to the unequal reproductive contributions of the sexes. This difference produces a stronger female choice since they have more risk in producing offspring. In some species, the male's contribution to reproduction ends at copulation, while in other species the male becomes the main caregiver. Plumage polymorphisms have evolved to reflect these differences and other measures of reproductive fitness, such as body condition or survival. The male phenotype sends signals to females who then choose the 'fittest' available male.
Sexual dimorphism is a product of both genetics and environmental factors. An example of sexual polymorphism determined by environmental conditions exists in the red-backed fairywren
The red-backed fairywren (''Malurus melanocephalus'') is a species of passerine bird in the Australasian wren family, Maluridae. It is endemic to Australia and can be found near rivers and coastal areas along the northern and eastern coastlines ...
. Red-backed fairywren males can be classified into three categories during breeding season
Seasonal breeders are animal species that successfully mate only during certain times of the year. These times of year allow for the optimization of survival of young due to factors such as ambient temperature, food and water availability, and cha ...
: black breeders, brown breeders, and brown auxiliaries. 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. The reproductive success
Reproductive success is an individual's production of offspring per breeding event or lifetime. This is not limited by the number of offspring produced by one individual, but also the reproductive success of these offspring themselves.
Reproduct ...
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.
Migratory patterns and behaviors also influence sexual dimorphisms. This aspect also stems back to the size dimorphism in species. It has been shown that the larger males are better at coping with the difficulties of migration and thus are more successful in reproducing when reaching the breeding destination. When viewing this in an evolutionary standpoint many theories and explanations come to consideration. If these are the result for every migration and breeding season the expected results should be a shift towards a larger male population through sexual selection. Sexual selection is strong when the factor of environmental selection is also introduced. 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. When the environment gives advantages and disadvantages of this sort, the strength of selection is weakened and the environmental forces are given greater morphological weight. The sexual dimorphism could also produce a change in timing of migration leading to differences in mating success within the bird population. When the dimorphism produces that large of a variation between the sexes and between the members of the sexes multiple evolutionary effects can take place. This timing could even lead to a speciation phenomenon if the variation becomes strongly drastic and favorable towards two different outcomes. Sexual dimorphism
Sexual dimorphism is the condition where the sexes of the same animal and/or plant species exhibit different morphological characteristics, particularly characteristics not directly involved in reproduction. The condition occurs in most anim ...
is maintained by the counteracting pressures of natural selection and sexual selection. For example, sexual dimorphism in coloration increases the vulnerability of bird species to predation by European sparrowhawks in Denmark. Presumably, increased sexual dimorphism means males are brighter and more conspicuous, leading to increased predation. Moreover, the production of more exaggerated ornaments in males may come at the cost of suppressed immune function. So long as the reproductive benefits of the trait due to sexual selection are greater than the costs imposed by natural selection, then the trait will propagate throughout the population. Reproductive benefits arise in the form of a larger number of offspring, while natural selection imposes costs in the form of reduced survival. This means that even if the trait causes males to die earlier, the trait is still beneficial so long as males with the trait produce more offspring than males lacking the trait. This balance keeps 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.
Such differences in form and reproductive roles often cause differences in behavior. As previously stated, males and females often have different roles in reproduction. The courtship and mating behavior of males and females are regulated largely by hormones throughout a bird's lifetime. Activational hormones occur during puberty and adulthood and serve to 'activate' certain behaviors when appropriate, such as territoriality during breeding season. 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. Such behavioral differences can cause disproportionate sensitivities to anthropogenic pressures. Females of the whinchat in Switzerland breed in intensely managed grasslands. Earlier harvesting of the grasses during the breeding season lead to more female deaths. Populations of many birds are often male-skewed and when sexual differences in behavior increase this ratio, populations decline at a more rapid rate. Also not all male dimorphic traits are due to hormones like testosterone, instead they are a naturally occurring part of development, for example plumage. In addition, the strong hormonal influence on phenotypic differences suggest that the genetic mechanism and genetic basis of these sexually dimorphic traits may involve transcription factors or cofactors rather than regulatory sequences.
Sexual dimorphism may also influence differences in parental investment during times of food scarcity. For example, in the blue-footed booby
The blue-footed booby (''Sula nebouxii'') is a marine bird native to subtropical and tropical regions of the eastern Pacific Ocean. It is one of six species of the genus '' Sula'' – known as boobies. It is easily recognizable by its distinct ...
, the female chicks grow faster than the males, resulting in booby parents producing the smaller sex, the males, during times of food shortage. This then results in the maximization of parental lifetime reproductive success. 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 may also only appear during mating season, some species of birds only show dimorphic traits in seasonal variation. The males of these species will molt into a less bright or less exaggerated color during the off breeding season. This occurs because the species is more focused on survival than reproduction, causing a shift into a less ornate state.
Consequently, sexual dimorphism has important ramifications for conservation. However, sexual dimorphism is not only found in birds and is thus important to the conservation of many animals. Such differences in form and behavior can lead to sexual segregation, defined as sex differences in space and resource use. Most sexual segregation research has been done on ungulates, but such research extends to bat
Bats are mammals of the order Chiroptera.''cheir'', "hand" and πτερόν''pteron'', "wing". With their forelimbs adapted as wings, they are the only mammals capable of true and sustained flight. Bats are more agile in flight than most bi ...
s, kangaroo
Kangaroos are four marsupials from the family Macropodidae (macropods, meaning "large foot"). In common use the term is used to describe the largest species from this family, the red kangaroo, as well as the antilopine kangaroo, eastern gre ...
s, and birds. Sex-specific conservation plans have even been suggested for species with pronounced sexual segregation.
The term sesquimorphism (the Latin numeral prefix
Numeral or number prefixes are prefixes derived from numerals or occasionally other numbers. In English and many other languages, they are used to coin numerous series of words. For example:
* unicycle, bicycle, tricycle (1-cycle, 2-cycle, 3-cyc ...
''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 Examples include Cape sparrow
The Cape sparrow (''Passer melanurus''), or mossie, is a bird of the sparrow family Passeridae found in southern Africa. A medium-sized sparrow at , it has distinctive plumage, including large pale head stripes in both sexes. Its plumage is m ...
(''Passer melanurus''), rufous sparrow (subspecies ''P. motinensis motinensis''), and saxaul sparrow
The saxaul sparrow (''Passer ammodendri'') is a passerine bird of the sparrow family Passeridae, found in parts of Central Asia. At and , it is among the larger sparrows. Both sexes have plumage ranging from dull grey to sandy brown, and pale ...
(''P. ammodendri'').
Mammals
In a large proportion of mammal species, males are larger than females. Bothgenes
In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a ba ...
and hormone
A hormone (from the Greek participle , "setting in motion") is a class of signaling molecules in multicellular organisms that are sent to distant organs by complex biological processes to regulate physiology and behavior. Hormones are required fo ...
s affect the formation of many animal brains before "birth
Birth is the act or process of bearing or bringing forth offspring, also referred to in technical contexts as parturition. In mammals, the process is initiated by hormones which cause the muscular walls of the uterus to contract, expelling the f ...
" (or hatching
Hatching (french: hachure) is an artistic technique used to create tonal or shading effects by drawing (or painting or scribing) closely spaced parallel lines. (It is also used in monochromatic representations of heraldry to indicate what the ...
), and also behaviour of adult individuals. Hormones significantly affect human brain formation, and also brain development at puberty. A 2004 review in ''Nature Reviews Neuroscience
''Nature Reviews Neuroscience'' is a monthly peer-reviewed review journal published by Nature Portfolio. It was established in 2000. The editor-in-chief is Darran Yates.
Abstracting and indexing
The journal is abstracted and indexed in:
*PubMed/M ...
'' observed that "because it is easier to manipulate hormone levels than the expression of sex chromosome genes, the effects of hormones have been studied much more extensively, and are much better understood, than the direct actions in the brain of sex chromosome genes." It concluded that while "the differentiating effects of gonadal secretions seem to be dominant," the existing body of research "support the idea that sex differences in neural expression of X and Y genes significantly contribute to sex differences in brain functions and disease."
Pinnipeds
Marine mammals
Marine mammals are aquatic mammals that rely on the ocean and other marine ecosystems for their existence. They include animals such as seals, whales, manatees, sea otters and polar bears. They are an informal group, unified only by their re ...
show some of the greatest sexual size differences of mammals, because of sexual selection and environmental factors like breeding location. The mating system of pinnipeds
Pinnipeds (pronounced ), commonly known as seals, are a widely distributed and diverse clade of carnivorous, fin-footed, semiaquatic, mostly marine mammals. They comprise the extant families Odobenidae (whose only living member is the walru ...
varies from polygamy to serial monogamy
Monogamy ( ) is a form of dyadic relationship in which an individual has only one partner during their lifetime. Alternately, only one partner at any one time (serial monogamy) — as compared to the various forms of non-monogamy (e.g., polyg ...
. Pinnipeds are known for early differential growth and maternal investment since the only nutrients for newborn pups is the milk provided by the mother. For example, the males are significantly larger (about 10% heavier and 2% longer) than the females at birth in sea lion pups. The pattern of differential investment can be varied principally prenatally and post-natally. ''Mirounga leonina'', the southern elephant seal
The southern elephant seal (''Mirounga leonina'') is one of two species of elephant seals. It is the largest member of the clade Pinnipedia and the order Carnivora, as well as the largest extant marine mammal that is not a cetacean. It gets its ...
, is one of the most dimorphic mammals.
Primates
Humans
According to Clark Spencer Larsen, modern day ''Homo sapiens
Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, culture, a ...
'' show a range of sexual dimorphism, with average body mass between the sexes differing by roughly 15%. Considerable discussion in academic literature concerns potential evolutionary advantages associated with sexual competition (both intrasexual and intersexual) and short- and long-term sexual strategies. According to Daly and Wilson, "The sexes differ more in human beings than in monogamous mammals, but much less than in extremely polygamous mammals."
The average basal metabolic rate is about 6 percent higher in adolescent males than females and increases to about 10 percent higher after puberty. Females tend to convert more food into fat
In nutrition, biology, and chemistry, fat usually means any ester of fatty acids, or a mixture of such compounds, most commonly those that occur in living beings or in food.
The term often refers specifically to triglycerides (triple est ...
, while males convert more into muscle
Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of muscle ...
and expendable circulating energy reserves. 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. The difference in strength relative to body mass is less pronounced in trained individuals. In Olympic weightlifting, male records vary from 5.5× body mass in the lowest weight category to 4.2× in the highest weight category, while female records vary from 4.4× to 3.8×, a weight adjusted difference of only 10–20%, and an absolute difference of about 40% (i.e. 472 kg vs 333 kg for unlimited weight classes; see Olympic weightlifting records). 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'.
In early adolescence, females are on average taller than males (as females tend to go through puberty earlier), but males, on average, surpass them in height in later adolescence and adulthood. In the United States, adult males are on average 9% taller and 16.5% heavier than adult females.
Males typically have larger tracheae and branching bronchi
A bronchus is a passage or airway in the lower respiratory tract that conducts air into the lungs. The first or primary bronchi pronounced (BRAN-KAI) to branch from the trachea at the carina are the right main bronchus and the left main bronchus ...
, with about 30 percent greater lung volume
Lung volumes and lung capacities refer to the volume of air in the lungs at different phases of the respiratory cycle.
The average total lung capacity of an adult human male is about 6 litres of air.
Tidal breathing is normal, resting breathin ...
per body mass
Human body weight is a person's Mass versus weight, mass or weight.
Strictly speaking, body weight is the measurement of weight without items located on the person. Practically though, body weight may be measured with clothes on, but without shoe ...
. On average, males have larger heart
The heart is a muscular organ in most animals. This organ pumps blood through the blood vessels of the circulatory system. The pumped blood carries oxygen and nutrients to the body, while carrying metabolic waste such as carbon dioxide to ...
s, 10 percent higher red blood cell
Red blood cells (RBCs), also referred to as red cells, red blood corpuscles (in humans or other animals not having nucleus in red blood cells), haematids, erythroid cells or erythrocytes (from Greek ''erythros'' for "red" and ''kytos'' for "holl ...
count, higher hemoglobin
Hemoglobin (haemoglobin BrE) (from the Greek word αἷμα, ''haîma'' 'blood' + Latin ''globus'' 'ball, sphere' + ''-in'') (), abbreviated Hb or Hgb, is the iron-containing oxygen-transport metalloprotein present in red blood cells (erythrocyt ...
, hence greater oxygen-carrying capacity. They also have higher circulating clotting factors
Coagulation, also known as clotting, is the process by which blood changes from a liquid to a gel, forming a blood clot. It potentially results in hemostasis, the cessation of blood loss from a damaged vessel, followed by repair. The mechanism o ...
(vitamin K
Vitamin K refers to structurally similar, fat-soluble vitamers found in foods and marketed as dietary supplements. The human body requires vitamin K for post-synthesis modification of certain proteins that are required for blood coagulation ...
, prothrombin
Thrombin (, ''fibrinogenase'', ''thrombase'', ''thrombofort'', ''topical'', ''thrombin-C'', ''tropostasin'', ''activated blood-coagulation factor II'', ''blood-coagulation factor IIa'', ''factor IIa'', ''E thrombin'', ''beta-thrombin'', ''gamma- ...
and platelet
Platelets, also called thrombocytes (from Greek θρόμβος, "clot" and κύτος, "cell"), are a component of blood whose function (along with the coagulation factors) is to react to bleeding from blood vessel injury by clumping, thereby ...
s). These differences lead to faster healing of wound
A wound is a rapid onset of injury that involves lacerated or punctured skin (an ''open'' wound), or a contusion (a ''closed'' wound) from blunt force trauma or compression. In pathology, a ''wound'' is an acute injury that damages the epider ...
s and lower sensitivity to nerve pain
Pain is a distressing feeling often caused by intense or damaging stimuli. The International Association for the Study of Pain defines pain as "an unpleasant sensory and emotional experience associated with, or resembling that associated with, ...
after injury. In males, pain-causing injury to the peripheral nerve
A nerve is an enclosed, cable-like bundle of nerve fibers (called axons) in the peripheral nervous system.
A nerve transmits electrical impulses. It is the basic unit of the peripheral nervous system. A nerve provides a common pathway for the ...
occurs through the microglia
Microglia are a type of neuroglia (glial cell) located throughout the brain and spinal cord. Microglia account for about 7% of cells found within the brain. As the resident macrophage cells, they act as the first and main form of active immune de ...
, while in females it occurs through the T cell
A T cell is a type of lymphocyte. T cells are one of the important white blood cells of the immune system and play a central role in the adaptive immune response. T cells can be distinguished from other lymphocytes by the presence of a T-cell re ...
s (except in pregnant women, who follow a male pattern).
Females typically have more white blood cell
White blood cells, also called leukocytes or leucocytes, are the cells of the immune system that are involved in protecting the body against both infectious disease and foreign invaders. All white blood cells are produced and derived from mult ...
s (stored and circulating), as well as more granulocyte
Granulocytes are
cells in the innate immune system characterized by the presence of specific granules in their cytoplasm. Such granules distinguish them from the various agranulocytes. All myeloblastic granulocytes are polymorphonuclear. They ha ...
s and B and T lymphocyte
A lymphocyte is a type of white blood cell (leukocyte) in the immune system of most vertebrates. Lymphocytes include natural killer cells (which function in cell-mediated, cytotoxic innate immunity), T cells (for cell-mediated, cytotoxic adap ...
s. Additionally, they produce more antibodies
An antibody (Ab), also known as an immunoglobulin (Ig), is a large, Y-shaped protein used by the immune system to identify and neutralize foreign objects such as pathogenic bacteria and viruses. The antibody recognizes a unique molecule of the ...
at a faster rate than males, hence they develop fewer infectious
An infection is the invasion of tissues by pathogens, their multiplication, and the reaction of host tissues to the infectious agent and the toxins they produce. An infectious disease, also known as a transmissible disease or communicable dis ...
diseases and succumb for shorter periods. Ethologists
Ethology is the scientific study of animal behaviour, usually with a focus on behaviour under natural conditions, and viewing behaviour as an evolutionarily adaptive trait. Behaviourism as a term also describes the scientific and objectiv ...
argue that females, interacting with other females and multiple offspring in social groups, have experienced such traits as a selective advantage. Females have a higher sensitivity to pain due to aforementioned nerve differences that increase the sensation, and females thus require higher levels of pain medication after injury. Hormonal changes in females affect pain sensitivity, and pregnant women have the same sensitivity as males. Acute pain tolerance is also more consistent over a lifetime in females than males, despite these hormonal changes. Despite differences in the physical feeling, both sexes have similar psychological tolerance to (or ability to cope
The cope (known in Latin as ''pluviale'' 'rain coat' or ''cappa'' 'cape') is a liturgical vestment, more precisely a long mantle or cloak, open in front and fastened at the breast with a band or clasp. It may be of any liturgical colour.
A co ...
with and ignore) pain.
In the human brain
The human brain is the central organ of the human nervous system, and with the spinal cord makes up the central nervous system. The brain consists of the cerebrum, the brainstem and the cerebellum. It controls most of the activities of the ...
, a difference between sexes was observed in the transcription
Transcription refers to the process of converting sounds (voice, music etc.) into letters or musical notes, or producing a copy of something in another medium, including:
Genetics
* Transcription (biology), the copying of DNA into RNA, the fir ...
of the PCDH11X/Y gene pair unique to ''Homo sapiens''. Sexual differentiation in the human brain from the undifferentiated state is triggered by testosterone from the fetal testis. Testosterone is converted to estrogen in the brain through the action of the enzyme aromatase. Testosterone acts on many brain areas, including the SDN-POA, to create the masculinized brain pattern. Brains of pregnant females carrying male fetuses may be shielded from the masculinizing effects of androgen through the action of sex hormone-binding globulin
Sex hormone-binding globulin (SHBG) or sex steroid-binding globulin (SSBG) is a glycoprotein that binds to androgens and estrogens. When produced by the Sertoli cells in the seminiferous tubules of the testis, it has also been called androgen-bi ...
.
The relationship between sex differences in the brain and human behavior is a subject of controversy in psychology and society at large. Many females tend to have a higher ratio of gray matter
Grey matter is a major component of the central nervous system, consisting of neuronal cell bodies, neuropil (dendrites and unmyelinated axons), glial cells (astrocytes and oligodendrocytes), synapses, and capillaries. Grey matter is distingui ...
in the left hemisphere of the brain in comparison to males. Males on average have larger brains than females; however, when adjusted for total brain volume the gray matter differences between sexes is almost nonexistent. Thus, the percentage of gray matter appears to be more related to brain size than it is to sex. Differences in brain physiology between sexes do not necessarily relate to differences in intellect. Haier ''et al.'' found in a 2004 study that "men and women apparently achieve similar IQ results with different brain regions, suggesting that there is no singular underlying neuroanatomical structure to general intelligence and that different types of brain designs may manifest equivalent intellectual performance". (See the sex and intelligence article for more on this subject.) 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
In graph theory, an expander graph is a sparse graph that has strong connectivity properties, quantified using vertex, edge or spectral expansion. Expander constructions have spawned research in pure and applied mathematics, with several applica ...
property, minimum vertex cover
In graph theory, a vertex cover (sometimes node cover) of a graph is a set of vertices that includes at least one endpoint of every edge of the graph.
In computer science, the problem of finding a minimum vertex cover is a classical optimiza ...
), the structural connectome of women are significantly "better" connected than the connectome of men. 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 was also described in the gene level and shown to extend from the sex chromosomes. Overall, about 6500 genes have been found to have sex-differential expression in at least one tissue. Many of these genes are not directly associated with reproduction, but rather linked to more general biological features. In addition, it has been shown that genes with sex-specific expression undergo reduced selection efficiency, which lead to higher population frequencies of deleterious mutations and contributing to the prevalence of several human diseases.
Immune function
Sexual dimorphism in immune function is a common pattern in vertebrates and also in a number of invertebrates. Most often, females are more 'immunocompetent' than males. This trait is not consistent among all animals, but differs depending on taxonomy, with the most female-biased immune systems being found in insects. In mammals this results in more frequent and severe infections in males and higher rates of autoimmune disorders in females. One potential cause may be differences in gene expression of immune cells between the sexes. Another explanation is that endocrinological differences between the sexes impact the immune system – for example, testosterone acts as an immunosuppressive agent.Cells
Phenotypic differences between sexes are evident even in cultured cells from tissues. For example, female muscle-derivedstem cells
In multicellular organisms, stem cells are undifferentiated or partially differentiated cells that can differentiate into various types of cells and proliferate indefinitely to produce more of the same stem cell. They are the earliest type o ...
have a better muscle regeneration efficiency than male ones. There are reports of several metabolic differences between male and female cells and they also respond to stress differently.
Reproductively advantageous
In theory, larger females are favored by competition for mates, especially in polygamous species. Larger females offer an advantage in fertility, since the physiological demands of reproduction are limiting in females. Hence there is a theoretical expectation that females tend to be larger in species that are monogamous. Females are larger in many species ofinsect
Insects (from Latin ') are pancrustacean hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body (head, thorax and abdomen), three pairs of ...
s, many spider
Spiders ( order Araneae) are air-breathing arthropods that have eight legs, chelicerae with fangs generally able to inject venom, and spinnerets that extrude silk. They are the largest order of arachnids and rank seventh in total species div ...
s, many fish
Fish are aquatic, craniate, gill-bearing animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish as well as various extinct related groups. Approximately 95% of li ...
, many reptiles, owl
Owls are birds from the order Strigiformes (), which includes over 200 species of mostly solitary and nocturnal birds of prey typified by an upright stance, a large, broad head, binocular vision, binaural hearing, sharp talons, and feathers a ...
s, birds of prey and certain mammals such as the spotted hyena
The spotted hyena (''Crocuta crocuta''), also known as the laughing hyena, is a hyena species, currently classed as the sole extant member of the genus ''Crocuta'', native to sub-Saharan Africa. It is listed as being of least concern by the IUC ...
, and baleen whales such as blue whale
The blue whale (''Balaenoptera musculus'') is a marine mammal and a baleen whale. Reaching a maximum confirmed length of and weighing up to , it is the largest animal known to have ever existed. The blue whale's long and slender body can b ...
. As an example, in some species, females are sedentary, and so males must search for them. Fritz Vollrath and Geoff Parker
Professor Geoffrey Alan Parker FRS (born 24 May 1944) is an emeritus professor of biology at the University of Liverpool and the 2008 recipient of the Darwin Medal. Parker has been called “the professional’s professional”.
He has a par ...
argue that this difference in behaviour leads to radically different selection pressures on the two sexes, evidently favouring smaller males. Cases where the male is larger than the female have been studied as well, and require alternative explanations.
One example of this type of sexual size dimorphism is the bat '' Myotis nigricans'', (black myotis bat) where females are substantially larger than males in terms of body weight, skull measurement, and forearm length. The interaction between the sexes and the energy needed to produce viable offspring make it favorable for females to be larger in this species. Females bear the energetic cost of producing eggs, which is much greater than the cost of making sperm by the males. The fecundity advantage hypothesis states that a larger female is able to produce more offspring and give them more favorable conditions to ensure their survival; this is true for most ectotherms. A larger female can provide parental care for a longer time while the offspring matures. The gestation and lactation periods are fairly long in ''M. nigricans'', the females suckling their offspring until they reach nearly adult size. They would not be able to fly and catch prey if they did not compensate for the additional mass of the offspring during this time. Smaller male size may be an adaptation to increase maneuverability and agility, allowing males to compete better with females for food and other resources.
Some species of anglerfish
The anglerfish are fish of the teleost order Lophiiformes (). They are bony fish named for their characteristic mode of predation, in which a modified luminescent fin ray (the esca or illicium) acts as a lure for other fish. The luminescence c ...
also display extreme sexual dimorphism. Females are more typical in appearance to other fish, whereas the males are tiny rudimentary creatures with stunted digestive systems. A male must find a female and fuse with her: he then lives parasitically, becoming little more than a sperm-producing body in what amounts to an effectively hermaphrodite composite organism. A similar situation is found in the Zeus water bug ''Phoreticovelia disparata
''Phoreticovelia disparata'', also called the Zeus bug, is a species of semi-aquatic bug from the family Veliidae (tribe Microveliini) with a unique form of sexual dimorphism. It is endemic to Australia (Queensland).Polhemus, D. A. & Polhemus, J ...
'' 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). This is taken to the logical extreme in the Rhizocephala
Rhizocephala are derived barnacles that parasitise mostly decapod crustaceans, but can also infest Peracarida, mantis shrimps and thoracican barnacles, and are found from the deep ocean to freshwater. Together with their sister groups Thoracic ...
crustaceans, like the Sacculina
''Sacculina'' is a genus of barnacles that is a parasitic castrator of crabs. They belong to a group called '' Rhizocephala''. The adults bear no resemblance to the barnacles that cover ships and piers; they are recognised as barnacles because t ...
, 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.
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''. There is some evidence that, in these genera, the dimorphism may be tied to a sex chromosome, or to chemical signalling from females.
Another complicated example of sexual dimorphism is in '' Vespula squamosa'', the southern yellowjacket. In this wasp species, the female workers are the smallest, the male workers are slightly larger, and the female queens are significantly larger than her female worker and male counterparts.
Evolution
In 1871, Charles Darwin
Charles Robert Darwin ( ; 12 February 1809 – 19 April 1882) was an English naturalist, geologist, and biologist, widely known for his contributions to evolutionary biology. His proposition that all species of life have descended fr ...
advanced the theory of sexual selection, which related sexual dimorphism with sexual selection
Sexual selection is a mode of natural selection in which members of one biological sex choose mates of the other sex to mate with (intersexual selection), and compete with members of the same sex for access to members of the opposite sex (int ...
.
The first step towards sexual dimorphism is the size differentiation of sperm and eggs (anisogamy
Different forms of anisogamy: A) anisogamy of motile cells, B) egg_cell.html"_;"title="oogamy_(egg_cell">oogamy_(egg_cell_and_sperm_cell),_C)_anisogamy_of_non-motile_cells_(egg_cell_and_spermatia)..html" ;"title="egg_cell_and_sperm_cell.html" ;" ...
). Anisogamy
Different forms of anisogamy: A) anisogamy of motile cells, B) egg_cell.html"_;"title="oogamy_(egg_cell">oogamy_(egg_cell_and_sperm_cell),_C)_anisogamy_of_non-motile_cells_(egg_cell_and_spermatia)..html" ;"title="egg_cell_and_sperm_cell.html" ;" ...
and the usually large number of small male gametes relative to the larger female gametes usually lies in the development of strong sperm competition
Sperm competition is the competitive process between spermatozoa of two or more different males to fertilize the same egg during sexual reproduction. Competition can occur when females have multiple potential mating partners. Greater choice and ...
, because small sperm enable organisms to produce a large number of sperm, and make males (or male function of hermaphrodites) more redundant.
This intensifies male competition for mates and promotes the evolution of other sexual dimorphism in many species, especially in vertebrates
Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, ...
including mammals
Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or ...
. However, in some species females compete for mates in ways more usually associated with males (usually species in which males invest a lot in rearing offspring and thus are no longer considered as so redundant).
Sexual dimorphism by size is evident in some extinct species such as the velociraptor
''Velociraptor'' (; ) is a genus of small dromaeosaurid dinosaur that lived in Asia during the Late Cretaceous epoch, about 75 million to 71 million years ago. Two species are currently recognized, although others have been assigned in the pa ...
. 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.
Volvocine algae have been useful in understanding the evolution of sexual dimorphism and species like the beetle '' C. maculatus'', where the females are larger than the males, are used to study its underlying genetic mechanisms.
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. In these species, there is a selection pressure
Any cause that reduces or increases reproductive success in a portion of a population potentially exerts evolutionary pressure, selective pressure or selection pressure, driving natural selection. It is a quantitative description of the amount of ...
for whatever traits enable a male to have more matings. The male may therefore come to have different traits from the female.
These traits could be ones that allow him to fight off other males for control of territory or a harem
Harem (Persian: حرمسرا ''haramsarā'', ar, حَرِيمٌ ''ḥarīm'', "a sacred inviolable place; harem; female members of the family") refers to domestic spaces that are reserved for the women of the house in a Muslim family. A hare ...
, such as large size or weapons; or they could be traits that females, for whatever reason, prefer in mates. Male–male competition
Sexual selection is a mode of natural selection in which members of one biological sex choose mates of the other sex to mate with (intersexual selection), and compete with members of the same sex for access to members of the opposite sex ( ...
poses no deep theoretical questions but mate choice
Mate choice is one of the primary mechanisms under which evolution can occur. It is characterized by a "selective response by animals to particular stimuli" which can be observed as behavior.Bateson, Paul Patrick Gordon. "Mate Choice." Mate Choic ...
does.
Females may choose males that appear strong and healthy, thus likely to possess "good allele
An allele (, ; ; modern formation from Greek ἄλλος ''állos'', "other") is a variation of the same sequence of nucleotides at the same place on a long DNA molecule, as described in leading textbooks on genetics and evolution.
::"The chro ...
s" and give rise to healthy offspring. 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). Two hypotheses for explaining this fact are the sexy son hypothesis
The sexy son hypothesis in evolutionary biology and sexual selection, proposed by Patrick J. Weatherhead and Raleigh J. Robertson of Queen's University in Kingston, Ontario in 1979, states that a female's ideal mate choice among potential mate ...
and the handicap principle
The handicap principle is a hypothesis proposed by the biologist Amotz Zahavi to explain how evolution may lead to "honest" or reliable Signalling theory, signalling between animals which have an obvious motivation to bluff or deceive each other. ...
.
The sexy son hypothesis states that females may initially choose a trait because it improves the survival of their young, but once this preference has become widespread, females must continue to choose the trait, even if it becomes harmful. Those that do not will have sons that are unattractive to most females (since the preference is widespread) and so receive few matings.
The handicap principle states that a male who survives despite possessing some sort of handicap thus proves that the rest of his genes are "good alleles". If males with "bad alleles" could not survive the handicap, females may evolve to choose males with this sort of handicap; the trait is acting as a hard-to-fake signal of fitness.
See also
*Bateman's principle
Bateman's principle, in evolutionary biology, is that in most species, variability in reproductive success (or reproductive variance) is greater in males than in females. It was first proposed by Angus John Bateman (1919–1996), an English geneti ...
* List of homologues of the human reproductive system
This list of related male and female reproductive organs shows how the male and female reproductive organs and the development of the reproductive system are related, sharing a common developmental path. This makes them biological homologues. T ...
* Sex differences in humans
Sex differences in humans have been studied in a variety of fields. Sex determination occurs by the presence or absence of a Y in the 23rd pair of chromosomes in the human genome. Phenotypic sex refers to an individual's sex as determined by the ...
* Sex differences in human psychology
* Sexual differentiation
Sexual differentiation is the process of development of the sex differences between males and females from an undifferentiated zygote. Sex determination is often distinct from sex differentiation; sex determination is the designation for the deve ...
* Sexual dimorphism in dinosaurs
Sexual dimorphism in dinosaurs refers to the different physical characteristics of male and female dinosaurs of the same species. This means that the male and female dinosaurs of a species may differ in size, color, shape, or they may even look lik ...
* Sexual dimorphism in non-human primates
Sexual dimorphism describes the morphological, physiological, and behavioral differences between males and females of the same species. Most primates are sexually dimorphic for different biological characteristics, such as body size, canine tooth ...
* Sexual dimorphism measures
* Sexually dimorphic nucleus
Sex is the biological distinction of an organism between male and female.
Sex or SEX may also refer to:
Biology and behaviour
*Animal sexual behaviour
**Copulation (zoology)
**Human sexual activity
**Non-penetrative sex, or sexual outercourse
** ...
* Gynandromorphism
References
Sources
* * *Further reading
* * *External links
* {{DEFAULTSORT:Sexual Dimorphism Animal anatomy Sexual selection Polymorphism (biology) Asymmetry