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The sex ratio (or gender ratio) is usually defined as the ratio of
male Male (symbol: ♂) is the sex of an organism that produces the gamete (sex cell) known as sperm, which fuses with the larger female gamete, or ovum, in the process of fertilization. A male organism cannot reproduce sexually without access to ...
s to females in a
population Population typically refers to the number of people in a single area, whether it be a city or town, region, country, continent, or the world. Governments typically quantify the size of the resident population within their jurisdiction usi ...
. As explained by
Fisher's principle Fisher's principle is an evolutionary model that explains why the sex ratio of most species that produce offspring through sexual reproduction is approximately 1:1 between males and females. A. W. F. Edwards has remarked that it is "probably ...
, for evolutionary reasons this is typically about 1:1 in species which reproduce sexually. Many species deviate from an even sex ratio, either periodically or permanently. Examples include parthenogenic species, periodically mating organisms such as aphids, some eusocial wasps,
bee Bees are winged insects closely related to wasps and ants, known for their roles in pollination and, in the case of the best-known bee species, the western honey bee, for producing honey. Bees are a monophyletic lineage within the superfami ...
s, ants, and termites. The human sex ratio is of particular interest to anthropologists and demographers. In human societies, sex ratios at birth may be considerably skewed by factors such as the age of mother at birth and by sex-selective abortion and infanticide. Exposure to pesticides and other environmental contaminants may be a significant contributing factor as well. As of 2014, the global sex ratio at birth is estimated at 107 boys to 100 girls (1,000 boys per 934 girls)..


Types

In most species, the sex ratio varies according to the age profile of the population. It is generally divided into four subdivisions: * — ratio at fertilization * — ratio at birth * — ratio in sexually mature organisms :This is equivalent to the (ASR), which is defined as the ratio of adult males to females in a population. :*The operational sex ratio (OSR) is the ratio of ''sexually active'' males to females in a population, and is therefore derived from a subset of the individuals included when calculating the ASR. Although some research has equated these two values, they are conceptually different. * — ratio in post-reproductive organisms These definitions can be somewhat subjective since they lack clear boundaries.


Sex ratio theory

Sex ratio theory is a field of academic study which seeks to understand the sex ratios observed in nature from an evolutionary perspective. It continues to be heavily influenced by the work of
Eric Charnov Eric Lee Charnov (born October 29, 1947) is an American evolutionary ecologist. He is best known for his work on foraging, especially the marginal value theorem, and life history theory, especially sex allocation and scaling/allometric rules. ...
. He defines five major questions, both for his book and the field in general (slightly abbreviated here): #For a dioecious species, what is the equilibrium sex ratio maintained by natural selection? #For a
sequential hermaphrodite Sequential hermaphroditism (called dichogamy in botany) is a type of hermaphroditism that occurs in many fish, gastropods, and plants. Sequential hermaphroditism occurs when the individual changes its sex at some point in its life. In particular ...
, what is the equilibrium sex order and time of
sex change Sex change is a natural or artificial process in which an individual's sex is changed. Sex change may also refer to: Biology and medicine *Sex reassignment therapy * Sex reassignment surgery * Sequential hermaphroditism, a phenomenon whereby so ...
? #For a
simultaneous hermaphrodite Simultaneous hermaphroditism is one of the two types of hermaphroditism, the other type being sequential hermaphroditism. In this form of hermaphroditism an individual has sex organs of both sexes and can produce both gamete types even in the s ...
, what is the equilibrium allocation of resources to male versus female function in each breeding season? #Under what conditions are the various states of hermaphroditism or dioecy evolutionarily stable? When is a mixture of sexual types stable? #When does selection favour the ability of an individual to alter its allocation to male versus female function, in response to particular environmental or life history situations? Biological research mostly concerns itself with sex ''allocation'' rather than sex ratio, sex allocation denoting the allocation of energy to either sex. Common research themes are the effects of local mate and resource competition (often abbreviated LMC and LRC, respectively).


Fisher's principle

Fisher's principle (1930) explains why in most species, the sex ratio is approximately 1:1. His argument was summarised by W. D. Hamilton (1967) as follows, assuming that parents invest the same whether raising male or female offspring: :# Suppose male births are less common than female. :# A newborn male then has better mating prospects than a newborn female, and therefore can expect to have more offspring. :# Therefore parents genetically disposed to produce males tend to have more than average numbers of grandchildren born to them. :# Therefore the genes for male-producing tendencies spread, and male births become more common. :# As the 1:1 sex ratio is approached, the advantage associated with producing males dies away. :# The same reasoning holds if females are substituted for males throughout. Therefore 1:1 is the equilibrium ratio. In modern language, the 1:1 ratio is the evolutionarily stable strategy (ESS). This ratio has been observed in many species, including the bee ''
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 '' Sphaeralc ...
''. A study performed by Danforth observed no significant difference in the number of males and females from the 1:1 sex ratio.


Examples in non-human species


Environmental and individual control

Spending equal amounts of resources to produce offspring of either sex is an evolutionarily stable strategy: if the general population deviates from this equilibrium by favoring one sex, one can obtain higher reproductive success with less effort by producing more of the other. For species where the cost of successfully raising one offspring is roughly the same regardless of its sex, this translates to an approximately equal sex ratio. Bacteria of the genus '' Wolbachia'' cause skewed sex ratios in some arthropod species as they kill males. Sex-ratio of adult populations of pelagic copepods is usually skewed towards dominance of females. However, there are differences in adult sex ratios between families: in families in which females require multiple matings to keep producing eggs, sex ratios are less biased (close to 1); in families in which females can produce eggs continuously after only one mating, sex ratios are strongly skewed towards females. Several species of reptiles have temperature-dependent sex determination, where incubation temperature of eggs determines the sex of the individual. In the American alligator, for example, females are hatched from eggs incubated between , whereas males are hatched from eggs . In this method, however, all eggs in a clutch (20–50) will be of the same sex. In fact, the natural sex ratio of this species is five females to one male. In birds, mothers can influence the sex of their chicks. In peafowl, maternal body condition can influence the proportion of daughters in the range from 25% to 87%. In several groups of fish, such as
wrasses The wrasses are a family, Labridae, of marine fish, many of which are brightly colored. The family is large and diverse, with over 600 species in 81 genera, which are divided into 9 subgroups or tribes. They are typically small, most of them ...
, parrotfish and clownfish,
dichogamy Sequential hermaphroditism (called dichogamy in botany) is a type of hermaphroditism that occurs in many fish, gastropods, and plants. Sequential hermaphroditism occurs when the individual changes its sex at some point in its life. In particular ...
— or sequential hermaphroditism — is normal. This can cause a discrepancy in the sex ratios as well. In the bluestreak cleaner wrasse, there is only one male for every group of 6-8 females. If the male fish dies, the strongest female changes its sex to become the male for the group. All of these wrasses are born female, and only become male in this situation. Other species, like clownfish, do this in reverse, where all start out as non-reproductive males, and the largest male becomes a female, with the second-largest male maturing to become reproductive.


Domesticated animals

Traditionally, farmers have discovered that the most economically efficient community of animals will have a large number of females and a very small number of males. A herd of cows with a few bulls or a flock of hens with one rooster are the most economical sex ratios for domesticated livestock.


Dioecious plants secondary sex ratio and amount of pollen

It was found that the amount of fertilizing pollen can influence secondary sex ratio in dioecious plants. Increase in pollen amount leads to decrease in number of male plants in the progeny. This relationship was confirmed on four plant species from three families – '' Rumex acetosa'' (
Polygonaceae The Polygonaceae are a family of flowering plants known informally as the knotweed family or smartweed—buckwheat family in the United States. The name is based on the genus ''Polygonum'', and was first used by Antoine Laurent de Jussieu in ...
), ''Melandrium album'' (Caryophyllaceae), '' Cannabis sativa'' and ''
Humulus japonicus ''Humulus japonicus'', known as Japanese hop, is an ornamental plant in the family Cannabaceae. Some authorities have it as a synonym of ''Humulus scandens''. Originally native to East Asian countries such as China, Japan, Korea, and extending i ...
'' ( Cannabinaceae).


Polyandrous and cooperatively breeding homeotherms

In
charadriiform Charadriiformes (, from ''Charadrius'', the type genus of family Charadriidae) is a diverse order of small to medium-large birds. It includes about 390 species and has members in all parts of the world. Most charadriiform birds live near water a ...
birds, recent research has shown clearly that polyandry and sex-role reversal (where males care and females compete for mates) as found in
phalarope __NOTOC__ A phalarope is any of three living species of slender-necked shorebirds in the genus ''Phalaropus'' of the bird family Scolopacidae. Phalaropes are close relatives of the shanks and tattlers, the '' Actitis'' and Terek sandpipers ...
s, jacanas, painted snipe and a few
plover Plovers ( , ) are a widely distributed group of wading birds belonging to the subfamily Charadriinae. Description There are about 66 species in the subfamily, most of them called "plover" or "dotterel". The closely related lapwing subf ...
species is clearly related to a strongly male-biased adult sex ratio. Those species with male care and polyandry invariably have adult sex ratios with a large surplus of males, which in some cases can reach as high as six males per female. Male-biased adult sex ratios have also been shown to correlate with cooperative breeding in mammals such as
alpine marmot The alpine marmot (''Marmota marmota'') is a large ground-dwelling squirrel, from the genus of marmots. It is found in high numbers in mountainous areas of central and southern Europe, at heights between in the Alps, Carpathians, Tatras and No ...
s and wild canids. This correlation may also apply to cooperatively breeding birds, though the evidence is less clear. It is known, however, that both male-biased adult sex ratios and cooperative breeding tend to evolve where caring for offspring is extremely difficult due to low secondary productivity, as in
Australia Australia, officially the Commonwealth of Australia, is a sovereign country comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands. With an area of , Australia is the largest country by ...
and Southern Africa. It is also known that in cooperative breeders where both sexes are philopatric like the
varied sittella The varied sittella (''Daphoenositta chrysoptera'') is a small, around 10–11 cm long, songbird native to Australia. It is also known as the Australian nuthatch, orange-winged sittella and the barkpecker. Taxonomy The varied sittella was f ...
, adult sex ratios are equally or more male-biased than in those cooperative species, such as fairy-wrens,
treecreepers The treecreepers are a family, Certhiidae, of small passerine birds, widespread in wooded regions of the Northern Hemisphere and sub-Saharan Africa. The family contains eleven species in two genera, ''Certhia'' and ''Salpornis''. Their plumage i ...
and the noisy miner where females always disperse.


See also

* Evolution of sex * Operational sex ratio * Sex allocation * Trivers–Willard hypothesis * XY sex-determination system Humans: * Human sex ratio * List of countries by sex ratio * Bride kidnapping * Groom kidnapping * Demographic transition * Sex selection * Sex-selective abortion and infanticide * Youth bulge Institutions: *
Gender Balance Council The Gender Balance Council (GBC) is an Emirati federal entity responsible for developing and implementing the gender balance agenda in the United Arab Emirates. The Council is chaired by Manal bint Mohammed bin Rashid Al Maktoum, President of ...


Notes


References

* Also printed as * *{{cite journal , author1=Trivers R.L. , author-link1=Robert Trivers , author2=Willard D.E., author-link2=Dan Willard , title=Natural selection of parental ability to vary the sex ratio of offspring , journal=Science , volume=179 , pages=90–2 , year=1973 , doi = 10.1126/science.179.4068.90 , pmid=4682135 , issue=4068 , bibcode = 1973Sci...179...90T , s2cid=29326420 *Rath, R.M., and Mishra A.K. (2005). ''Techniques for Sex Ratio Analysis.'' Association of Professional Geographers.


External links


CIA listing of sex ratios for individual countries (including age divisions)


Population Selection