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The sex ratio is the
ratio In mathematics, a ratio indicates how many times one number contains another. For example, if there are eight oranges and six lemons in a bowl of fruit, then the ratio of oranges to lemons is eight to six (that is, 8∶6, which is equivalent to ...

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 sexual reproduction, reproduce sexually ...

male
s to
female Female (symbol: ♀) is the sex Sex is either of two divisions, typically male Male (♂) is the sex of an organism that produces the gamete known as sperm. A male gamete can fuse with a larger female gamete, or ovum, in the process of ...

female
s in a
population Population typically refers the number of people in a single area whether it be a city or town, region, country, or the world. Governments typically quantify the size of the resident population within their jurisdiction by a process called a ...

population
. In most sexually reproducing species, the ratio tends to be 1:1. This tendency is explained by
Fisher's principleFisher's principle is an evolution Evolution is change in the Heredity, heritable Phenotypic trait, characteristics of biological populations over successive generations. These characteristics are the Gene expression, expressions of genes that ...
. For various reasons, however, many species deviate from anything like an even sex ratio, either periodically or permanently. Examples include
parthenogenic Parthenogenesis (; from the Greek grc, παρθένος, translit=parthénos, lit=virgin, label=none + grc, γένεσις, translit=génesis, lit=creation, label=none) is a natural form of asexual reproduction Asexual reproduction is a typ ...
species, periodically mating organisms such as aphids, some
eusocial Eusociality (from Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10.7 mil ...
wasp A wasp is any insect of the narrow-waisted suborder Apocrita of the order Hymenoptera which is neither a bee nor an ant; this excludes the broad-waisted sawflies (Symphyta), which look somewhat like wasps, but are in a separate suborder. The ...

wasp
s such as ''
Polistes fuscatus ''Polistes fuscatus'', whose common name is the dark or northern paper wasp, is widely found throughout southern Canada, the United States, Mexico, and Central America.Metcalf, R.A.; J.C. Marlin; and G. S. Whitt. (1984). Genetics of Speciation w ...

Polistes fuscatus
'' and ''
Polistes exclamans ''Polistes exclamans'', the Guinea paper wasp, is a social Social organisms, including humans, live collectively in interacting populations. This interaction is considered social whether they are aware of it or not, and whether the exchange is ...
'', bees, ants, and termites. The
human sex ratio File:Sex ratio total population per country 2020 (ages over 65).svg, 300x300px, Sex ratio by country for the over-65 population. Blue represents more men, red more women than the world average of 0.81 males/female. In anthropology and demography ...

human sex ratio
is of particular interest to anthropologists and demographers. In human societies, however, sex ratios at birth may be considerably skewed by factors such as the age of mother at birth, and by
sex-selective abortion Sex-selective abortion is the practice of terminating a pregnancy based upon the predicted sex of the infant. The selective abortion of female fetuses is most common where male children are valued over female children, especially in parts of Eas ...
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: * primary sex ratio — ratio at fertilization * secondary sex ratio — ratio at birth * tertiary sex ratio — ratio in sexually mature organisms ** Also called ''adult sex ratio'' and abbreviated to ''ASR''. ASR is defined as the proportion of adults in a population that are male. **''
Operational sex ratio In the evolutionary biology Evolutionary biology is the subfield of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular ...
'' abbreviated as ''OSR'' is the proportion of adults in the sexually active population that are males. 'OSR' has often been confused with 'ASR' although these are conceptually different. * quaternary sex ratio — ratio in post-reproductive organisms These definitions can be somewhat subjective since they lack clear boundaries.


Sex ratio theory

The theory of sex ratio is a field of study concerned with the accurate prediction of sex ratios in all sexual species, based on a consideration of their
natural history Natural history is a domain of inquiry involving organisms, including animals, fungus, fungi, and plants, in their natural environment, leaning more towards observational than experimental methods of study. A person who studies natural history ...

natural history
. The field continues to be heavily influenced by
Eric CharnovEric L. Charnov (born October 29, 1947) is an American evolutionary ecologist. He is best known for his work on foraging Foraging is searching for wild food resources. It affects an animal's fitness because it plays an important role in an anima ...
’s 1982 book, ''Sex Allocation''. He defines five major questions, both for his book and the field in general (slightly abbreviated here): #For a
dioecious Dioecy (; Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10.7 million as o ...
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, a ...
, what is the equilibrium sex order and time of
sex change Sex change is a process by which a living being changes sex – that is, by which female sexual characteristics are substituted for male ones or vice versa. Sex change may occur naturally, as in the case of the sequential hermaphroditism Sequentia ...
? #For a
simultaneous hermaphrodite In reproductive biology, a hermaphrodite () is an organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the Life#Biology, properties of life ...
, 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 explains why for most species, the sex ratio is approximately 1:1. Bill Hamilton expounded Fisher’s argument in his 1967 paper on “Extraordinary sex ratios” as follows, given the assumption of equal parental expenditure on offspring of both sexes. :# 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 North America is a continent entirely within the Northern Hemisphere and almost all ...
. 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 Bacteria (; common noun bacteria, singular bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typ ...
of the genus ''
Wolbachia ''Wolbachia'' is a genus Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank In biological classification In biology Biology is the natural science that studies life and living organisms, including their ...

Wolbachia
'' cause skewed sex ratios in some
arthropod An arthropod (, (gen. ποδός)) is an invertebrate animal having an exoskeleton, a Segmentation (biology), segmented body, and paired jointed appendages. Arthropods form the phylum Euarthropoda,Reference showing that Euarthropoda is a phylum: ...
species as they kill males. Sex-ratio of adult populations of pelagic
copepods Copepods (; meaning "oar-feet") are a group of small crustaceans found in nearly every freshwater and saltwater habitat (ecology), habitat. Some species are planktonic (inhabiting sea waters), some are benthos, benthic (living on the ocean floor) ...
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 Temperature-dependent sex determination (TSD) is a type of environmental sex determination Environmental sex determination is the Sex-determination system, establishment of sex by a non-genetic cue, such as nutrient availability, experienced within ...
, where incubation temperature of eggs determines the sex of the individual. In the
American alligator The American alligator (''Alligator mississippiensis''), sometimes referred to colloquially as a gator or common alligator, is a large crocodilian reptile native to the Southeastern United States and northeastern Mexico. It is one of two extan ...

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 Peafowl is a common name for three bird species in the genera ''Pavo (genus), 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 pe ...

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 Fish are Aquatic animal, aquatic, craniate, gill-bearing animals that lack Limb (anatomy), limbs with Digit (anatomy), digits. They form a sister group to the tunicates, together forming ...
,
parrotfish Parrotfishes are a group of about 95 fish species regarded as a family (Scaridae), or a subfamily (Scarinae) of the wrasses. With about 95 species, this group's largest species richness is in the Indo-Pacific. They are found in coral reefs, ...

parrotfish
and
clownfish '') in their anemone home File:Clown Fish Swimming.jpg, A clownfish swimming Clownfish or anemonefish are fish Fish are Aquatic animal, aquatic, craniate, gill-bearing animals that lack Limb (anatomy), limbs with Digit (anatomy), digits. ...

clownfish
,
dichogamy Sequential hermaphroditism (called dichogamy in botany Botany, also called , plant biology or phytology, is the science of plant life and a branch of biology. A botanist, plant scientist or phytologist is a scientist who specialises in this ...
— or sequential hermaphoditism — 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 Sorrel (''Rumex acetosa''), also called common sorrel or garden sorrel, is a perennial A perennial plant or simply perennial is a plant that lives more than two years. The term ('' per-'' + '' -ennial'', "through the years") is often u ...
'' (
Polygonaceae The Polygonaceae are a family In human society A society is a Social group, group of individuals involved in persistent Social relation, social interaction, or a large social group sharing the same spatial or social territory, typicall ...
), ''Melandrium album'' (Caryophyllaceae), ''
Cannabis sativa ''Cannabis sativa'' is an annual indigenous to , but now of cosmopolitan distribution due to widespread cultivation. It has been cultivated throughout , used as a source of , , , , and . Each part of the plant is harvested differently, depend ...

Cannabis sativa
'' and '''' ( Cannabinaceae).


Polyandrous and cooperatively breeding homeotherms

In birds, recent research has shown clearly that
polyandry Polyandry (; from grc-gre, πολυ- ''poly-'', "many" and ἀνήρ ''anēr'', "man") is a form of polygamy Polygamy (from Late Greek Late Greek means writings in the Greek language Greek (modern , romanized: ''Elliniká'', Ancient ...
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 In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as ...
s, jacanas,
painted snipe The Rostratulidae, commonly known as the painted-snipes, are a family In human society A society is a Social group, group of individuals involved in persistent Social relation, social interaction, or a large social group sharing th ...
and a few
plover Plovers ( or ) are a widely distributed group of wader, wading birds belonging to the subfamily Charadriinae. Description There are about 66 species in the subfamily, most of them called "plover" or "Dotterel (disambiguation), dotterel". Th ...
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 Cooperative breeding is a social system characterized by alloparental care: offspring receive care not only from their parents, but also from additional group members, often called helpers. Cooperative breeding encompasses a wide variety of group st ...
in mammals such as
alpine marmot The alpine marmot (''Marmota marmota'') is a large Ground squirrel, 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, Carpathian ...
s and wild
canids Canidae (; from Latin, ''canis'', "dog") is a biological family Family ( la, familia, plural ') is one of the eight major hierarchical taxonomic ranks in Linnaean taxonomy; it is classified between order (biology), order and genus. A famil ...
. 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 state, sovereign country comprising the mainland of the Australia (continent), Australian continent, the island of Tasmania, and numerous List of islands of Australia, sma ...

Australia
and
Southern Africa Southern Africa is the south South is one of the cardinal directions or compass points. South is the opposite of north and is perpendicular to the east and west. Etymology The word ''south'' comes from Old English ''sūþ'', from earlier Pr ...
. 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 fir ...
, adult sex ratios are equally or more male-biased than in those cooperative species, such as fairy-wrens, treecreepers and the
noisy miner The noisy miner (''Manorina melanocephala'') is a bird in the honeyeater family, Meliphagidae, and is endemic Endemism is the state of a species In biology, a species is the basic unit of biological classification, classification and ...
where females always disperse.


See also

* Evolution of sex *
Operational sex ratio In the evolutionary biology Evolutionary biology is the subfield of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular ...
*
Sex allocation Sex allocation is the allocation of resources to male versus female reproduction in sexual species.Charnov EL. (1982). ''The Theory of Sex Allocation''. Princeton University Press, Princeton, NJ. Sex allocation theory tries to explain why many speci ...
*
Trivers–Willard hypothesisIn evolutionary biology Evolutionary biology is the subfield of biology that studies the evolution, evolutionary processes (natural selection, common descent, speciation) that produced the Biodiversity, diversity of life on Earth. In the 1930s, ...
*
XY sex-determination system The XY sex-determination system is a sex-determination system A sex-determination system is a biological system that determines the development of sexual characteristics in an organism In biology, an organism (from Ancient Greek, Gree ...
Humans: *
Human sex ratio In anthropology Anthropology is the of ity, concerned with , , , and , in both the present and past, including . studies patterns of behaviour, while studies cultural meaning, including norms and values. studies how language influenc ...

Human sex ratio
*
List of countries by sex ratio File:Sex ratio total population per country 2020 (age 0-14).svg, 300x300px, Sex ratio by country for population aged below 15. Red represents more girls, blue more boys than the world average of 1.07 males/females. (2020) The human sex ratio F ...
*
Bride kidnapping Bride kidnapping, also known as bridenapping, marriage by abduction or marriage by capture, is a practice in which a man abducts the woman he wishes to marry. Bride kidnapping has been practiced around the world and throughout prehistory and ...
* Groom kidnapping *
Demographic transition In demography Demography (from prefix ''demo-'' from Ancient Greek δῆμος (''dēmos'') meaning 'the people', and ''-graphy'' from γράφω (''graphō'') meaning 'writing, description or measurement') is the statistics, statistical s ...
*
Sex selection Sex is either of two divisions, typically male Male (♂) is the sex of an organism that produces the gamete known as sperm. A male gamete can fuse with a larger female gamete, or ovum, in the process of fertilization. A male cannot sexual r ...
* Sex-selective abortion and infanticide *
Youth bulge A population pyramid or "age-gender-pyramid" is a graphical illustration of the distribution of a population In biology, a population is a number of all the organisms of the same group or species In biology, a species is the basic uni ...
Institutions: * Gender Balance Council


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 Ratios Selection