A sex ratio is the ratio of males to females in a population. As explained by Fisher's principle, for evolutionary reasons this is usually about equal in species which reproduce sexually.
Types
In most species, the sex ratio varies according to the age profile of the population.
It is generally divided into four subdivisions:
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— ratio at fertilization
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— ratio at birth
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— ratio in sexually mature organisms
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The tertiary sex ratio is equivalent to the (ASR), which is defined as the ratio of adult males to females in a population.
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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.
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— 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.
He defines five major questions, both for his book and the field in general (slightly abbreviated here):
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For a dioecious species, what is the equilibrium sex ratio maintained by natural selection?
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For a sequential hermaphrodite, what is the equilibrium sex order and time of sex change?
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For a simultaneous hermaphrodite, what is the equilibrium allocation of resources to male versus female function in each breeding season?
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Under what conditions are the various states of hermaphroditism or dioecy evolutionarily stable? When is a mixture of sexual types stable?
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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:
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Suppose male births are less common than female.
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A newborn male then has better mating prospects than a newborn female, and therefore can expect to have more offspring.
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Therefore parents genetically disposed to produce males tend to have more than average numbers of grandchildren born to them.
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Therefore the genes for male-producing tendencies spread, and male births become more common.
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As the 1:1 sex ratio is approached, the advantage associated with producing males dies away.
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The same reasoning holds if females are substituted for males throughout. Therefore 1:1 is the equilibrium ratio.
This means that the 1:1 ratio is the evolutionarily stable strategy.
Human sex ratio
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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 2024, the global sex ratio at birth is estimated at 107 boys to 100 girls (1,000 boys per 934 girls).
[ (2023 estimates)] By old age, the sex ratio reverses, with 81 older men for every 100 older women; across all ages, the global population is nearly balanced, with 101 males for every 100 females.
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.
bacterium 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%.
Dichogamy (sequential hermaphroditism) is normal in several groups of fish, such as wrasses, parrotfish and clownfish. 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),
(Caryophyllaceae),
and
Humulus japonicus (
Cannabinaceae).
[ (cit.: Plant Breeding Abstr., 1934, 5, № 3, p. 248, ref. № 768).]
Polyandrous and cooperatively breeding homeotherms
In
Charadriiformes birds, recent research has shown clearly that polyandry and sex-role reversal (where males care and females compete for mates) as found in
,
Jacanidae,
painted snipe and a few
plover 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 and wild Canidae.[Allainé, Dominique; Brondex, Francine; Graziani, Laurent; Coulon, Jacques and Till-Bottraud, Irène; "Male-biased sex ratio in litters of alpine marmots supports the helper repayment hypothesis"] This correlation may also apply to cooperatively breeding birds,
See also
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Evolution of sex
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Operational sex ratio
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Sex allocation
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Trivers–Willard hypothesis
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XY sex-determination system
Humans:
Further reading
External links
