Sex ratio
Template:Short description Template:Redirect Template:Cs1 config 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.<ref name="Fisher">Template:Cite book</ref><ref name="Hamilton">Template:Cite journal</ref> However, many species deviate from an even sex ratio, either periodically or permanently. These include parthenogenic and androgenetic<ref name="Schwander-2016">Template:Cite journal</ref> species, periodically mating organisms such as aphids, some eusocial wasps, bees, ants, and termites.<ref>Template:Cite journal</ref>
Types
In most species, the sex ratio varies according to the age profile of the population.<ref>Template:Cite journal</ref>
It is generally divided into four subdivisions:
- Template:Vanchor — ratio at fertilization
- Template:Vanchor — ratio at birth
- Template:Vanchor — ratio in sexually mature organisms
- The tertiary sex ratio is equivalent to the Template:Vanchor (ASR), which is defined as the ratio of adult males to females in a population.<ref>Template:Cite journal</ref><ref name="Kvarnemo">Template:Cite book</ref>
- 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.<ref name="Kvarnemo" /> Although conceptually distinct, researchers have sometimes equated the ASR with the OSR, particularly in experimental studies of animals where the difference between the two values may not always be readily apparent.<ref name="Székely">Template:Cite journal</ref>
- Template:Vanchor — 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.<ref name="Charnov">Template:Cite book</ref> 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, what is the equilibrium sex order and time of sex change?
- For a simultaneous hermaphrodite, 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
Template:Main Fisher's principle (1930)<ref name="Fisher"/> explains why in most species, the sex ratio is approximately 1:1. His argument was summarised by W. D. Hamilton (1967)<ref name="Hamilton"/> 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.
This means that the 1:1 ratio is the evolutionarily stable strategy.<ref name=MaynardSmithPrice73>Template:Cite journal</ref> This ratio has been observed in many species, including the bee Macrotera portalis. A study performed by Danforth observed no significant difference in the number of males and females from the 1:1 sex ratio.<ref name="Danforth2">Template:Cite journal</ref>
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<ref name="cdc">Template:Cite web</ref> and by sex-selective abortion and infanticide. Some families also stop having children after they have both boys and girls.<ref name="Wang"/> Exposure to pesticides and other environmental contaminants may be a significant contributing factor as well.<ref>Template:Cite journal</ref> As of 2024, the global sex ratio at birth is estimated at 105 boys to 100 girls.<ref name=":0">Template:Cite web (2024 estimates)</ref> The sex ratio is reversed for people 65 and older, where there are 81 men for every 100 women. Across all ages, the global population is nearly balanced, with 101 males for every 100 females.<ref name=":0" />
A large 2025 epidemiological study of American nurses<ref name="Wang">Template:Cite journal</ref> found the chances of a specific couple giving birth to a boy or girl are not always 50-50, and the overall pattern follows a beta-binomial distribution. The number of families with three or more children that have all boys or all girls is higher than would be expected by random chance. Families with three girls had a 58% chance of having a fourth, and families with three boys had a 61% chance of having a fourth.<ref>Template:Cite news</ref> Women 29 or older at first birth had a 13% higher chance of having only boys or only girls.<ref>Template:Cite journal</ref> Some researchers questions why the same pattern was not seen in a study of the population of Sweden.<ref>A child’s biological sex may not always be a random 50-50 chance</ref>
Factors which may affect birth sex unconfirmed by large studies include vaginal pH, length of the follicular phase of the menstrual cycle, the menstrual cycle phase at conception, and genetic factors. Other unconfirmed correlations with inherited traits include attractiveness, wealth, aggression, and body size. A 2025 study of over 58,000 American women suggested a correlation with two single-nucleotide mutations in the mother — NSUN6 increasing the likelihood of only female children and TSHZ1 increasing the likelihood of only male children.<ref name="Wang" />
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.<ref>Template:Cite journal</ref>
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 Template:Convert, whereas males are hatched from eggs Template:Convert. 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.<ref>Template:Cite journal</ref>
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%.<ref>Template:Cite journal</ref>
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.Template:Citation needed
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),<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> Silene alba (Caryophyllaceae),<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> Cannabis sativa<ref>Template:Cite journal</ref> and Humulus japonicus (Cannabinaceae).<ref>Template:Cite journal (cit.: Plant Breeding Abstr., 1934, 5, № 3, p. 248, ref. № 768).</ref>
Polyandrous and cooperatively breeding homeotherms
In charadriiform birds, recent research has shown clearly that polyandry and sex-role reversal (where males care and females compete for mates) as found in phalaropes, jacanas, painted snipe and a few plover species is clearly related to a strongly male-biased adult sex ratio.<ref name="evolution">Template:Cite journal</ref> Those species with male care and polyandry invariably have adult sex ratios with a large surplus of males,<ref name="evolution"/> which in some cases can reach as high as six males per female.<ref>Template:Cite journal</ref>
Male-biased adult sex ratios have also been shown to correlate with cooperative breeding in mammals such as alpine marmots and wild canids.<ref>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"</ref> This correlation may also apply to cooperatively breeding birds,<ref>Template:Cite journal</ref> though the evidence is less clear.<ref name="evolution"/> It is known, however, that both male-biased adult sex ratios<ref name="parental">Template:Cite journal</ref> and cooperative breeding tend to evolve where caring for offspring is extremely difficult due to low secondary productivity, as in Australia<ref>Template:Cite journal</ref> and Southern Africa. It is also known that in cooperative breeders where both sexes are philopatric like the varied sittella,<ref>Template:Cite journal</ref> adult sex ratios are equally or more male-biased than in those cooperative species, such as fairy-wrens, treecreepers and the noisy miner<ref>Template:Cite journal</ref> where females always disperse.
Extreme female-biased ratios in parasitoid wasps
Some parasitoid wasps, particularly those in the genus Sclerodermus, exhibit extremely female-biased sex ratios, often with fewer than 10% of offspring being male. This is unusual because classical local mate competition theory predicts that as more females contribute offspring within a group, the sex ratio should become more balanced, since males are no longer only competing with their brothers for mates.<ref name=":1">Template:Cite journal</ref>
Recent research has shown that social interactions among egg-laying females) can explain this discrepancy. Mathematical models suggest that dominant females may kill the sons of subordinates or prevent them from producing sons altogether. These behaviors, forms of infanticide and reproductive dominance, can lead to highly skewed sex ratios at both the individual and group level, even when all females cooperate in raising the brood.<ref name=":1" />
See also
- Evolution of sex
- Operational sex ratio
- Sex allocation
- Trivers–Willard hypothesis
- XY sex-determination system
Humans:
- List of countries by sex ratio
- Bride kidnapping
- Groom kidnapping
- Demographic transition
- Sex selection
- Sex-selective abortion and infanticide
- Youth bulge
References
Further reading
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