Sex ratio
Encyclopedia
Sex ratio is the ratio
of male
s to female
s in a population
. The primary sex ratio is the ratio at the time of conception, secondary sex ratio is the ratio at time of birth, and tertiary sex ratio is the ratio of mature organisms.
The human sex ratio
is of particular interest to anthropologists and demographers. In humans the secondary sex ratio is commonly assumed to be 105 boy
s to 100 girl
s (which sometimes is shortened to "a ratio of 105"). In human societies, however, sex ratios at birth may be considerably skewed by natural reasons such as the age of mother at birth, and unnatural reasons such as sex-selective abortion
. The CIA estimates that the current world wide sex ratio at birth is 107 boys to 100 girls. In 2010, the global sex ratio was 986 females per 1,000 males and trended to reduce to 984 in 2011.
In biology
, sex ratio is defined as the proportion of males in the population.
. The field continues to be heavily influenced by Eric Charnov
's 1982 book, Sex Allocation. He defines five major questions, both for his book and the field in general (slightly abbreviated here):
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).
In modern language, the 1:1 ratio is the evolutionarily stable strategy
(ESS).
Measuring these is a problem since there are no clear boundaries between them.
s to 100 girl
s (which sometimes is shortened to "a ratio of 105"). In human societies, however, sex ratios at birth may be considerably skewed even in economically developed countries such as the United States by natural reasons such as the age of mother at birth and ethnicity, and unnatural reasons in other countries because of sex-selective abortion and infanticide.
: 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 27.7° to 30 °C, whereas males are hatched from eggs 32.2–33.8 °C. In this method, however, all eggs in a clutch (20–50) will be of the same gender. In fact, the natural sex ratio of this species is five females to one male.
), Melandrium album (Cariophyllaceae), Cannabis sativa
and Humulus japonicus
(Cannabinaceae).
, maternal body condition can influence the proportion of daughters in the range from 25% to 87%.
In several different groups of fish, such as the Wrasses, Parrotfish
, Clownfish
, dichogamy
— 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 wrasse 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.
Ratio
In mathematics, a ratio is a relationship between two numbers of the same kind , usually expressed as "a to b" or a:b, sometimes expressed arithmetically as a dimensionless quotient of the two which explicitly indicates how many times the first number contains the second In mathematics, a ratio is...
of male
Male
Male refers to the biological sex of an organism, or part of an organism, which produces small mobile gametes, called spermatozoa. Each spermatozoon can fuse with a larger female gamete or ovum, in the process of fertilization...
s to female
Female
Female is the sex of an organism, or a part of an organism, which produces non-mobile ova .- Defining characteristics :The ova are defined as the larger gametes in a heterogamous reproduction system, while the smaller, usually motile gamete, the spermatozoon, is produced by the male...
s in a population
Population
A population is all the organisms that both belong to the same group or species and live in the same geographical area. The area that is used to define a sexual population is such that inter-breeding is possible between any pair within the area and more probable than cross-breeding with individuals...
. The primary sex ratio is the ratio at the time of conception, secondary sex ratio is the ratio at time of birth, and tertiary sex ratio is the ratio of mature organisms.
The human sex ratio
Human sex ratio
In anthropology and demography, the human sex ratio is the sex ratio for Homo sapiens . Like most sexual species, the sex ratio is approximately 1:1. In humans the secondary sex ratio is commonly assumed to be 105 boys to 100 girls, an assumption that is a subject of debate in the scientific...
is of particular interest to anthropologists and demographers. In humans the secondary sex ratio is commonly assumed to be 105 boy
Boy
A boy is a young male human , as contrasted to its female counterpart, girl, or an adult male, a man.The term "boy" is primarily used to indicate biological sex distinctions, cultural gender role distinctions or both...
s to 100 girl
Girl
A girl is any female human from birth through childhood and adolescence to attainment of adulthood. The term may also be used to mean a young woman.-Etymology:...
s (which sometimes is shortened to "a ratio of 105"). In human societies, however, sex ratios at birth may be considerably skewed by natural reasons such as the age of mother at birth, and unnatural reasons such as sex-selective abortion
Sex-selective abortion
Sex-selective abortion is the practice of terminating a pregnancy based upon the predicted sex of the baby. The selective abortion of female fetuses is most common in areas where cultural norms value male children over female children, especially in parts of People's Republic of China, India,...
. The CIA estimates that the current world wide sex ratio at birth is 107 boys to 100 girls. In 2010, the global sex ratio was 986 females per 1,000 males and trended to reduce to 984 in 2011.
In biology
Biology
Biology is a natural science concerned with the study of life and living organisms, including their structure, function, growth, origin, evolution, distribution, and taxonomy. Biology is a vast subject containing many subdivisions, topics, and disciplines...
, sex ratio is defined as the proportion of males in the population.
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 historyNatural history
Natural history is the scientific research of plants or animals, leaning more towards observational rather than experimental methods of study, and encompasses more research published in magazines than in academic journals. Grouped among the natural sciences, natural history is the systematic study...
. The field continues to be heavily influenced by Eric Charnov
Eric Charnov
Eric L. Charnov 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 is a MacArthur Fellow and a Fellow of the American Academy of Arts and...
's 1982 book, Sex Allocation. He defines five major questions, both for his book and the field in general (slightly abbreviated here):
- For a dioeciousDioeciousDioecy is the property of a group of biological organisms that have males and females, but not members that have organs of both sexes at the same time. I.e., those whose individual members can usually produce only one type of gamete; each individual organism is thus distinctly female or male...
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 changeSex changeSex change is a term often used for gender reassignment therapy, that is, all medical procedures transgendered people can have, or specifically to sexual reassignment surgery, which usually refers to genitalia surgery only...
? - 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
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 through-out. Therefore 1:1 is the equilibrium ratio.
In modern language, the 1:1 ratio is the evolutionarily stable strategy
Evolutionarily stable strategy
In game theory and behavioural ecology, an evolutionarily stable strategy , which is sometimes also called an evolutionary stable strategy, is a strategy which, if adopted by a population of players, cannot be invaded by any alternative strategy that is initially rare. An ESS is an equilibrium...
(ESS).
Types of sex ratio
The sex ratio varies according to the age profile of the population. It is generally divided into four:- primary sex ratio — ratio at fertilization
- secondary sex ratio — ratio at birth
- tertiary sex ratio — ratio in sexually active organisms
- quaternary sex ratio — ratio in post-reproductive organisms
Measuring these is a problem since there are no clear boundaries between them.
Human sex ratio
Humans have a Fisherian sex ratio. In humans the secondary sex ratio is commonly assumed to be 105 boyBoy
A boy is a young male human , as contrasted to its female counterpart, girl, or an adult male, a man.The term "boy" is primarily used to indicate biological sex distinctions, cultural gender role distinctions or both...
s to 100 girl
Girl
A girl is any female human from birth through childhood and adolescence to attainment of adulthood. The term may also be used to mean a young woman.-Etymology:...
s (which sometimes is shortened to "a ratio of 105"). In human societies, however, sex ratios at birth may be considerably skewed even in economically developed countries such as the United States by natural reasons such as the age of mother at birth and ethnicity, and unnatural reasons in other countries because of sex-selective abortion and infanticide.
Environmental
Spending equal amounts of resources to produce offspring of either sex is an evolutionarily stable strategyEvolutionarily stable strategy
In game theory and behavioural ecology, an evolutionarily stable strategy , which is sometimes also called an evolutionary stable strategy, is a strategy which, if adopted by a population of players, cannot be invaded by any alternative strategy that is initially rare. An ESS is an equilibrium...
: 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
Wolbachia
Wolbachia is a genus of bacteria which infects arthropod species, including a high proportion of insects , as well as some nematodes. It is one of the world's most common parasitic microbes and is possibly the most common reproductive parasite in the biosphere...
cause skewed sex ratios in some arthropod
Arthropod
An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...
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
Temperature-dependent sex determination
Temperature-dependent sex determination is type of environmental sex determination in which the temperatures experienced during embryonic development determine the sex of the offspring. It is most prevalent and common among amniote vertebrates that are classified under the reptile class, but is...
, where incubation temperature of eggs determines the sex of the individual. In the American Alligator
American Alligator
The American alligator , sometimes referred to colloquially as a gator, is a reptile endemic only to the Southeastern United States. It is one of the two living species of alligator, in the genus Alligator, within the family Alligatoridae...
, for example, females are hatched from eggs incubated between 27.7° to 30 °C, whereas males are hatched from eggs 32.2–33.8 °C. In this method, however, all eggs in a clutch (20–50) will be of the same gender. In fact, the natural sex ratio of this species is five females to one male.
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 (PolygonaceaePolygonaceae
Polygonaceae is a family of flowering plants known informally as the "knotweed family" or "smartweed family"— "buckwheat family" in the United States. The name is based on the genus Polygonum and was first used by Antoine Laurent de Jussieu in 1789 in his book, Genera Plantarum. The name refers...
), Melandrium album (Cariophyllaceae), Cannabis sativa
Cannabis sativa
Cannabis sativa is an annual herbaceous plant in the Cannabaceae family. Humans have cultivated this herb throughout recorded history as a source of industrial fibre, seed oil, food, recreation, spiritual enlightenment and medicine...
and Humulus japonicus
Humulus japonicus
Humulus japonicus is an ornamental plant in the Cannabaceae family.This species is a host plant for Polygonia c-aureum.-External links:***...
(Cannabinaceae).
Individual
In birds, mothers can influence the gender of their chicks. In peafowlPeafowl
Peafowl are two Asiatic species of flying birds in the genus Pavo of the pheasant family, Phasianidae, best known for the male's extravagant eye-spotted tail, which it displays as part of courtship. The male is called a peacock, the female a peahen, and the offspring peachicks. The adult female...
, maternal body condition can influence the proportion of daughters in the range from 25% to 87%.
In several different groups of fish, such as the Wrasses, Parrotfish
Parrotfish
Parrotfishes are a group of fishes that traditionally had been considered a family , but now often are considered a subfamily of the wrasses. They are found in relatively shallow tropical and subtropical oceans throughout the world, but with the largest species richness in the Indo-Pacific...
, Clownfish
Clownfish
Clownfish or anemonefish are fishes from the subfamily Amphiprioninae in the family Pomacentridae. Twenty-eight species are recognized, one in the genus Premnas, while the remaining are in the genus Amphiprion. In the wild they all form symbiotic mutualisms with sea anemones...
, dichogamy
Dichogamy
Sequential hermaphroditism is a type of hermaphroditism that occurs in many fish, gastropods and plants. Here, the individual is born one sex and changes sex at some point in their life. They can change from a male to female , or from female to male...
— or sequential hermaphoditism — is normal. This can cause a discrepancy in the sex ratios as well. In the Bluestreak cleaner wrasse
Bluestreak cleaner wrasse
The bluestreak cleaner wrasse is one of several species of cleaner wrasse found on coral reefs in the Indian Ocean and much of the Pacific Ocean, as well as many seas, including the Red Sea and those around Southeast Asia...
, 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 wrasse 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.
Economical
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 prize bulls or a flock of hens with one rooster are the most economical sex ratios for domesticated livestock.See also
- Bride kidnappingBride kidnappingBride kidnapping, also known as marriage by abduction or marriage by capture, is a practice throughout history and around the world in which a man abducts the woman he wishes to marry...
- Demographic transitionDemographic transitionThe demographic transition model is the transition from high birth and death rates to low birth and death rates as a country develops from a pre-industrial to an industrialized economic system. The theory is based on an interpretation of demographic history developed in 1929 by the American...
- Evolution of sexEvolution of sexThe evolution of sexual reproduction is currently described by several competing scientific hypotheses. All sexually reproducing organisms derive from a common ancestor which was a single celled eukaryotic species. Many protists reproduce sexually, as do the multicellular plants, animals, and fungi...
- Human sex ratioHuman sex ratioIn anthropology and demography, the human sex ratio is the sex ratio for Homo sapiens . Like most sexual species, the sex ratio is approximately 1:1. In humans the secondary sex ratio is commonly assumed to be 105 boys to 100 girls, an assumption that is a subject of debate in the scientific...
- List of countries by sex ratio
- Operational sex ratioOperational sex ratioIn the evolutionary biology of sexual reproduction, the operational sex ratio is the ratio of sexually competing males that are ready to mate to sexually competing females that are ready to mate...
- Sex allocationSex allocationSex allocation is the allocation of resources to male versus female reproduction in sexual species . Sex allocation depends upon the breeding system of a species, as well as how reproduction is carried out within each breeding system. Breeding systems can be categorised as dioecious, in which...
- Sex selectionSex selectionSex selection is the attempt to control the sex of the offspring to achieve a desired sex. It can be accomplished in several ways, both pre- and post-implantation of an embryo, as well as at birth...
- Sex-selective abortion and infanticide
- XY sex-determination systemXY sex-determination systemThe XY sex-determination system is the sex-determination system found in humans, most other mammals, some insects and some plants . In this system, females have two of the same kind of sex chromosome , and are called the homogametic sex. Males have two distinct sex chromosomes , and are called...
- Youth bulge