Sex ratio

Sex ratio is the ratio of males to females 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 boys to 100 girls (which sometimes is shortened to "a ratio of 105"). In human societies, however, sex ratios at birth or among infants may be considerably skewed by sex-selective abortion and infanticide.

In biology, 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 history. 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):

  1. For a dioecious species, what is the equilibrium sex ratio maintained by natural selection?
  2. For a sequential hermaphrodite, what is the equilibrium sex order and time of sex change?
  3. For a simultaneous hermaphrodite, what is the equilibrium allocation of resources to male versus female function in each breeding season?
  4. Under what conditions are the various states of hermaphroditism or dioecy evolutionarily stable? When is a mixture of sexual types stable?
  5. 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. Assuming equal parental investment:

W.D. Hamilton gave the following basic explanation in his 1967 paper on "Extraordinary sex ratios", given the condition that males and females cost equal amounts to produce:

# 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 commoner.
# 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 (ESS)

Types of sex ratio

The sex ratio varies according to the age profile of the population. It is generally divided into four:

Measuring these is a problem since there are no clear boundaries between them.

Human sex ratio

In anthropology and demography, the human sex ratio is the sex ratio for Homo sapiens. Humans have a Fisherian sex ratio. In humans the secondary sex ratio is commonly assumed to be 105 boys to 100 girls (which sometimes is shortened to "a ratio of 105"). In human societies, however, sex ratios at birth or among infants may be considerably skewed by sex-selective abortion and infanticide.

Examples in non-human species


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.

The bacterium wolbachia causes skewed sex ratios in some arthropod species as it kills 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 which females require multiple matings to keep producing eggs, sex ratios are less biased (close to 1); and in families 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, where 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 (Polygonaceae), Melandrium album (Cariophyllaceae), Cannabis sativa and Humulus japonicus (Cannabinaceae).


In birds, mothers can influence the gender of their chicks. In peafowl, 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 it's 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.


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 and a few prize bulls or a flock of chickens and one rooster are the most economical sex ratios for domesticated livestock.

See also


External links

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