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.[1]
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 may be considerably skewed by factors such as the age of mother at birth[2], and by sex-selective abortion and infanticide. The CIA estimates that the current world wide sex ratio at birth is 107 boys to 100 girls.[3] In 2010, the global sex ratio was 986 females per 1,000 males and trended to reduce to 984 in 2011.[4]
In biology, sex ratio is defined as the proportion of males in the population.
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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.[5] 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).
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"[6] as follows, given the assumption of equal parental expenditure on offspring of both sexes.
In modern language, the 1:1 ratio is the evolutionarily stable strategy (ESS).[7]
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.
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.[8]
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.[9]
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),[10][11] Melandrium album (Cariophyllaceae),[12][13] Cannabis sativa[14] and Humulus japonicus (Cannabinaceae).[15]
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%.[16]
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.
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.