Sex ratio
A sex ratio is the ratio of males to females in a population. As explained by Fisher's principle, for evolutionary reasons this is typically about 1:1 in species which reproduce sexually.[2][3] However, many species deviate from an even sex ratio, either periodically or permanently. Examples include parthenogenic species, periodically mating organisms such as aphids, some eusocial wasps, bees, ants, and termites.[4]
"Gender balance" redirects here. For gender balance as a socio-political issue, see Gender equality.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[5] and by sex-selective abortion and infanticide. Exposure to pesticides and other environmental contaminants may be a significant contributing factor as well.[6] As of 2024, the global sex ratio at birth is estimated at 107 boys to 100 girls (1,000 boys per 934 girls).[7] 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.[7]
In most species, the sex ratio varies according to the age profile of the population.[8]
It is generally divided into four subdivisions:
These definitions can be somewhat subjective since they lack clear boundaries.
Examples in non-human species[edit]
Environmental and individual control[edit]
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.[15]
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 (81.9 to 86.0 °F), whereas males are hatched from eggs 32.2 to 33.8 °C (90.0 to 92.8 °F). 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.[16]
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%.[17]
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[edit]
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[edit]
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),[18][19] Melandrium album (Caryophyllaceae),[20][21] Cannabis sativa[22] and Humulus japonicus (Cannabinaceae).[23]
Polyandrous and cooperatively breeding homeotherms[edit]
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.[24] Those species with male care and polyandry invariably have adult sex ratios with a large surplus of males,[24] which in some cases can reach as high as six males per female.[25]
Male-biased adult sex ratios have also been shown to correlate with cooperative breeding in mammals such as alpine marmots and wild canids.[26] This correlation may also apply to cooperatively breeding birds,[27] though the evidence is less clear.[24] It is known, however, that both male-biased adult sex ratios[28] and cooperative breeding tend to evolve where caring for offspring is extremely difficult due to low secondary productivity, as in Australia[29] and Southern Africa. It is also known that in cooperative breeders where both sexes are philopatric like the varied sittella,[30] adult sex ratios are equally or more male-biased than in those cooperative species, such as fairy-wrens, treecreepers and the noisy miner[31] where females always disperse.
Humans:
Institutions: