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How do mammalian parents manipulate the sex of their offspring

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Is China going to eat up the whole world?
Is China going to eat up the whole world?

Most of the human body’s abilities are still hidden from us. Where do sudden bursts of energy come from when a woman in a panic can lift a car? How can one person learn twenty languages? How can yoga teach you to have different temperatures in different parts of the body? What predisposes a person to get cancer? Millions of experiments later, this temporary home for our spirit is still a mysterious entity.

One of the major areas of ignorance is why some people have only girls, some only boys and some both. How does the body decide which sperm to choose?

Are we the only mammals to whom the sex of their children comes as a surprise. It would seem so. Animals who are so much more comfortable with their bodies can apparently choose the sex of their children.

Non-mammals can do it quite easily.

Ants, bees and wasps, which live in complex societies, choose the sex of their babies. Queen honey bees are not just mindless egg layers. Every young queen goes on a mating flight and stores the sperm she collects from multiple matings for the rest of her life, using it up bit by bit as she lays eggs. Males, called drones, emerge from unfertilized eggs, and females emerge from fertilized ones and become the workers. If the queen adds sperm to an egg, it will produce a female; if she withholds sperm, the egg will produce a male. Mothers prefer to have daughters because they help with bringing up babies, so large workforces of female helpers are created with very few males. That means the queen controls the sex of her offspring. Only in termite societies do both male and females help to raise offspring so there is an equal balance of sexes.

Yellow dung flies collect sperm from different mates and then selectively choose the “best” sperm according to the dung on which the eggs will be laid.

Reptiles control the sex of their babies by lowering or raising the temperature of their eggs.  Eggs are affected by the temperature at which they are incubated. In turtles males are produced at lower incubation temperatures than females with as little difference as 1-2 deg.C. In lizards and crocodiles, this pattern is reversed. That both males and females come out of a nest is because incubation temperatures vary widely. Eggs at the top of a nest incubate at different temperatures than eggs in the middle or at the bottom.

The female of the Southern Water Skink in Australia controls the sex of her offspring with her own body temperature.  When temperatures are warm, female lizards give birth exclusively to males. Which means that, as global warming increases, the females will stop giving birth to females and the species will become extinct.

Now, a 2013 study at Stanford University School of Medicine published in the journal PLOSONE says that mammals can choose the sex of their children as well. The researchers analyzed 90 years of breeding records of 198 species and 2,300 animals from the San Diego Zoo and proved that mammals manipulate the sex ratios of their offspring as part of an evolutionary strategy. All major mammal groups were represented from primates, lions, bears, wolves, cows, buffalo and deer, horses and rhinos.

The scientists found that grandmothers and grandfathers were able to strategically choose to give birth to sons, if those sons would be high-quality and in turn reward them with more grandchildren. The process is believed to be largely controlled by the females who make the decisions of gender. According to the study, when a mother mostly gave birth to boys, those sons had 2.7 times more offspring than males whose moms had a more equal number of boys and girls. The daughters of moms who mostly had girls produced 1.2 times as many offspring as daughters whose moms had an equal mix of boys and girls. “This is one of the holy grails of modern evolutionary biology — finding the data which definitively show that when females choose the sex of their offspring, they are doing so strategically to produce more grandchildren,” says Joseph Garner, Stanford professor and senior author of the study.

Garner adds “We like to think of reproduction as being all about the males competing for females, with females dutifully picking the winner. But in reality females are making highly strategic decisions about their reproduction based on the environment, their condition and the quality of their mate. Amazingly, the female is somehow picking the sperm that will produce the sex that will serve her interests the most: The sperm are really just pawns in a game that plays out over generations.”

The “vaal rhebok” is a South African antelope. Herds are made up of several females and one dominant male who mates with all of them. That male will defend his harem from solitary males often fighting to the death. Therefore it is safer for these antelopes to have daughters, who will reproduce each breeding season. A son may never mate at all. So most bear females with an occasional male that guarantees a number of grandchildren.

Francois’ Langur is an  Asian primate living in matriarchal groups led by females. Groups usually only have one adult male and young males set off to make groups of their own when they reach maturity.

Having daughters leads to more grandchildren than having a balanced sex ratio. Females high up on the social totem pole have more daughters. It is the lower ranked ones that have the occasional son.

The study builds on a theory first proposed in 1973 by scientists Robert Trivers and Dan Willard, founders of the field of evolutionary biology. They challenged the conventional wisdom that sex determination in mammals is random, with parents investing equally in their offspring to generate a 50-50 sex ratio in the population. Instead, they hypothesized that mammals manipulate the sex of their offspring in order to get better reproductive success. Parents in good condition, based on health, size, dominance, hierarchy produce sons, whose inherited strength can help them better compete in the mating market and give them greater opportunities to produce more offspring. Conversely, mothers in poor condition produce more daughters. The hypothesis was reinforced in 1984 in a paper by T.H. Clutton-Brock at the University of Cambridge, who found that among wild red deer, dominant mothers produced significantly more sons than deer who held a subordinate position within the herd.

How do mammalian parents manipulate the sex of their offspring? The mechanism is not known, though one theory holds that females can control the speed of “male” and “female” sperm as they move through the reproductive tract.

Can humans do it? Here is some weird evidence that might show that humans unconsciously, manipulate their children’s sexes as well. For instance, the top-ranking wives in polygamous societies are more likely to have sons than lower-ranking wives, according to an analysis of Mormons. A 2009 survey of 400 U.S. billionaires found they were more likely to have sons than daughters, keeping the wealth in the family. A study published in 1988  found that mothers with an inherited speech disorder had three times as many sons as daughters, in theory because a son with a speech impediment would have an easier time finding a mate than a speech-impaired daughter.

A few years from now, I am sure we will catch up with the ability of the humble dung fly and learn how to consciously control the sex of our children.

Maneka Sanjay Gandhi

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