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Testosterone matters

Is it a boy or a girl? This is perhaps the most intriguing question every expectant mother ponders over, particularly in countries where pre-natal sex determination is not allowed legally. Now a study from New Zealand says that the gender of a newborn is more than a matter of the toss of a coin and that the emotional status of the mother at the time of conception may have a role to play.

The controversial theory, put forward by Valerie Grant and her colleagues at the University of Auckland, says that if a woman is stressed out at the time of conception, the chances of her delivering a male baby are relatively high. This would also be true if she is of a dominating nature, the scientists point out.

Evolutionary and reproductive biologists as well as demographers, at least a section of them, have always thought that sex selection of the offspring is much more than a matter of chance — that is, whether a sperm bearing a male sex chromosome or one bearing a female sex chromosome reaches the ovum first. Their hunch stems mainly from the evidence they gathered in recent years to support their view that within many mammalian species, including humans, the sex ratio of the offspring can vary significantly. For instance, during the world wars, they found that there was an increase in the number of male children born.

The Kiwi scientists, who used cow embryos for the latest study, reached this conclusion by measuring the levels of testosterone (the male sex hormone) in the fluid (or follicular fluid) that surrounds the ovum and correlating it with the sex of the offspring. High levels of testosterone in women are strongly linked to either stress or their dominating nature, they argue.

“In women, testosterone is a barometer of stress levels. We know for sure that chronic stress elevates their testosterone levels,” says Larry Chamley, a co-author of the paper that appeared in the May 2008 issue of the journal Biology of Reproduction.

According to Grant, there is a biological reason for that. While in males testosterone is produced by the testes, in females it is generated in the peripheral tissues that are controlled by the adrenal glands. “When stressed, the testes put a brake on testosterone production, whereas the adrenal gland — associated with the production of the stress hormone, cortisol — releases more testosterone as a reaction to stress,” Grant told KnowHow.

The study has found that bovine female eggs exposed to high levels of testosterone are more likely to produce male embryos when fertilised in test tubes. The scientists assume that this could be because eggs under the influence of testosterone would choose to fertilise with a sperm that carries a Y-chromosome (male sex chromosome) rather than one carrying an X-chromosome.

But what is the mechanism behind such a choice? It is not yet very clear, Grant admits.

A dominant section of reproductive biologists, however, is not convinced. “Whatever they have shown has been done under laboratory conditions. In nature, it doesn’t work that way,” says Atmaram Bandivdekar, a scientist with the National Institute for Research in Reproductive Health, Mumbai.

Grant has been studying the subject of sex selection of embryos for at least three decades. Another hypothesis she put forward nearly 10 years ago argues that there is a strong correlation between the mother’s nature and the sex of her baby. In other words, women who are more confident, assertive, influential and with a strong sense of self have high levels of testosterone and would most likely produce sons, whereas mothers who tend to be more nurturing, empathic and tolerant would have daughters.

Interestingly, she found, the latter have lower testosterone levels.

So it is not without reason that the primary sex ratio (the sex ratio at the embryo stage) is tilted towards males – a healthy average sex ratio being 110 male embryos to 100 female ones. This is because males are more vulnerable than their female counterparts. "Hence, this is Nature’s way of maintaining a healthy sex ratio at the reproductive age," Grant observes.

In her book Maternal Personality, Evolution and the Sex Ratio: Do Mothers Control the Sex of the Infant, published in 1998, she argued that dominance is a core personality trait whose biological basis is testosterone. The action of this hormone within the reproductive system leads to the favouring of sperm carrying male chromosomes. Thus the mother bears the gender that she is most suited to raise. Grant has all along been arguing that female testosterone levels play a role — albeit a small one – in determining the sex of the offspring.

Reproductive biologists may take years to resolve their differences relating to the sex selection of embryos. In the meanwhile, those children who want their next sibling to be a sister can try out something: don’t aggravate your mother!

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