Is it worth losing your left testicle?

ime of insemination determined a child's sex, while other ancient Greek philosophers thought it had something to do with the left and right sides of the body.

Two millennia later, an 18th-century French surgeon - writing under the pseudonym of Procope Couteau - took up the idea and advised men who wish to have baby boys to cut off their left testicle.

In more recent times, prospective parents who wish for either a boy or a girl have been offered all manner of remedies and food supplements to affect a baby's sex. But none of these folk recipes have been able to alter the fundamental biology that determines the 50:50 sex ratio.

A study published last week reveals a new twist to an ancient story. Scientists have found the probability of giving birth to a girl rather than a boy increases significantly the nearer the mother lives to the equator.

Conversely, the higher the latitude - and the further away from the equator - the greater the chances of a woman having a baby boy.

Kristen Navara of the University of Georgia in Athens studied the sex ratio of newborn boys to girls in 202 countries - from northern Europe to equatorial Africa - and found a clear link between latitude and a skewed sex ratio. The nearer to the equator, the greater the probability of baby girls, says the study.

The natural sex ratio at birth is slightly biased towards males in humans, with about 106 boys born to every 100 girls.

This sex ratio of 51.5 percent in favour of boys is believed to be nature's way of balancing the slightly increased risk of premature death in young males and so bringing the overall sex ratio in the child-rearing age groups nearer to the natural balance of 50:50.

But Navara found this average sex ratio at birth masks an underlying geographical trend. Using data on global birth rates compiled by the US's Central Intelligence Agency, Navara found countries in tropical latitudes produced fewer boys - 51.1 percent males - compared to countries in temperate and subarctic regions, where the sex ratio is 51.3 percent in favour of boys.

The difference may seem small, but it is nevertheless statistically significant, Navara said. It was even larger between some of the countries in the study. For instance, in tropical Central African Republic the sex ratio was 49 percent boys, whereas in more temperate China it was 53 percent in favour of boys, she said.

Sex in mammals is determined by the type of sperm that fertilises the egg. A sperm carrying the man's X chromosome will become a female embryo, whereas a sperm carrying the Y chromosome will produce a female embryo at conception. In theory, men produce equal numbers of X and Y sperms, which means the sex ratio at birth should be 50:50.

An exceptions to this rule is if male embryos and newborn boys are more likely to die prematurely. Another could come about if food is at risk of being in short supply. Now an underlying biological trend might be showing itself up more clearly in the latest study on latitude.

Navara said the difference in the birth sex ratio between higher and lower latitudes might reflect an ancient evolutionary mechanism caused by food resources in more northerly regions being more varied than in the tropics. - Foreign Service

• This article was originally published on page 30 of The Sunday Tribune on April 05, 2009

Source : Babynet