Page last updated at 06:39 GMT, Wednesday, 1 April 2009 07:39 UK

Hot climate produces baby girls

A beach in Zanzibar, Tanzania
Tropical climates produce more girls

People who live in the tropics have more baby girls compared with those living in other parts of the world, work reveals.

It may be down to the hotter weather or the longer days, says US researcher Dr Kristen Navara in the Royal Society journal Biology Letters.

She says this climate may change miscarriage rates and sperm quality.

Or there may be some evolutionary advantage to having more girls than boys if you live by the equator.

Experts already know that the birth rates of boys and girls vary across the globe.

The results could indicate an adaptive strategy employed by humans
Dr Navara

While some of this can be explained by society - in countries like China baby boys are favoured and many unborn girls are electively aborted - there are natural processes at work.

Research suggests the female foetus is less fragile than the male foetus, which is more prone to the effects of the environment on pregnant women.

At times of extreme environmental stress, including war, the birth rate of girls outstrips that of boys.

Wide variations

Experts have suspected that latitude could have an effect. Past work has shown that the chances of giving birth to a boy increase as you head south - at least in Europe.

But it is difficult to draw conclusions looking at regions in isolation because of wide variations in things like culture, society and economy, to name but a few.

Dr Navara, of the University of Georgia, set out to gain a global perspective by looking a the sex ratios at birth of 202 countries over a 10-year period and taking into account socio-economic differences between nations and continents.

The accepted global average is slightly male biased, at 106 males per 100 females, or 51.5 %.

"The only country in the world which produces more females than males is the Central African Republic," she told the BBC's Network Africa.

In her study, Dr Navara found countries closer to the equator produced significantly fewer boys annually than those at temperate and subarctic latitudes - 51.1% and 51.3%, respectively.

This pattern held strong despite "enormous continental variation in lifestyle and socio-economic status", said Dr Navara.

"The results could indicate an adaptive strategy employed by humans, or there may be another non-adaptive strategy.

"Perhaps male ejaculate quality or miscarriage rates vary on a latitudinal scale," she said.

Survival advantage

Dr Bill James of University College London, who has spent his career studying sex ratio patterns, said although the differences found were statistically significant, it was not as meaningful as other factors that have been linked to sex ratios at birth.

"In general, when people face hard times they tend to have more girls than boys, although there are exceptions.

"For example, women who carry hepatitis B virus are more likely to have boys. So are women who have pre-eclampsia in pregnancy."

He said there were evolutionary explanations for sex ratio changes.

"The idea is that, in mammals, males have a greater variance in their reproductive success.

"Some have lots of offspring and others have none, whereas most females will have at least one offspring.

"So it pays a women who is reproductively fit in good times to have a boy because he may well give her more grandchildren.

"But when times are hard and if she is less reproductively fit, she is better off having a girl because in this way she should gain at least one grandchild."

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