Hot climates make for more baby girls, colder boys

London, Apr 1 (ANI): If you and your husband want a baby boy, then go live in the hills, suggests a new study.

A research has found that women living in cooler climates are more likely to give birth to boys than those in the tropics.

The study, which has for the first time demonstrated that latitude affects the sex of human babies, was based on global birth data collected by the CIA between 1997 and 2006 in 202 countries.

The global average from the data worked out at 51.3 per cent of all births being boys, or 105 for every 100 girls born. At tropical latitudes, the ratio of males born fell to 51.1 per cent, compared with 51.3 per cent in temperate and subarctic latitudes - the same as the global average.

"Of the 20 countries with the lowest ratios, 18 were at tropical latitudes," New Scientist quoted Kristen Navara of the University of Georgia in Athens, as saying.

And even though the differences might appear small, they translate into large numbers of babies.

Of the "girl-rich" tropical countries, the Central African Republic was the only one where more girls than boys were born in the 10-year study, with a ratio of 0.49 per cent. Navara pointed out that although this figure appears small, it translated in 2006, for example, into 1400 fewer boys than if the ratio was 50:50.

The reason for the result, however, remains elusive.

"It's very difficult to explain," says Navara.

She speculates that the most likely drivers are the longer, less variable day lengths, and higher temperatures at tropical latitudes compared with others.

"We think this may be mediated by the hormone melatonin, which is responsible for causing major reproductive changes in response to day lengths," says Navara.

"Treatment of rodents with melatonin results in pre-natal skews in the sex ratio, and it's possible something similar is happening with humans," she added.

Other possible explanations might be that temperature alters the survival of "male" or "female" sperm in semen, or perhaps favour the ability of one or the other to fertilise the egg.

The study has been published in the journal Biology Letters. (ANI)