WOMEN living in tropical latitudes have a greater chance of giving birth to a girl, according to the first global study to link geographical latitude with the sex ratio of births. Normally, slightly more boys are born than girls.
Countries at tropical latitudes enjoy warmer temperatures and longer, more constant day lengths, but it鈥檚 not clear why their maternity wards are more 鈥済irl-rich鈥�, says Kristen Navara of the University of Georgia in Athens. She analysed birth data between 1997 and 2006 from 202 countries collected by the US Central Intelligence Agency.
鈥淥f the 20 countries where fewest boys were born, 18 were at tropical latitudes,鈥� she notes. Navara鈥檚 analysis of the CIA data suggests that worldwide, boys account for 51.3 per cent of births, with just over 105 boys born for every 100 girls. But in tropical countries only 51.1 per cent of births are boys. And in one country, the Central African Republic, just 49 per cent of births were boys, so more of the newborns were actually girls.
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Although the differences appear small, they translate into thousands of babies every year, says Navara, whose results appear in Biology Letters ().
In the Central African Republic, for example, 1400 fewer boys were born in 2006 than if the sex ratio had been 50:50. Other 鈥済irl-rich鈥� tropical countries included Grenada, Mauritius and Bermuda. Navara says the results echo those of other studies in mammals, including meadow voles and Siberian hamsters, which have fewest males when days are shortest. She thinks variation in the circadian hormone melatonin is a factor, but there鈥檚 no clear explanation.