Hot climate produces baby 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.
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.