Steve Connor: Why sexual equality is nature's ideal

People often ask me if there is a biological reason for there being a roughly 50:50 ratio of males to females in the population. If one man can inseminate hundreds of women in his lifetime, surely there is no biological need for so many males to be born.

The question came up again the other day with a study showing that there was a very slight, but statistically significant, increase in the probability of a woman giving birth to a baby girl the nearer the mother lives to the equator, compared to mothers who live in more northerly or southerly latitudes.

The slight bias towards girls as you move to the equator may be a remnant trait of the seasonal nature of human reproduction when we lived a hunter-gatherer existence. In any case, the 50:50 ratio is still biased overall to males, with about 106 boys in the world being born for every 100 girls, because males are more likely to die in childhood.

But that still leaves the 50:50 question. The answer was in fact solved in the 1930s by the mathematical geneticist Ronald Aylmer Fisher. In essence, it's because in a population heavily weighted to females, it becomes an advantage for a mother to give birth to males, and vice versa if the population gets skewed towards males. In effect there is a perpetual balancing act in evolution that keeps the human sex ratio at about 50:50.

Milking a whale of a tale

A story popped up last week on the Planet Earth website of the Natural Environment Research Council about a nutritious alternative to cow's milk. Scientists at its Marine Research Institute in "Mid Glamorgan" devised a way to milk Minke whales, producing 40 times more milk than cows. They trained pods of the whales to be milked in a Norwegian fjord. It sounds implausible... because it is. Now 1 April is behind us, we can give a little sigh of relief for another year.

Riot of a man

While we are on the subject of April Fools' day, the anti-capitalist demonstration in London threw up an old face in the shape of Professor Chris Knight who has been suspended from his post at the University of East London for inciting people to eat bankers. I remember Chris from years ago when he came up with the "sex strike" theory for human menstruation. It's too complicated to be explained here but it involved stone age man (and woman), a full moon, red ochre, hunting meat and making whoopee.