A mother's milk may contain different levels of nutrients depending on the sex of her baby to meet different growth needs
Baby formula should be tailored for boys and girls to reflect the differences in milk that mothers produce depending on their baby's sex, researchers say.
Tests on mothers' milk in both monkeys and humans have showed that levels of fat, protein, vitamins, sugars, minerals and hormones vary enormously, but there is evidence that milk made for female and male babies is consistently different.
The make-up of the milk has a direct impact on the child's growth, but also on his or her behaviour and temperament, which may last for the rest of their life. Scientists suspect that breast milk may be tailored by nature to meet the different growth needs of the sexes.
The findings have led some researchers to suggest that baby formula should come in boy and girl formulations to match the differences seen in breast milk.
"We have good reason to be sceptical of a one-size-fits-all formula," said Prof Katie Hinde, an evolutionary biologist at Harvard University.