A mother's milk may contain different levels of nutrients depending on the sex of her baby to meet different growth needs, according to a new study which suggests formula milk should be gender-specific.
Scientists believe that mothers make breast milk differently for male and female babies, suggesting that baby formula should be different for boys and girls to match the differences seen in breast milk.
"We have good reason to be sceptical of a one-size-fits-all formula," said Professor Katie Hinde, an evolutionary biologist at Harvard University.
Speaking at the American Association for the Advancement of Science meeting in Chicago, Hinde described her work in rhesus monkeys that showed mothers produce milk with 35 per cent more fat and protein for male babies, and even richer milk when the male was first-born.
However, when mothers fed female babies, their milk was less fatty and had more calcium, probably to support the faster growth of their skeletons, 'The Guardian' reported.
Mothers produced more milk overall for females, and over the course of their breast feeding, they received the same amount of fat as the males.
"The recipes for milk for sons and daughters may be different, and the difference may be greater depending on where the mother is at in her reproductive career," said Hinde.
"Boys and girls have different developmental trajectories, so if they are not getting what they need, their development will not be optimal," Hinde said.
Hinde also looked at levels of the stress hormone cortisol in mothers' milk and how they affected the babies' behaviour.
Previous research has found that in humans milk with higher concentrations of cortisol made baby girls more irritable and harder to calm down.
Hinde measured levels of cortisol in breast milk for 108 baby monkeys at one month old, and later when the animals were three or four months old.
She found some subtle but important differences. Female monkeys became more nervous when cortisol was high early on in their breast feeding. Male monkeys behaved more nervously when cortisol rose over time.
