Animal mothers tailor milk for offspring of different sexes
A special blend of mother's milk just for girls? New research shows animal mothers are customising their milk in surprising ways, depending on whether they have male or female offspring.
The studies raise questions for human babies, too - about how hospitals choose the donor milk used for premature arrivals, or whether we should explore gender-specific infant formula.
"There's been this myth that mother's milk is pretty standard," said Harvard University evolutionary biologist Katie Hinde, whose research suggests that's far from true - in monkeys and cows, at least.
Instead, "the biological recipes for sons and daughters may be different", she told a meeting of the American Association for the Advancement of Science.
Paediatricians have long stressed that breast milk is best when it comes to a baby's first food.
Breast-fed infants are healthier, suffering fewer illnesses such as diarrhoea, earaches or pneumonia during the first year of life and are less likely to develop asthma or obesity later on.
But beyond general nutrition, there have been few studies of the content of human breast milk, and how it might vary from one birth to the next or even over the course of one baby's growth. That research is difficult to conduct in people.
So Hinde studied the milk that rhesus monkey mothers make for their babies.
The milk is richer in fat when monkeys have male babies, especially when it is the first birth. But they make a lot more milk when they have daughters.
Hinde also found that milk produced for monkey daughters contains more calcium.
Because high-quality breast milk is particularly important to the most vulnerable infants, Hinde wonders whether premature babies in intensive care might fare better with gender-matched donor milk.
Then there's the formula question.
"We think it's important - and it's not - to make different deodorants for men and women, and yet we kind of approach formula as though boys and girls have the same developmental priorities," she said.