Mothers may say they don't care whether they have a son or a daughter, but their breast milk says otherwise.
"Mothers are producing different biological recipes for sons and daughters," says Katie Hinde, an evolutionary biologist at Harvard University. Studies in humans, monkeys and other mammals have found a variety of differences in both the content and the quantity of milk produced.
One common theme: baby boys often get milk that is richer in fat or protein — and thus energy — while baby girls often get more milk.
There are a lot of theories as to why this happens, says Hinde, who presented her findings at the American Association for the Advancement of Science's annual meeting.
Rhesus monkeys, for instance, tend to produce more calcium in the milk they feed to daughters who inherit social status from their mothers.
"It could be adaptive in that it allows mothers to give more milk to daughters which is going to accelerate their develop and allow them to begin reproducing at early ages," says Hinde.
Males don't need to reach sexual maturity as quickly as females because the only limit on how often they reproduce is how many females they can win over. "While the food aspects of milk to some extent are replicated in formula, the immunological factors and medicine of milk are not and the hormonal signals are not," she says.
Getting a better understanding of how milk is personalized for specific infants will also help hospitals find better matches for breast milk donated to help nourish sick and premature infants in neo natal units, she adds.