CHICAGO- 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,” said 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, Hinde said Friday 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,” Hinde said.

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.

The females also nurse for longer than male monkeys, who spend more time playing off on their own and thus need more energetically dense milk.

It’s not yet clear why human mothers produce such different milk for their babies, Hinde said.