Nursing monkeys make different milk for male and female offspring, a finding that has direct implications for understanding previously unknown variations in human breast milk, researchers said.
Males and females respond differently to cortisol, a hormone found in mothers' milk but not in formula, said Katie Hinde, with Harvard University's Department of Human Evolution.
"There is this prevailing myth that mother's milk is standard," Hinde told reporters at the American Association for the Advancement of Science meeting in Chicago on Friday.
"There's evidence that mothers are producing different biological recipes of milk for sons and daughters and the magnitude of this effect varies across their reproductive careers," Hinde told Discovery News.
Breast milk calcium content, for example, is higher for females than males.
The handful of studies that have looked at variations in human breast milk based on the baby's gender tend to focus on the constituents of the milk and their concentrations -- how much fat, protein, sugar, calcium, etc., but have not accounted for overall milk production, which affects concentrations.
There is no data on how breast milk may vary for nursing mixed-gender twins, she added.
Formulas can copy mothers' milk food value, but not its hormonal content, Hinde said.
Related studies on 1.4 million cows also showed that mothers carrying females produce more milk than those gestating males.
The research may one day lead to better matching between donor breast milk and babies in neo-natal units, much like blood and bone marrow donors are matched with recipients, Hinde said.
"If we can better understand how milk is personalized for specific infants, we can do a better job of finding the closest match for donors," Hinde said.