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Originally published Friday, February 14, 2014 at 5:00 PM

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Animal mothers’ milk differs by infant’s sex

Rhesus monkey mothers make milk richer in fat for their sons, but for daughters they make a lot more milk.


The Associated Press

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WASHINGTON — New research shows animal moms customize their milk in surprising ways, depending on whether they have a boy or a girl.

The studies raise questions for human babies, too, including about how to choose donor milk that’s used for hospitalized preemies 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 on Friday.

Pediatricians have long stressed that breast milk is best when it comes to baby’s first food. Breast-fed infants are healthier, suffering fewer illnesses such as diarrhea, 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. That research is difficult to conduct in people.

So Hinde studies the milk that rhesus monkey mothers make for their babies. The milk is richer in fat when monkeys have male babies, especially when it’s mom’s first birth, she found.

But they made a lot more milk when they had daughters, Hinde discovered. Do daughters nurse more, spurring production? Or does something signal mom prenatally to produce more?

To tell, Hinde paired with Kansas State University researchers to examine lactation records of nearly 1.5 million Holstein cows. Unlike monkey babies, calves are separated from their mothers early, meaning any difference should be prenatal.

Sure enough, cows that bore daughters produced about 1.6 percent more milk. Since cows lactate for 305 days, that adds up. More interesting, cows often lactate while pregnant — and those that bore a second daughter in a row produced almost 1,000 more pounds of milk over nearly two years than those that produced only sons, Hinde calculated.

Back to the monkeys, where Hinde found still more differences in the quality of the milk.

Milk produced for monkey daughters contains more calcium, she found. One explanation: Female monkeys’ skeletons mature faster than males’ do, suggesting they need a bigger infusion of the bone-strengthening mineral.

Mothers’ milk also affects babies’ behavior, Hinde said. Higher levels of the natural stress-hormone cortisol in milk can make infants more nervous and less confident. But boys and girls appear sensitive to the hormone’s effects at different ages, her latest monkey research suggests.

One previous study of human babies has linked higher cortisol levels in breast milk to cranky daughters but not sons, although Hinde cautioned that testing cortisol reactions at only one point in time could have missed an effect on younger or older boys.

What about boy-and-girl twins? Hinde can’t answer; the monkeys she studies seldom have twins. Nor can she explain why the animals show these gender differences.

“It’s something highly personalized for that mother and that infant at that time point. That’s an exquisite thing,” said Hinde, who wants to see similar study of human breast milk.



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