Animals customize milk depending on baby's sex

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WASHINGTON -- A special blend of mother's milk just for girls? New research shows animal moms are customizing their milk in surprising ways, depending on whether they have a boy or a girl.

The studies raise questions for human babies, too -- about how to choose the donor milk that is 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 is far from true -- in monkeys and cows, at least. Instead, "the biological recipes for sons and daughters may be different," she told an American Association for the Advancement of Science meeting Friday.

Pediatricians have long stressed that breast milk is best as 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 being 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 Ms. 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 is mom's first birth, she found.

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

To answer those questions, Ms. 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 on, 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, Ms. Hinde calculated.

As for the monkeys, Ms. Hinde found still more differences in the milk quality. Milk produced for monkey daughters contains more calcium, she found.

Mothers' milk even affects babies' behavior, she 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, not sons, but Ms. Hinde cautioned that testing cortisol reactions at only one point in time could have missed an effect on younger or older boys.

Because high-quality breast milk is particularly important to the most vulnerable infants, she wonders whether premature babies in intensive care might fare better with gender-matched donor milk.

Then there's the infant 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," Ms. Hinde said with a laugh.


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