Animal Study Reveals Intriguing Answers

Recent studies in cows and monkeys reveal that mothers may make different milk depending on the sex of their babies. Is this echoed in human mothers?

Breast milk

Harvard University evolutionary biologist Katie Hinde conducted research recently on the milk composition in rhesus monkeys and Holstein cows and she discovered that the composition — in these animals, at least — changed according to the baby’s sex. Is that true for humans too?

Mother’s milk in action

Hinde researched milk production and composition of rhesus monkeys and found that the milk that was produced when the mother had a male baby was richer in fat. This was especially true if it was her first birth. She also found that the quantity of milk was greater if they had female babies.

She then paired up with Kansas State University scientists to go through the data of approximately 1.5 million Holstein cows. Monkey babies stay with their mothers after birth, but Holstein dairy calves are separated from their moms shortly after they are born. They wanted to find out if the findings were similar to the monkey studies and wanted to discover if it was the babies themselves that spurred on the differences or if the differences were prenatal in origin. The answer? They discovered that the Holstein mothers of female babies produced 1.6 percent more milk, even though the babies weren’t around to nurse.

Another difference in rhesus monkey moms and their boy and girl babies was that moms of females had more calcium than the milk of moms of males.

Reflected in humans?

Can these studies be correlated to human mothers and their babies? "It’s hard to say," says Hinde. There have been no in-depth studies on humans and their breast milk composition according to their baby’s sex, but that could be on the horizon. She says that she can’t answer why these animals have such differences according to the sex of their babies, but maybe research in the future can give us more clues. It would make a difference for preemie babies and donor milk, for example, if the sex of the babies were matched up when donations were administered. That is still speculation at this point in time.

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