Nursing gender differences

From booties to blankets and everything in between, mothers tend to customize their babies’ surroundings by gender. Now science is showing that moms’ mammary glands do the same thing. While breast milk doesn’t come in pink and blue, it does appear to have different properties depending on the baby’s gender.

Harvard researcher Katie Hinde spoke about the growing evidence of gender-specific mother’s milk at last month’s meeting of the American Association for the Advancement of Science. Discoveries in the field shed light on how each person—and animal, too—receives the biological ingredients he or she needs to fulfill a specific role in creation.

For example, the skeletons of both human and monkey females develop faster than males. Hinde’s research has shown that mothers’ milk for female monkey offspring in her lab contains more calcium than milk for males, which could be support the females’ rapid skeletal development.

Other studies have shows similar results. Scientists have discovered that among certain populations, milk for boys has a higher energy density and a higher fat content. The results vary between cultures, though, so the explanation for the differences isn’t as simple as saying boys need more fat and energy, and their moms are giving it to them.

“The work that’s been done is, I think, incredibly intriguing and exciting, but it’s the first chapter,” Hinde told me.

Scientists do know that male and female mammals have different “developmental priorities,” meaning their growth patterns are customized for their reproductive roles. Males grow up to father a large number of children. Females have more limited reproductive capacity. In Hinde’s field of evolutionary biology, scientists have come up with nine different hypotheses for how and why those differences came to exist, all of which rely on the influence of natural selection. What they don’t question is the existence of differences between the sexes that go deeper than what can be seen from the outside.

Mother’s milk also varies by birth order, scientists have found. The question they can’t answer yet is whether differences in milk contribute to differences between genders or vice versa. Hinde says it’s probably a little of both.

Hinde is now extending her research to humans. Her passion is helping mothers better understand and provide for their babies’ needs.

“Humans all across the world are what we call cooperative breeders,” Hinde said. “They have family and friends and community around them that help them with raising their kids. A chimpanzee mom, she can do it on her own. … A human mom cannot do it on her own. She needs help and support.”

Research by Hinde and her colleagues could impact future recipes for infant formula. It could also change the use of donor milk in Neonatal Intensive Care Units.