Female rhesus monkeys produce a different formulation of breast milk to feed their daughters than they do to feed their sons. Mama cows do the same thing. These findings have led a Harvard University evolutionary biologist to rethink human breast milk. If human mothers produce gender-specific breast milk formulas to feed their children, is it time to take another look at donor milk and infant formulas for human babies?

Katie Hinde, the Harvard biologist, wanted to prove or disprove the myth that mother’s milk is pretty standard but her recent study suggests different biological recipes based on sex of the offspring. In monkeys, the milk from a first pregnancy was different from the milk produced in subsequent pregnancies, too.

Hinde turned to cows and monkeys to test her theory since there are reliable lactation records available for some cows and because monkeys have a lifespan so much shorter than humans. There has been very little study to date on human breast milk content and production.

Holstein Cows

Holstein cows are bred primarily for milk to use in dairy products. Records of their milk production is big business and they produce milk for about 305 days after giving birth. Hinde enlisted the help of researchers at Kansas State University to analyze lactation records of approximately 1.5 million Holsteins. Holstein calves are separated from their mothers very early on, a factor Hinde says points to milk formula differences being prenatal rather than something that develops after birth as the offspring suckles. She found that:

• 1.6% more milk was produced when a female was born.
• Cows lactate during pregnancy so cows that bear two daughters in a row produce about 1,000 pounds more milk during a two-year period than when they bear two sons in a row.

Rhesus Monkeys

Rhesus monkeys do not typically get separated from their offspring during the nursing phase of motherhood. The content and production of their milk varied significantly between offspring genders:

• A lot more milk is produced when the offspring is female.
• When the baby is male, the milk is richer in fat.
• When the first baby is male, the milk is richer in fat than ever.
• When the child is female, the milk contains more calcium; the extra intake of the bone-building mineral could facilitate the female’s earlier skeletal maturity. In humans, too, girls mature faster than boys.
• Mother’s milk (human and monkey) contains the stress hormone, cortisol, when mom is stressed and, in monkeys, infants get nervous and lose confidence when they consume cortisol-laced mother’s milk. The stress hormone affects female offspring at a different stage of maturity than it affects the males.

Hinde’s research doesn’t address differences in milk content when male-female fraternal twins are born because monkeys usually don’t have twins. She describes the gender personalization as “an exquisite thing” and important to development.

Gender-specific donor milk could be highly beneficial to human preemies and development of gender-matched infant formulas, too. After all, says Hinde, we make deodorants in different formulas for men and women. Why not infant formulas, too?


Source: Klotz, Irene. “Breast Milk Varies for Males, Females.” Discovery News. Discovery Communications, LLC. Feb 14, 2014. Web. Feb 18, 2014.