Oh my, milk is really amazing! Respect the boob
“Mother’s milk is food; mother’s milk is medicine; and mother’s milk is signal,” says Katie Hinde, an assistant professor of human evolutionary biology at Harvard. (She also writes the fascinating blog Mammals Suck, which I suspect is the only place on the Internet where you can fill out a Mammal Madness bracket.) “When people find out I study milk, they automatically think we already know about it, or it’s not important. And I’m like, ‘No, we don’t know about it, and it’s super important.’”
But first, a disclaimer—because conversations about lactation always seem to require disclaimers, especially if you happen to be someone who will never ever lactate. (I’m pretty sure.) In my new book Baby Meets World, I write about how, contrary to myth, not nursing has never been a death sentence. Hundreds of years before halfway-decent formula, infants were fed gruesome substitutes for breast milk (mushed bread and beer, say)—and although many more died than those who were nursed, many also survived. So the lesson of the new science of milk isn’t that formula is some sort of modern evil. (It isn’t modern or evil.) It’s that milk is really complicated—and evolutionarily amazing.
Here’s how complicated: Some human milk oligosaccharides—simple sugar carbohydrates—were recently discovered to be indigestible by infants. When my son was nursing, those oligosaccharides weren’t meant for him. They were meant for bacteria in his gut, which thought they were delicious. My wife was, in a sense, nursing another species altogether, a species that had been evolutionarily selected to protect her child. (A relationship immortalized in the paper titled “Human Milk Oligosaccharides: Every Baby Needs a Sugar Mama.”) In effect, as Hinde and UC-Davis chemist Bruce German have written, “mothers are not just eating for two, they are actually eating for 2 × 1011 (their own intestinal microbiome as well as their infant’s)!” That’s what’s meant by milk serving as medicine, and that’s only scratching the surface.
But Hinde primarily studies the food and the signal elements of milk. “The signal is in the form of hormones that are exerting physiological effects in the infant,” she explains. “Infants have their own internal hormones, but they’re also getting hormones from their mother. They’re binding to receptors in the babies, and we’re just starting to understand what those effects are.”
Hinde works with rhesus macaques, and she’s tracked the effects of the hormone cortisol in their milk. Cortisol is often thought of as the stress hormone, but its function is far more varied, and Hinde has found that the amount and especially the variation of cortisol successfully predicts how the infant macaques go on to behave. It’s a stunning finding: The composition of early milk seems to mold infant temperament. But—and here’s the twist—the males were much more sensitive than the females. Roughly, the more cortisol, the more bold and exploratory the male rhesus macaques were.
Such sex-specific variations in milk, possibly “programmed” by the placenta during gestation, may be common. In humans, there’s early data suggesting that mothers produce fattier milk for boys than girls. But that may be only part of the story, as Hinde has found with rhesus macaques. “Just because sons are getting better milk doesn’t mean they’re getting more. It looks like they’re getting very similar total calories.” So why do sons get fattier milk? “In rhesus macaques, daughters stay in their social groups their whole lives,” Hinde notes. “They form a bond with their mother that only ends when one of them dies. So it might be that mothers are nursing their daughters more frequently and that helps establish this bond.” In contrast, the sons end up leaving the group—and fattier milk means they nurse less often, which means they can spend more time playing with strangers, developing skills they’ll need later in life. The milk, in other words, reflects and cements the social structure of rhesus macaques.”
The Secrets of Breast Milk