
The Amazing New Science of Breast Milk
How Babies Work
Emergent Thinking About Emergent Humans
March 20 2013 4:46 AM

The Secrets of Breast Milk

Mother breastfeeding her child.
The more we learn about breast milk, the more we learn how much more we have to learn.

Photo by Endopack/iStockphoto








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.






