Does Breastmilk Contain Hidden Messages?

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Breastmilk can protect against common childhood diseases and decrease a child’s chance of developing allergies, but can it affect a baby’s temperament? If a new study of baby monkeys is any indication, the answer is yes.

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The study, which was published in the journal Behavioral Ecology, found that the level of cortisol present in a mother’s milk had a significant effect on a baby monkey’s development.

Cortisol, known as the stress hormone, “curbs functions that would be non-essential or detrimental in a fight-or-flight situation,” according to the Mayo Clinic. “This complex natural alarm system also communicates with regions of your brain that control mood, motivation and fear.”

The researchers found that different monkey mothers had different levels of cortisol in their milk. New mothers, who have lactated less and thus produce less milk, generally had high levels, while cortisol levels were lower in mothers who had delivered 10 or more babies. Higher levels of cortisol “predicated a more nervous, less confident temperament,” but also faster weight gain, according to the study.

The study analyzed 108 nursing rhesus macaque mothers at the California National Primate Research Center. High levels of cortisol, researchers suggest, are a message to baby monkeys to prioritize growth over “the development of an exploratory, playful, confident temperament,” the researchers write. On her blog, Dr. Katie Hinde, lead author of the study, explains it this way: “We think that cortisol in milk functions to tell kiddo ‘buddy, there is limited milk coming down this pipeline, you need to prioritize growing, not burning precious calories playing and exploring.’ In this way babies are seemingly calibrating to expected resources from the mom across lactation, having a more nervous, less confident temperament.”

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But even though the baby monkeys were nervous, the researchers found they gained more weight, faster. “This generates a hierarchy of biological imperatives for the growing kiddo,” Hinde writes. “First order of business: STAY ALIVE! aka ‘maintenance.’ Resources on top of that? GROW! Even more resources? Get your behavioral activity on!”

Hinde points out that while people generally consider nervousness and low confidence as negatives, that isn’t the case here. “Infants at both ends of this distribution are seemingly calibrating to the mom’s resources and environment, and generally speaking calibration is a good thing,” she writes.

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So what does this mean for humans? Will your first baby, if nursed, be more nervous and less confident? Will she gain weight faster than later babies? It’s way too early to make that leap, according to Hinde. “We know hardly anything about this biobehavioral system in humans,” she writes. “Monkeys and humans share many features relative to other mammals – we typically produce socially complex, large-brained, singleton infants upon whom we lavish care, so the monkey results are informative for motivating more human research.” Plus, it’s more difficult to test these kinds of things on humans due to ethical concerns.

But for those who can’t help but wonder about their own cortisol – too high? too low? — Hinde has a reminder. “This one hormone, in one aspect of parenting, is just one of the rivets holding the plane wing on,” she writes. “One rivet can break, and the plane still flies… even two or three and the plane can get off the ground. Development is a multi-factorial system, and rarely is any one single aspect the linchpin. So moms, don’t let these findings stress you out.”