Milk is not just food. The more closely scientists examine it, the more complexity they find.

Along with nutrients like protein and calcium, milk contains immune factors that protect infants from disease. It hosts a menagerie of microbes, too, some of which may colonize the guts of babies and help them digest food. Milk even contains a special sugar that can fertilize that microbial garden.

Now, it turns out, milk also contains messages.

A new study of monkeys, published in the journal Behavioral Ecology, demonstrates that a hormone present in milk, cortisol, can have profound effects on how babies develop. Infant monkeys rely on cortisol to detect the condition of their mothers, the authors suggest, then adjust their growth and even shift their temperaments.

Jeffrey French, a neuroendocrinologist at the University of Nebraska at Omaha who was not involved in the study, praised its “remarkable sophistication” and said that it helped to change how we think about breast milk. “Milk serves almost like a pheromone, a chemical signal sent from one individual to another,” he said.

Katie Hinde, a behavioral biologist at Harvard and lead author on the new study, and her colleagues studied 108 rhesus macaque mothers nursing infants at the California National Primate Research Center. The researchers collected samples of milk, measuring how much energy each provided and the cortisol it contained.

Dr. Hinde and her colleagues also measured how much weight each nursing monkey gained and tracked its behavior.

Cortisol in breast milk may influence human infants as well. But Melissa Emery Thompson, an anthropologist at the University of New Mexico, cautioned that the differences between monkeys and humans make comparisons difficult.

Infant monkeys, for example, cling to their mothers and nurse whenever they want. Human mothers balance breast-feeding with many other tasks.

“We should expect the relationship between maternal stress, breast milk and infant temperament in humans to be relatively complex,” said Dr. Thompson.

Dr. Hinde agreed: “It’s going to be a bear to unpack all of that.”

But Ben Dantzer, a biologist at the University of Michigan, said that it was important to explore the implications for humans.

Scientists know much less about cortisol’s effects on human babies, because it is not possible to run carefully controlled experiments on them the way Dr. Hinde and her colleagues do on monkeys. Still, what little they do know is intriguing.

In a 2013 study, for example, researchers found that babies who drank high-cortisol breast milk tended to be more fearful and harder to soothe. But scientists can’t say whether human babies are using the same strategy as baby monkeys are.


Real the full article "In a Mother’s Milk, Nutrients, and a Message, Too" at the New York Times.