In a Mother’s Milk, Nutrients, and a Message, Too

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 serves many functions in mammals, but it is best known as a stress hormone. When cortisol courses through our bodies, it prepares us to handle alarming or fearful situations, increasing the brain’s consumption of glucose and suppressing the digestive system.

The cortisol in a mother’s body can also end up in her milk. Babies appear to be remarkably sensitive to the hormone as they nurse. Scientists have found that drinking milk causes infants to rapidly build receptors in their intestines for detecting cortisol. The same shift doesn’t happen when babies drink formula.

Among the macaques, some mothers delivered a lot of cortisol to their babies, the scientists found, while others delivered only a little. High-cortisol milk made babies put on weight faster, and they were more nervous and less confident.

To make sense of these results, the scientists looked for factors that might determine how much cortisol a mother produced in her milk. One stood out: how many other offspring she had

New mothers had high cortisol levels in their milk, Dr. Hinde found. Hormone levels were much lower in mothers who had had about 10 babies.

When female monkeys start having babies, Dr. Hinde noted, they can’t store as much energy in their milk. New mothers are still small, and so their bodies can’t provide many of the raw ingredients for milk. Their mammary glands are also underdeveloped, so they can’t convert those ingredients efficiently into milk.

Monkey mothers who have had more babies are able to supply new infants with more energy. Dr. Hinde suspects that the cortisol that newer mothers give their babies serves as a warning that they shouldn’t expect a lot of milk, or energy.

She sums up the message this way: “Prioritize growth, kiddo. You can’t really afford to be exploratory and playful. Once you spend a calorie on that, it’s a calorie you can’t use to grow.”

The babies fed high-cortisol milk develop a nervous temperament, focusing their limited energy on putting on weight. As a result, they grow faster, despite getting less energy from their inexperienced mothers.

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.

Deciphering the signals that babies detect in milk might lead someday to changes in formula. Right now, manufacturers try to replicate the nutrition, and even the microbes, in natural breast milk. But they may also need to consider the messages the formula is — or isn’t — sending.

“We’re really missing the breadth of what mother’s milk is,” said Dr. Hinde.