Nicholas Day describes the astonishing findings of Katie Hinde, an assistant professor of human evolutionary biology at Harvard. Here is just one example:
….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.