In polygynous primate species, males tend to be larger than females, and this sex difference in body size is evident starting very early in life. In humans, for example, boys are (on average) heavier than girls from birth. Anthropologist Katie Hinde thus suspected that nursing mothers may produce milk differently for sons vs. daughters. She investigated this hypothesis by examining the energy density and quantity of milk produced by captive rhesus macaques. Hinde found that the nutritional profile of a monkey mother’s milk does differ in both quality and quantity depending on the sex of her infant. Mothers of sons produced more energy-rich milk – higher in fat and protein – than mothers of daughters, who produced more milk with a higher sugar content.
Dr. Sylvia Amsler is a lecturer in anthropology at the University of Arkansas at Little Rock.
Hinde K. 2009. Richer Milk for Sons but More Milk for Daughters: Sex-Biased Investment during Lactation Varies with Maternal Life History in Rhesus Macaques. American Journal of Human Biology 21:512-519.
Smith RJ and Leigh SR. 1998. Sexual dimorphism in primate neonatal body mass. Journal of Human Evolution 34: 173–20.