In many mammals, humans included, the composition of a mother's breast milk varies with the sex of her offspring, with sons receiving nutritionally richer milk than daughters. Interestingly, the opposite has been found to be true in impoverished mothers.
Researchers at Michigan State University and other institutions found that among 72 economically sufficient mothers in rural Kenya, women with sons generally gave richer milk (2.8 percent fat compared with 1.74 percent for daughters). Poor women, however, favored daughters with creamier milk (2.6 versus 2.3 percent). These findings, published in the American Journal of Physical Anthropology in September, echo previous work that showed milk composition varying with infant gender in gray seals and red deer and with infant gender and the mother's condition in rhesus macaques. The new study also follows findings that affluent, well-nourished moms in Massachusetts produced more energy-dense milk for male infants.
Together the studies provide support for a 40-year-old theory in evolutionary biology. The Trivers-Willard hypothesis states that natural selection favors parental investment in daughters when times are hard and in sons when times are easy.
According to Fessenden, it stands to reason that the imbalance would be most common in polygamous populations, where males can father offspring with multiple females. The line of reasoning is as follows:
In those societies, a son can grow to be a strong, popular male with many wives and children, or he can end up with neither. Well-off parents who can afford to invest in sons should do so because their gamble could give them many grandchildren. Conversely, poor parents should not heavily invest in sons because it is unlikely to pay off—their offspring start at the bottom of the socioeconomic ladder. For those families, daughters are a safer bet because as long as they survive to adulthood, they are likely to produce young.
Given the hypothesized link to polygamous social systems, it would be interesting see how breast milk nutrition varies across a variety of monogamous and polygamous mammalian species. Social monogamy is fairly rare among mammals, having been observed in 27% of primates, and on the order of 5% of mammals overall. Scientists still aren't sure how monogamy evolved in the first place, but it would be fascinating to investigate whether a differential nutrient profile in maternal breast milk co-evolved with its practice. Understanding this connection could help us better understand the selective pressures of parental investment (while shedding light on the ways polygamy might affect our evolution moving forward).