Mom’s Milk Adapts: Breastmilk may be the first food for most babies, but it’s not the same for boys and girls.
Researchers at Michigan State University found that among 72
moms in rural Kenya, women with sons tended to give fattier milk. The boys drank breastmilk with 2.8
percent fat compared with 0.6 percent for girls. Among poor women, however, daughters got the creamier milk (2.6 versus 2.3 percent).
The finding, published in the American Journal of Physical Anthropology jives with previous studies on macaque monkeys, red deer and gray seals, as well as with research on healthy affluent moms in Massachusetts. All the research appears to back up a 40-year-old evolutionary theory that natural selection favors parental investment in daughters when times are tough and in sons when times are good.
The thinking is that raising a popular male can lead to many grandchildren (at least in societies where males have many sexual partners). On the other hand, poorer families are better off raising a daughter who is sure to have at least some grandchildren, while a son of low ranking may not manage to have many or even any.