Fetal Daughters Influence Milk Production in Cows

Functional development of the mammary gland occurs during pregnancy. When dairy cows and goats are gestating twins, mammary gland development is amplified due to hormonal signals from the much larger fetal-placental unit. Carrying twins seemingly programs higher milk production to meet the needs of “double the trouble” (Nielen et al., 1989, Hayden et al., 1979). But what if fetal-placental signals aren’t just about the number of offspring, what if other features are signaled that influence milk production, features like infant gender (sidebar: dairy scientists use the term “gender,” evolutionary biologists use the term “sex”)? Sex-biased lactation has received substantial research effort in the last few years, has been documented in humans, monkeys, rodents, deer, and marsupials, and was featured in a 2012 SPLASH column. But how does the mammary gland “know” that milk is being produced for a son or a daughter?