By Corrie Whisner, PhD
I recently ran across an interesting article in the PLOS ONE journal entitled, “Holsteins Favor Heifers, Not Bulls: Biased Milk Production Programmed during Pregnancy as a Function of Fetal Sex” by Katie Hinde and colleagues at Harvard and Kansas State Universities. After the passing of International Women’s Day on March 8 and recently finishing the book Lean In by Sheryl Sandberg, I felt inspired to share Hinde’s findings and highlight the scientific side of “girl power!”
In this large retrospective analysis of dairy cow lactation records, researchers found that Holsteins produced 1.6% more milk for female versus male offspring. More interesting, is the finding that fetal sex of first-time mothers influenced milk production in subsequent pregnancies. The advantage of having a female fetus during the first pregnancy carried over to the second pregnancy, resulting in greater milk production across both lactations regardless of whether the second fetus was male or female. The benefit of having a heifer during the first pregnancy was further documented when cows giving birth to a male in their first pregnancy showed significant increases in milk production if their second pregnancy was a female. This increase was evident when comparing data to milk production in dairy cows that gave birth to two consecutive male offspring; however, the milk production of male-female sequence mothers, did not increase to the level of female-first mothers.
According to Hinde, biologists have been interested in studying how mothers differentially allocate resources to male versus female fetuses during pregnancy. To date, little work has been done to investigate sex-biases in milk production which make this article very exciting! Reading this article definitely reminded me of the Barker Hypothesis and the earlier publication Boys Live Dangerously in the Womb, which both provide evidence for fetal programming in humans and suggest that male and female fetuses respond differently in utero to maternal cues.
Maternal milk is an ever-evolving elixir which fuels important changes throughout early development, so it isn’t hard to believe that milk production might differ between species. After reading this article I amazed to learn that mothers may allocate resources, such as milk, to the offspring sex that will receive maximal benefit. A current belief is that males from species with male-male competitive mating rituals will receive a greater investment across gestation and lactation from their mothers. Additionally, Rhesus monkeys have been found to produce energetically-dense milk, higher in fat, for their male offspring; however, female offspring received a greater milk volume which made up for the lack in energetic-density.
As research continues in this area, I wonder if we will start seeing different types of baby formula for boys and girls. Only time will tell…and to that, I say, “Let’s get MOO-ving, so we have more to talk about soon!”
References
1. Katie Hinde, Abigail J. Carpenter, John S. Clay, Barry J. Bradford. Holsteins Favor Heifers, Not Bulls: Biased Milk Production Programmed during Pregnancy as a Function of Fetal Sex. PLoS ONE, 2014; 9 (2): e86169 DOI:10.1371/journal.pone.0086169
2. Trivers RL, Willard DE (1973) Natural selection of parental ability to vary the sex ratio of offspring. Science 179(4068): 90–92. doi: 10.1126/science.179.4068.90
3. Hinde K (2007) First-time macaque mothers bias milk composition in favor of sons. Curr Biol 17(22): R958–R959. doi: 10.1016/j.cub.2007.09.029
4. 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. Am J Hum Biol 21(4): 512–519. doi: 10.1002/ajhb.20917
