Milk straight from the source contains a variation of vitamins, minerals, fats, sugars, and other factors that promote optimal growth, development, and behavior in babies. Not only does the nutrient content of the milk change over time as the baby grows, but the milk’s composition will actually differ based on the gender of the child.

In a variety of mammals, including humans, gender also plays an important part in milk composition. Males, who tend to be more muscular, require additional fat and protein. While the fat content in the milk females drink isn’t as high, they tend to consume more milk per meal and will nurse longer. A 2012 study led by Masako Fujita from Michigan State University published in the American Journal of Physical Anthropology showed that human mothers produce milk with 2.8% fat for sons and 1.74% for daughters. In extremely impoverished locations where infant mortality is high, the fat content is higher for girls.

A new study has shown that these differences may very well begin during fetal development. The study was led by evolutionary biologist Katie Hinde from Harvard University and was published in PLOS One. She presented her results this past Saturday at the AAAS 2014 Annual Meeting. The differences appear to begin during fetal development, according to Hinde’s study. Hinde analyzed 2.39 million lactation records from 1.49 million dairy cows and determined that those who had birthed females produced an average of about 445 kg (980 lbs) more milk than those who birthed males, over a two year lactation span. Even if the mother and calf were separated shortly after birth, the volume differences persisted. For cows, however, gender does not impact nutrient content.

For some animals, such as rhesus macaques, social status within the group is passed down from generation to generation. Previous research from Hinde has found that young female rhesus macaques get higher levels of calcium than their brothers, giving them stronger bones and potentially allowing them to reach sexual maturity more quickly. Because females cannot reproduce throughout their entire lives, starting early is an advantage. However, the males receive more cortisol, which helps to regulate metabolism and temperament, allowing them to grow up strong and sire more offspring.

Breast milk is hardly the static, homogenous liquid we are most familiar with from the store. Though the ability to nurse our young is one of the defining features that makes mammals distinct from every other class of animal, there is still an incredible amount about it that we are just discovering. Learning more about how the breast milk is formulated inside the mother’s body will allow us to better understand the ever-changing nutritional needs of babies and could even allow commercial formulas to be reconfigured to better nourish infants when breastfeeding is not an option.