Producing milk for their offspring costs female mammals a lot of energy. To produce milk, bodily reserves are mobilized to feed the developing young. The question always remained whether mothers invest any different in their sons and daughters.
Time to put it to the test, the researchers thought. They collected 2.39 million lactation records from 1.49 million dairy cows to check if the mammary gland that produces milk during lactation was more active by either of the sexes of the cow’s offspring. And they found a clear difference. Cows seem to favor daughters, producing significantly more milk for their female offspring.
Not only cows…
It was also found that the gender of the fetus directly enhances or diminishes the milk production. The effects on the milk production persist for a longer time. When the first calf of a cow is female, the milk production over the first two lactations is increased by around 445 kilogram.
Previously it was often assumed that after-birth conditions mainly influenced the milk production, but Holstein calves are separated from their mothers directly after birth. It has now become clear that the conditions in utero are key. Other animals, including humans, also have a so-called sex-biased milk synthesis.