Many human parents try hard to treat their sons and daughters equally. But when it comes to making milk, many mammals can be a bit unfair. Cows, for example, make significantly more milk for their daughters than their sons. Dr. Katie Hinde, an evolutionary biologist at Harvard University, discovered this, when she and her colleagues examined millions of dairy records. The study suggests that the bias begins during pregnancy, since dairy calves are separated from their moms at birth. The result seems to contradict a popular theory that moms with extra resources will favour their sons over their daughters, since the fittest males have the potential to create far more offspring than the fittest females. But Hinde thinks humans may also tailor their milk production for sons or daughters and that could have implications for how we feed our babies.

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