For decades, the dairy industry has used data to supercharge the humble black-and-white Holstein cow into a milk-producing machine. Across the US, thousands of dairy farmers keep assiduous records about how much milk their cows produce, and the volume and composition of that milk. All of this information feeds into mathematical models that predict the total amount of milk a cow makes over its lifetime. Farmers use this information every day to decide how to care for and breed their animals. As a result, cows today make four times more milk than they did in the 1940s.

To Katie Hinde, the dairy records were a goldmine. Hinde studies the biology of milk; her Twitter handle, delightfully, is @mammals_suck. By analysing around 2,390,000 lactation records covering 1,490,000 cows, she found a clear pattern: Cows produce more milk for their daughters than their sons. The sex of the first calf is particularly important, and can influence how much milk their later siblings get.

These results suggest an easy way in which dairy farmers could squeeze more milk out of their herds. But they also contradict a longstanding hypothesis about how animals invest in their young, one which predicts that creatures like cows ought to favour sons over daughters.

Since the 1970s, biologists have shown that animal mothers don’t treat all of their offspring equally. Often, they invest more in one sex than the other. The most famous explanation for this pattern was proposed by Robert Trivers and Dan Willard. They argued that females in rude health should devote more resources to the sex that would benefit most from them.

That would usually be males, especially in species where males compete for females. A really strong fit son could potentially impregnate dozens of females in a short space of time, while a weak one will impregnate none. A bit of extra parental effort could make a huge difference. By contrast, daughters spend a lot of time being pregnant and raising their existing young. This sets a ceiling on how many offspring they can raise, regardless of their physical condition.

So if you’re a baboon, a red deer or an elephant seal, and you want more grandkids, you should spend more effort on making your sons as competitive as possible. There are many ways of doing that. You could have more sons, or give birth to bigger sons.

Or, you could use milk. Milk is the main source of nutrients for mammal babies, but comes at great cost to mothers. As Hinde puts it, “Females dissolve parts of themselves to nourish their babies”. They could offer more milk, or more nutritious milk, to one sex over the other, and a few studies have looked for such biases in deer, seals, monkeys and even humans.

Hinde summarised this work in 2012, in a lucid and witty blog post that you should read. She concluded that “mothers make milk differently for males and females”, but noted that this was a niche field of research with many unanswered questions. For example, can male and female foetuses program their mother’s mammary glands in different ways, while they are still in the womb?

When dairy scientist Barry Bradford read Hinde’s post, he realised that cows could help to address her question. The industry’s careful record-keeping would easily reveal how much milk cows offer to calves of either sex. And since calves are separated from their mothers shortly after birth, you could cleanly measure the influence of the foetus, without having to worry about whether the calves had an influence after birth.

Bradford contacted Hinde through Twitter and asked if she wanted to collaborate. She leapt at the chance. “He said, ‘I can get the records but I can’t interpret the data from a theoretical perspective’. I said, ‘Well, that’s great because that’s where my skillset is!’”

The team retrieved every lactation record from 1995 to 1999, and cleaned them up to get rid of incomplete or duplicated entries. They ended up with 2.39 million records, covering 1.49 million cows—a welcome change from the small samples that animal behaviour scientists normally settle for.

The results were very clear: Cows make more milk when they give birth to daughters than sons. The milk doesn’t contain any more fat or protein so its quality is the same—there’s just more of it. “Things turned out so elegantly,” says Hinde.

The first pregnancy is crucial. It kicks off the development of the cow’s mammary glands, and creates a baseline that affects all later pregnancies. And by focusing on records for 113,750 cows, Hinde’s team showed that the sex of the first calf has long-lasting effects.

View Images Image credit: Scott Bauer, USDA Creative Commons license

After giving birth, a female cow makes milk for 305 days and during that time, farmers often impregnate her again. This means that she’s pregnant with Calf 2 while she’s still breast-feeding Calf 1. If Calf 1 is a son, mothers are permanently handicapped in how much milk they can make, especially if they have another son the second-time round. If they have a daughter next, that partly reverses the deficit, but not completely.

By contrast, having a daughter the first time round ‘protects’ a cow from the negative effects of a second-round son. All in all, a cow that has a daughter first time round makes around 445 kilograms more milk across her first two lactations than a cow with back-to-back sons. That’s a sizeable amount, equivalent to a production boost of 2.7 percent.

View Images The effect of sons and daughters on mum’s milk production, across two lactations. S = son, D = daughter, numbers along x-axis indicate order of pregnancy. Credit: Hinde et al, 2014, PLOS ONE.

These results are a blow for the Trivers-Willard hypothesis, which cows really ought to uphold. Dairy cows are domesticated versions of now-extinct cattle called aurochs. Bull aurochs were much bigger than females, and must have competed intensely for mates. They’re a classic example of the animal societies that Trivers and Willard were talking about. And yet, their modern descendants invest more in daughters than sons.

This doesn’t mean the hypothesis is “wrong”, just that it isn’t as universal as it’s sometimes made out to be. “The Trivers-Willard hypothesis has been such a dominating force in the literature, and the data for it is quite equivocal,” says Hinde. “Lots of people don’t want to believe that there could be a bias in favour of daughters, but this study was so clean.

So, why do cows invest more in daughters? It’s possible that the extra milk could speed up a daughter’s development, allowing her to reproduce at a younger age and amass more babies over her lifetime. Bulls aren’t in such a rush. They start reproducing later than females anyway, so they’ve got more time to make up for any shortfall in milk. This, says Hinde, means that “mothers may be able “under-invest” in a son with relatively less consequence for the number of offspring he will go on to sire.”

But Hinde cautions that people usually assume that these sorts of sex biases are adaptive for the mothers. But maybe it’s not about the mothers at all. Maybe the right question is: Why are foetal daughters better at manipulating their mothers into making more milk?

It could be that they make the right hormones. Female foetuses produce more oestrogens that could cross the placenta into the mother’s bloodstream, and influence the development of her breast glands. Sons can produce oestrogens too, but it might muck with their own genital development. Perhaps daughters can send more hormones into their mothers without any risk to themselves.

Either way, Hinde’s results have implications for the dairy industry. If they wanted to, dairy managers could ensure that most of the calves they breed are females, but they’d need to separate semen by sex to do so. In the past, some people have argued that this isn’t cost-effective, but it might be worth it if it leads to a 2.7 percent bump in milk production.

What about humans? A couple of small studies have found that women produce more nutritious milk for sons than daughters, but neither one measured milk volume. Indeed, it’s very hard to do that. One option would be to weight babies before and after feeding, but it would be hard to do that on a large enough scale to get reliable data.

Still, it’s a question that Hinde wants to address eventually. “Right now, formulas are the same for sons and daughters, and recommendations for neonatal intensive care units are the same,” she says. “We make different deodorants for men and women but we’re not thinking about whether the developmental needs of daughters and sons are identical.”