If an alien life form landed on Earth tomorrow and called up some of the planet’s foremost experts on lactation, it would have a heck of time figuring out what, exactly, humans and other mammals are feeding their kids.
The trouble is, no one can really describe what milk is—least of all the people who think most often about it. They can describe, mostly, who makes it: mammals (though arguably also some other animals that feed their young secretions from their throat or their skin). They can describe, mostly, where it comes from: mammary glands via, usually, nipples (though please note the existence of monotremes, which ooze milk into abdominal grooves). They can even describe, mostly, what milk does: nourish, protect, and exchange chemical signals with infants to support development and growth.
But few of these answers get at what milk, materially, compositionally, is actually like. Bridget Young, an infant-nutrition researcher at the University of Rochester, told me milk was an “ecological system”; Alan S. Ryan, a clinical-research consultant, called it a “nutritional instrument.” Bruce German, a food scientist at UC Davis, told me milk was “the result of the evolutionary selective pressure on a unique feeding strategy,” adding, by way of clarification, that it was “a biological process.” A few researchers defaulted to using milk to explain something else. “It’s the defining feature of mammals,” says Melanie Martin, an anthropologist at the University of Washington. None of these characterizations were bad. But had I been that alien, I would have no idea what these people were talking about.
What these experts were trying to avoid was categorizing milk as a “food”—the way that most people on Earth might, especially in industrialized countries where dairy products command entire supermarket aisles. “Overwhelmingly, when we think about milk, when we talk about milk, we think of…
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