Soy, skim … spider. Are any of these technically milk?

Take a walk down the refrigerated aisle at your local grocery store, and you’ll find cow’s milk available in an array of fat contents, from whole to skim, as well lactose-free versions.

Goat and sheep milk are also now increasingly available, as are milks made from oats, soy, almonds, cashews, coconuts, rice, bananas, hemp, and even peas.

Scientists have even found evidence of milk-like secretions produced by everything from spiders and amphibians to birds.

All of which might lead an inquisitive mind to wonder: Are all these products actually milk? What even is milk?

(Here’s how your favorite plant-based milk impacts the planet.)

We know more about milk now than any other moment in history.

“We can measure almost everything there is in milk,” says Michael Power, an animal scientist who maintains the milk repository at the Smithsonian’s National Zoo and Conservation Biology Institute—a collection that includes about 15,000 milk samples from more than 200 species of mammals.

“We can measure all the peptides that are in milk. You can measure all the microbes that are in milk. We can look at all the genetic stuff that’s in milk,” says Power, who is also the author of Milk: The Biology of Lactation.

But even as science sheds light on the true nature of milk, new questions are arising. Here’s what we’re learning.

Before milk, there was poison.

If you really want to know about milk, says Power, you need to go back to a time when milk did not exist.

Somewhere between 250 and 300 million years ago, there was a small, scaly-skinned creature known as a synapsid. This is the ancient lineage that would eventually give rise to mammals—and synapsids are also thought to be the first of our ancestors capable of nourishing their young with secretions from glands on their abdomen.

This wasn’t the suckling you’re used to. Synapsids laid eggs and then slathered them with discharge from their bellies.

(Some spiders nurse their young with milk.)

“We think the original purpose is mostly going to be water balance,” says Power.

In addition to H2O, scientists suspect that synapsid liquids would have contained essential minerals, such as calcium, phosphate, and sodium—as well as poisons that protected the delicate eggs against bacteria and fungi.

Whatever was in that synapsid home brew, it worked, because they made it through what’s known as the Permian-Triassic Extinction Event, which extinguished nine out of every 10 plant and animal species on the planet.

“To me, that means that lactation is an incredibly important evolution and adaptation,” says Power.

Milk is nourishment.

Today, there are as many kinds of milks as there are species of mammals, and each is unique.

Hooded seal milk contains up to 60 percent fat, while black rhino milk has just 0.2 percent fat. Blue whale milk is the consistency of runny cottage cheese, the better to transfer from mom to calf underwater.

(Millions of Americans drink raw milk. But is it safe?)

Platypuses and echidnas don’t have teats, but rather exude their milk from patches of skin, not unlike a sweat gland weeping perspiration.

Suffice it to say, each kind of mammal has gone down an evolutionary path that helps it survive, and that has spurned evolution in milk chemistry, consistency, and delivery mechanism.

Milk is a mammal thing. (Or is it?)

But are we mammals alone in our liquid magic? Scientists are starting to uncover evidence that suggests we might not be.

“Pour yourself a frosty glass of pigeon milk. Or maybe don’t,” writes Rosemary Mosco in A Pocket Guide To Pigeon Watching.

For the first few days of life, both mom and dad pigeons vomit a curd-like secretion from their throat lining into the mouths of their young—accomplishing the very same purpose as the first few days of nursing in humans. Penguins, flamingoes, and a handful of other birds also produce this substance known as crop milk.

Likewise, jumping spiders, nematodes, and discus fish have also been found to produce nutrient-rich secretions that sustain their young. And at least one species of legless amphibian known as caecilians was more recently found to do the same—but it does so while the young are still inside the mother, secreting its milk from the oviduct. Scientists think great white sharks employ a similar method, with milky white uterine fluid nourishing their pups.

(This might be the first newborn great white shark ever recorded.)

But are these substances actually milk?

Carlos Jared and Marta Antoniazzi—the scientists who discovered the caecilian’s oviduct milk—tell me they were surprised to find it contained protein, carbohydrates, and lipids, just like mammalian milk.

“The fatty acids are very similar to those we find in regular milk,” says Antoniazzi, of the Brazilian biological research center known as Instituto Butantan.

Power points out, though, that it’s only ever an occasional species of amphibian, arachnid, bird, or fish that can produce these substances.

“Mammals are completely defined as a huge taxonomic group by milk,” he says.

And to him, that means mammal milk stands alone.

Milk is food.

The global milk market is expected to exceed $1.2 trillion by 2028—and so it’s also important for regulators to define what constitutes milk on grocery store shelves.

In its Code of Federal Regulations, the U.S. Food and Drug Administration officially stipulates that “milk” comes from cows—milk is defined as “the lacteal secretion, practically free from colostrum, obtained by the complete milking of one or more healthy cows.”

(We still don’t know why humans started drinking cow’s milk.)

It also outlines a few acceptable variations: “Milk that is in final package form for beverage use shall have been pasteurized or ultrapasteurized, and shall contain not less than 8 1/4 percent milk solids not fat and not less than 3 1/4 percent milkfat.”

Interestingly, the FDA does not maintain “standards of identity” when it comes to other popular mammal milks, such as goat or sheep’s milk.

In 2018, the agency set out to determine if the rise in plant-based milks was confusing consumers. After receiving more than 13,000 comments from the public, the agency decided that people generally understand plant-based “milks” do not contain cow’s milk. Even still, it recommends that any product with “milk” in the name also include information about how it is nutritionally different from cow’s milk.

But are plant-milks truly milk?

“To me, milk fundamentally is about parents providing for young,” says Katie Hinde, a bioanthropologist at Arizona State University and co-editor for the International Milk Genomics Consortium’s publication, Splash! Milk Science Update.

So no, says Hinde, almonds and oats need not apply.

Milk is information.

By far one of the most interesting things about milk science today is the emerging understanding of how complex the liquid can be.

“I think of milk as sort of an information conduit from mother to offspring,” says Power.

Through a balance of nutrients, hormones, and even microbes, milk guides the baby’s growth and development, but also reflects the mother’s body condition—some ingredients, such as calcium, literally come from the mother mammal’s own bones.

“Milk can change over time,” says Power. The biochemical structure of a mother’s milk is different in the morning and at night, he says—and can even change by the time you finish one feeding.

Hinde points to tammar wallabies, which are born with their internal organs only partially developed. “Marsupial milk has proteins that helps them develop their lungs so that they can start breathing air through their lungs instead of respirating through their skin,” says Hinde.

Researchers are working to identify those proteins to see if they might lead to treatments for premature human babies, who often suffer from underdeveloped lungs and pneumonia.

Milk is still full of mysteries.

While it’s no understatement to say that we humans wouldn’t exist without milk—and that it continues to wow scientists every day with its intricacy, flexibility, and medical potential—there is one thing you won’t find Power calling milk.

“Some people say, ‘Well, [milk is] the perfect food,’” says Power. “And I laugh and say, ‘Evolution makes nothing that’s perfect.’”

Milk is naturally deficient in iron and Vitamin D, he points out. It also represents a compromise between what the baby requires and what the mother can afford to produce. In other words, a “perfect” milk would contain even more nutritional resources, but this would endanger the female that produced it.

“You can’t kill the mom to feed the baby,” Power explains.

Finally, milk is full of things we have yet to understand.

“There are huge numbers of other things that are going on in there,” says Power. “We find things in milk and sometimes, we’re scratching our head…does it have a purpose or not?”

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