Look, you wouldn’t be here without milk.

You may not have depended on it as a baby, but one of your ancestors did. And also even now, many of us continue to indulge in cheese, ice cream, and all of the good things dairy gives us as adults.

But we’re not the only animals that drink milk. And we’re not the only animals that produce milk. Because guess what?

You’re a mammal. The only thing that’s weird about us is that we don’t exclusively drink the milk of our own species. But why do we rely so heavily on milk and why do we favor the milk of cows and goats, for example, over other animals?

Well, some of the alternatives are… pretty extreme. Let me give you a taste. If you’ve had a cup of coffee with cream lately, or maybe cereal with milk, you’ve come to the right place.

Because, while you may not have thought of it then, milk is weird. Well, the liquid itself isn’t that weird. It’s super useful.

But the way that we use it? That’s weird. I mean, milk is for babies, that’s why it was made.

Plenty of adult humans drink it anyway, like myself, and the milk we drink mostly comes from cows, but there are lots of other options out there -- including the fattiest and skimmest milks known. You just probably wouldn’t want to drink them. Milk is produced by modified sweat glands in mammals called mammary glands, and it’s used to give baby animals the nutrients they need before they can digest other food.

All mammals produce milk--it’s where we get the name Mammalia, and it’s one of the three defining features of mammals, along with hair and three middle ear bones. Humans, being mammals, also make and drink milk. But we’ve done something no other animal has:.

We’ve taken this nutrient-rich baby food and, through genetic mutation, turned it into a digestible, dietary staple for humans of all ages. But most people don’t continue to drink human milk throughout their lives. If they did, it would mean a human would have had to sacrifice the calories necessary to make the milk, which would have totally defeated the purpose back when we were starting to use it as an extra source of nutrients.

So instead, we drink the milk of other animals. People around the world drink milk -- and make other food products from animals like yak, reindeer, water buffalo, elk, and horses. In the US and Europe, dairy consumption is mostly limited to the mammary secretions of sheep, goats, and cows.

But really, mostly cows. Why? Well, first, picture a cow.

They’re big and they’re pretty nice. Over generations of selective breeding, we’ve created creatures who can give between six and seven gallons of milk a day, and they’re domesticated enough to just line up to do it. Goats and sheep, while they’ve also been bred to produce more milk and be more willing to do so, can only produce about a gallon of milk a day.

By economics alone, cows are the better milk producers. But it’s also a matter of taste. Cow’s milk has a fat content similar to human milk -- about 3.5% versus a human’s 4.5% which makes it familiar, at least if you want to drink whole milk.

By comparison, the water buffalo, which produces the milk that gives the world buffalo mozzarella, has a fat content of almost 7 percent -- great for cheese, but maybe not the best for drinking. These animal milks all have different fat content, because it depends on the animal’s environment and the way that they nurse. If you’re looking for the very fattiest known milk, you’ll have to find a hooded seal -- their milk is about 60% fat.

That’s because hooded seal pups are born on floating sea ice in the North Atlantic and Arctic oceans. The harsh temperatures and the instability of the environment mean that the pups have to put on a lot of fat really fast. They only nurse for about four days, but almost double their weight in that time.

By comparison, heavy cream is usually around 36% fat. Even if you wanted to hang out in the freezing, icy ocean, drinking this stuff would be like trying to drink really fatty peanut butter. At the opposite extreme, the skimmest known milk comes from the black rhinoceros, with a fat content of around 0.2%.

The watery milk probably has to do with their long nursing period. Black rhino moms nurse their young for almost two years, and pouring a ton of fat into their milk for that long would use up a lot of resources. But if you wanted to try some out, you might have some trouble tracking it down.

For one thing, black rhinos are critically endangered, so it would be tough to find one in the first place. And even if you did find one, it’s still a rhinoceros. Have you seen one of those things?

About a third of the females and half the males die from fights with other animals. Unless you’re a baby rhinoceros, a nursing rhino mom is probably not going to let you walk up and take a swig. Now, before we sign off, I want to clear up one milky myth that gets repeated a lot.

Hippopotamuses are mammals, and they make milk. You may have heard that it is pink, but it is not. Hippo milk is whitish, just like all other milk.

This misconception probably comes from the fact that hippos do secrete a super useful mucus on their skin that helps protect against UV rays and works as an antibiotic. And the mucus is a reddish color. But it is not milk!

Though, again, you probably wouldn’t want to go up to a hippo and check. But, ok, knowing that there’s so many other animals that make milk, and even knowing that some of it isn’t exactly appetizing, I still want to know:. Does it all taste the same?

If I had to pick one to try, I’d probably try polar bear milk. Just so I could have the story. But alas, I will probably never get the chance to taste polar bear milk, so we’ll have to settle for Stefan’s explanation of what it would taste like if we could drink it:.

Science is all about asking important questions.

Like: What does polar bear milk taste like? And the best way to find that out is… well… to taste it. So that’s exactly what polar bear scientist Andrew Derocher did. Turns out it’s got a creamy texture, and it’s a bit chalky, salty and even fishy.

And that reveals a lot about its composition, which gives researchers like Derocher insights into the lives of polar bears. Milk production is one of the defining features of mammals. That means all mammals, from platypus to humans, make some variety of the stuff.

Each species’ milk is tuned to the unique needs of their babies, so the composition—and resulting taste—can tell us a lot about that species. Of course, getting milk from a mother polar bear is no easy task, but intrepid researchers like Derocher have actually collected samples—and even tried them, just for good measure. Polar bear milk is pretty fishy tasting because it has lots of fishy-smelling oils, which come from the seal blubber mom is eating.

And it’s also salty or chalky because it has a lot of minerals, including sodium, in it. Saltiness is a common feature of marine mammal milks because sodium binds with other chemical compounds to help the cub absorb the fat and fat-soluble vitamins in the milk. And boy is there a lot of fat—that’s why it’s so creamy.

Analyses show that when the cubs are young, fat makes up about 40 percent of the mama bear’s milk, while another 11 percent is protein. Those hefty levels of fat and protein allow the cubs to drink relatively small amounts per day. A 50 kilogram, 8 month old cub only downs about 470 grams a day.

When our babies are that age, they’re only about 9 kilograms, yet they’ll drink around 750 grams a day. Mind you, human milk is much less rich. It has only a tenth of the fat of polar bear milk and a sixth of the protein.

So it’s much less creamy, but a lot sweeter, because it contains about seven times as much lactose or ‘milk sugar,’ which helps nurture the right intestinal microbes and boost baby’s immune system. Don’t look at me like that. You know you were wondering what human milk tastes like.

And while human milk isn’t quite as fatty as polar bear milk, it is fattier than cow’s milk. That’s because both human babies and polar bear cubs are born underdeveloped and helpless, or altricial, so they need a certain amount of fat and nutrients to finish their development outside the womb. The quicker that happens for the bears, the better, because that means less time when the young are vulnerable to predators or rivals.

Our species has family groups and tools to keep babies safe— polar bear mamas don’t. So they fast-track their cubs’ development by feeding them the bear version of half and half. The fact that mama bears produce such fatty milk is even more impressive when you realize that they produce it while starving.

In an extreme effort to keep their cubs safe, these moms hunker down with them in an underground snow den for 4 to 6 months. During that time the family never leaves, and mom has to slowly use up her own fat reserves to make the milk. And because those reserves are eventually limited, the milk actually changes flavor and becomes less creamy over time.

In addition to helping the cubs grow nice and rotund, that fat is also there as biological fuel. As you can imagine, it’s not exactly tropical inside a snow den. Cubs are born almost totally hairless, so they need a way to keep warm in those freezing temperatures.

A high metabolic rate allows their bodies act like mini furnaces and generate heat by burning some of the milk’s fat until they grow enough fur and fat to weather the cold. A lot of protein could also provide the needed caloric fuel, but it turns out protein is kind of troublesome for polar bears. Protein digestion creates waste products like urea or uric acid, which the body needs to dilute to make safe.

Despite all the ice around, there’s actually not that much drinkable water available to the bears, so if they eat too much protein, they can poison themselves. That’s why, if you look really closely at a nature documentary you’ll see that polar bears munching on seals aren’t actually eating much of the meat. They’re going for blubber, because it contains a lot of fat and fluid.

Polar bear milk is also a little more sour at first because it contains more nutrients, immune cells and antibodies, which kick-start the newborns’ immune systems. And it contains a pretty hefty dose of vitamin D, which helps those little bear bodies absorb the calcium from the milk and grow strong bones. That’s important because polar bear cubs grow really fast so their bones need to be able to support all that new weight.

Vitamin D is sometimes called the sunshine vitamin because we can boost synthesis of it by going out in the sun but since the baby bears are cooped up, they have to get theirs from mom. Unfortunately, those aren’t only things a mother bear passes along in her milk. Scientists are increasingly concerned that polar bear milk may be becoming bitter due to persistent pollutants.

Everything from flame retardants to pesticides is drifting up to the Arctic, and these chemicals, which can cause damage to the immune, digestive and reproductive systems of Arctic animals, collect in fatty tissues like blubber. So polar bears are particularly at risk because they eat so much contaminated fat. Bear moms then unknowingly pass these chemicals on to their cubs.

Scientists don’t yet know what impacts these pollutants have on cub growth or health but, needless to say, they might not be good. So they’re continuing monitor the levels of these contaminants and study their effects. And, strange as it may sound, further sampling of polar bear milk could help them keep an eye on that, as well tell them if there are any changes to the animals’ diet or overall health.

Who knew you could learn so much from a sip of milk? So human environmental impacts are so far-reaching, they’re even affecting polar bear milk. Great.

We’ve also contributed to the rise of antibiotic-resistant bacteria, to our own peril. So where’s the good news? Here’s Michael to tell us how milk might hold the key to fighting antibiotic resistance.

And you’ll never guess what kind. A huge milestone in medicine was the discovery of antibiotics in the early 1900s, arming doctors against bacterial infections like never before. But the thing about bacteria is they reproduce really quickly, which means more genetic mutations, and the chance of developing resistance to our drugs.

In fact, MRSA is a strain of bacteria that runs rampant in hospitals nowadays because of its resistance to lots of antibiotics. So scientists are turning to unusual places to find new compounds to fight off these drug-resistant “superbugs.” Including... Tasmanian devil milk.

In a paper published this month, researchers at the University of Sydney reported that this marsupial’s milk is full of antimicrobial compounds called cathelicidins. These compounds have been found in mammals and birds, and they basically work by poking holes in the cell membrane of a bacterium or fungus, which kills them. So far, we know that humans have one cathelicidin protein, but Tasmanian devils have six of them, which these scientists named Saha-CATH1 through 6.

And they’re really important in developing the immune system of newborn Tasmanian devil joeys, which initially don’t have any protection from all the microbes in their mom’s pouch. So the researchers synthesized those six cathelicidins and pitted them against 25 strains of bacteria and 6 strains of fungi. Three of the compounds didn’t show any antimicrobial properties, so they might be involved in regulating other parts of the Tasmanian devil immune system.

But Saha-CATH3, 5, and 6 did. Saha-CATH3 was specifically out to kill one strain of fungus. On the other hand, Saha-CATH6 killed several strains of Streptococcus bacteria and a strain of antibiotic-resistant bacteria called VREF.

And Saha-CATH5 seems to be the most promising compound when it comes to potential drug development, since it wiped out just over half of the bacteria and fungi that were tested, including the superbugs VREF and MRSA. So there’s a lot we can learn from other animals when it comes to human medicine. Wow, see, milk is great!

I don’t know about you, I’m really looking forward to this bacteria-busting technology hitting the mainstream. But if we’re being honest, for all the great stuff that milk does for us, we were never supposed to drink this much of it. And some of us only persist in our dairy delight because we’re actually... mutants.

Have you ever found yourself, I don’t know, thinking about the X-Men, secretly wishing you were a mutant who could control the weather, or read minds, or do something cool like that? Let me ask you this, then: Do you drink and enjoy milk? How about ice cream?

Can you do that without getting ill? Well then, congratulations, you are a mutant with a special superpower after all! Milk is produced by mammalian mammary glands, mainly to feed babies until they can digest other foods.

Although milk is very nutritious -- full of protein, calcium, potassium, vitamin-D -- it’s also full of lactose, or milk sugar, which can be hard to digest. Luckily, baby mammals, including human babies, produce tons of the lactose-digesting enzyme lactase. If they didn’t, they wouldn’t be able to process their sole food source, and they’d die.

Back in the day, when a child was four or five years old, their bodies started easing off the production of lactase. And by the time the kid was seven or eight years old, nursing or drinking another animal’s milk would have made him really sick. If you or someone you know is lactose intolerant, you know what kind of miserable stomach-cramping bathroom blowout awaits you if you dare succumb to a double-scoop of chocolate chip ice cream.

And those sufferers are actually in the majority -- about 70 percent of the world’s population cannot produce lactase after childhood. And without lactase around to digest lactose, milk basically becomes toxic. So what changed?

How did milk go from being a weird food that only babies could appreciate to a supermarket staple? Two words: Mutant. Farmers.

Humans started to domesticate animals around 11,000 years ago in the Middle East. And traces of milk fat have been discovered on artifacts in the Fertile Crescent going back about 8,500 years ago, and in central Europe, around 7,000 years ago. The chemistry of these traces suggested that Neolithic herders had discovered a neat new way to reduce the concentrations of lactose in milk -- by fermenting it. -- turning it into cheese and yogurt.

But that only got them so far. They still could not drink the actual milk, be it goat, or cow, or whatever. And then, everything changed, when a unique genetic mutation popped up.

It’s known as the lactase persistence trait, carried by what’s called the LP allele, and scientists think it first appeared about 7,500 years ago in central Europe. That one little gene variant allowed its bearers to continue producing lactase into adulthood. It probably spread as those Neolithic groups trekked north and west through Europe.

The allele did particularly well in the north, probably for several reasons. For one thing, dairy products store well in colder climates, and they’re extremely handy in places where food may have been harder to come by, or grow. And it may even be that milk’s high concentrations of vitamin D provided a health advantage in areas with little winter sun, since our bodies typically need sunlight to make vitamin D.

So the lactase persistence trait may have helped make this wave of human migration possible, but still, it wasn’t necessary everywhere. Today, in Britain and Scandinavia, nearly 90 percent of adults can chug all the milk they want, whereas down toward the Mediterranean, probably less than 40 percent of people have lactase persistence. And in some populations in Africa and Asia, it shows up in less than 10 percent.

So if you’re one of the 30 percent of the world’s mutants who can eat ice cream with impunity, enjoy that evolutionary perk for everybody else! Well, we’re mutants, but at least we’re mammals. Like, we have some justification for enjoying milk.

At least we’re not creepy, crawly, horrible little…. I mean, what gives cockroaches the right?! Just….

Olivia, you take it from here. You, me, dogs, cats, horses, guinea pigs, we're all mammals. All part of one big, happy, milk-producing family.

Other animals generally don't share the ability to feed their young with a substance secreted from their own bodies. With a few exceptions. Like the Pacific beetle cockroach.

Yes, I said cockroach. Now, let me start by clarifying that I'm talking about a milk-producing insect, not a milk substitute made from insects, which does exist but is a whole other, unrelated thing. And I should also note that this stuff is not exactly milk at least not by the standards of the dairy industry since the roaches don't have mammary glands.

Plus, their babies don't nurse after birth the way mammalian babies do. so you can't just like, squeeze them to collect a tube full of milk. But these insects do produce a special food for their young. And that's how they're able to avoid doing something that most other insects do: laying eggs.

Eggs can't exactly run from all the predators that might make a fine meal of them. So young insects that can move about the moment they leave their mother have a bit of a leg up in life or, six of them, to be exact. And the Pacific beetle cockroach is the only known viviparous cockroach, which means that females give birth to live babies.

They actually have a uterus of sorts called a brood sac. And it's while a female is incubating her babies in this sac that she produces a milk-like substance for her embryos to ingest. This quote “milk” is a complete source of nutrition for the young cockroaches.

It's almost 46% protein, including all of the essential amino acids, and about 25% carbs. And it's a whopping 16 to 22% fat, which includes omega-3s and other “healthy” fats. Plus it's got vitamins and minerals.

So basically, it's got a lot of everything, which is why it has three times as many calories per gram as buffalo milk, and some researchers say it's among the world's most nutritious substances. And that gives the soon-to-be cockroach babies an evolutionary advantage. During gestation, Pacific beetle cockroach babies undergo a 50-fold increase in dry mass from the time they arrive in the brood sac as fertilized eggs to the moment they leave their mother's body.

That's so big that, at birth, the brood typically numbering around 12 nymphs can be one and a half times the weight of their mother. And this whole process from embryo to nymph happens three times faster than in other cockroach species! They keep growing fast, too.

Males will reach adulthood after just three to four molts. By contrast, German cockroaches molt six times before reaching adulthood. And since the bugs are very vulnerable during and just after the molting process, fewer molts means they spend less of their lives in this exposed state.

In fact, this milk is considered so nutritious that well, you may have already guessed where we're headed. Yes, some have tossed around the idea of mass producing cockroach milk for human consumption. But alas, scientists and health-food moguls have yet to come up with a practical way to harvest this stuff.

The roaches are pretty small, and since the fluid is excreted into the brood sac not out into the world, you can't exactly attach them to a milking machine. To get the milk from the mama bugs, researchers inserted filter paper into their brood sacs. That soaks up the goods, which the scientists can then extract from the paper.

They can also cut open the young roaches to get at the stuff. see, the liquid turns into crystals in the embryos' digestive tracts. And those crystals can then be cut from their stomachs, a process the roaches don't survive. Either way, each roach only gives a tiny amount.

Experts estimate it'd take upwards of 1000 cockroaches to get 100 grams of milk. That means you'd have to milk and probably kill countless cockroaches to produce an actual bottle of this stuff, let alone enough bottles to sell to the masses. Also, we don't actually know it's safe to consume in any quantity.

So, you won't see it on store shelves any time soon. Researchers may one day be able to synthesize cockroach milk, though. Still, even if that happens, the milk is more likely to have medical applications than it is to end up as a substitute coffee creamer.

Which is fine, because we can always drink that milk that's made from insects instead. It seems that for many animals, milk promotes growth in babies. Even if they’re not mammals!

It may taste different or be delivered differently depending on your needs as an organism, but in the end it’s an investment in a species’ next generation. Thanks for watching this episode of SciShow. If you’d like to keep learning more, you might like our video about just why babies are so great.

Check it out, and we’ll see you next time.