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All mammals produce milk, but you probably wouldn't want to dip your cookies in all of them.

Milk, and the Mutants That Love It

Hosted by: Hank Green
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[SciShow Intro plays]

Hank: 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.   Thank you for watching this episode of SciShow, which was brought to you by our patrons on Patreon. If you want to help us spread more tasty science, you can go to And don’t forget to go to and subscribe!