【高级听力】（文末附视频）Milk Is Just Filtered Blood

Milk Is Just Filtered Blood

《文 末 附 视 频》

This may not be a word-for-word transcript.

Hi, this is Emily from MinuteEarth.

Supermarkets have lots of milks to choose from these days, many of which are the product of blending up and straining various nuts and seeds.

Plain old milk milk, on the other hand, is also a strained product – it’s filtered cow blood. As weird as that seems, all mammal milk is, in fact, made from blood, because blood contains lots of nutrients, and baby mammals need lots of sugar, fat, and protein to grow complex brains and bodies.

But mammal mammas can’t just open an artery; that would be dangerous. Plus, most of the useful nutrients in blood are too dilute and blood has too much iron for most babies to process. This is where the mammary gland comes in: It’s full of thousands of tiny sacs whose walls have special cells that grab water and nutrients from passing blood, do some chemistry on them, and pass them to the inside of the sacs where they mix together to become milk. When a baby starts to suckle on the teat, its sucking pattern tells the mom’s brain to release the hormone oxytocin, which attaches to the sacs, causing them to squeeze out the tiny droplets of milk into the baby’s mouth.

Each mammal species tailors its blood-filtering recipe to the needs of its babies. For example, to help their Arctic-dwelling pups pack on the blubber, hooded seal moms produce a milk with 15 times the fat of cow milk. Cottontail moms make high-protein milk for their bunny babies, allowing them to develop their hopping muscles quickly. The tammar wallaby can make two different types of milk in two different teats at the same time – one high in sugars for a newborn in the pouch and one high in fat and protein for her waddler.

And while selectively bred modern dairy cows don’t have a particularly wacky recipe for their milk, they sure make a lot of it. The current record holder – a Holstein named Aftershock – can produce a bathtub full of milk every day.

That’s utterly impressive.

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