Not only do they need a whole lot of milk, but because they need space in their abdomen for a rumen, they don't really have space for a great big milk cistern in there. This is why they have developed an udder: there isn't space for all the necessary milk internally.

What is the purpose of a cow's udder?

A cow's udder is the part of its body that produces milk. When you milk a cow, you empty its udder by squeezing the milk into a bucket. Moo! To milk any four-legged ruminant, you need to carefully squeeze and pull each teat after cleaning the udder.

Why do cows have four udders?

It's a common misconception that cows have four udders. In fact, cows only have one udder, but each udder is split up in to four quarters, each of which has one teat and separate storage for milk. Cows have four separate teats on each udder so that they can feed multiple calves at the same time.