Every other Friday on Morning Edition, Outside/In host Sam Evans-Brown tackles a question from a listener.

Clair from Plymouth asks: “Why do cows have four teats and just about every other animal that doesn’t have litter has two?

Note: This edition of Ask Sam originally aired in February, 2020.

Ask Sam - Teats

This has been a topic of scientific inquiry going back all the way to Aristotle who first posited the idea that in mammals, the number of teats is — as a rule — double the number of offspring in the average litter.

This came to be known as the one-half rule, which is perhaps is a misnomer since it’s not really a rule... just a strong correlation. Generally speaking mammals have enough nipples that they’ve got double the capacity for their typical needs with enough overhead in case they have a big litter.

However, there are a couple of species that break the one half rule and notable among them is the naked mole rat, which have litters of as many as 28 pups, but only have 12 nipples.

(We’re not here to do a deep dive into naked mole rat society, which is fascinating. However, the short version is that they have a queen, which is a dominant female who’s the only one allowed to breed in a colony and the rest of the colony feeds her and supports her which seems to help her have enough milk for all those babies.)

But back to cows...

Cows break the rule in the other direction: more teats, fewer offspring. The answer to this question came to me from Russ Hovey, a professor of animal science at UC Davis in California. It's is an evolutionary story, which starts with the size of the calves.

“These species have a much larger offspring than some other species, so there’s now a considerable requirement for a large amount of milk,” he says.

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.

The fact that you have this big bag of milk dangling down there, means physically it needs to be supported.

Unlike goats and sheep, which divide their udders in half, cows divide it into quarters. They have two ligaments that suspend the udder and are arranged kind of in a cross.

Russ suspects evolution has pushed towards these four individual milk bags in order to avoid injury.

“You can imagine that there’s more suspension,” he says, “like trying to carry four small shopping bags instead of trying to carry two large ones.”

Natural Selection Vs. Bred Selection

Natural selection is always pushing certain traits to be more common, but in domesticated species natural selection begins to work in tandem with the choices made by their human wardens. The number of teats on a cow might be a perfect example of this.

For instance, humans have certainly been selecting for cows that produce more milk for millennia. As such, is it possible that at one point cows were like goats, with two teats, but then as they were pushed to larger and larger udders, eventually the four-baggers started to survive longer because the two-teated cows would tend to get more injury and infection?

Russ says we can’t know the answer to that question (in part, because nipples aren’t preserved in the fossil record), but it’s a plausible evolutionary story.

But it’s worth noting that the human factor here might be the real determinant. Animals breeders can, in just a few generations, select for animals that are more likely to have twin calves. So much so that the cattle industry recommends keeping track of cows that give birth to twins, in order to ensure you aren’t selecting for that trait by accident.

You can even select for animals that have more nipples! Extra teats is a fairly common trait, even among humans, and Alexander Graham Bell was interested in this that he raised a flock of sheep that had as many as six teats each.

So, why do cows have four teats? At least part of the answer to that question is because we want them too.

Sam Evans-Brown, is host of NHPR’s Outside/In which you can subscribe to where-ever you get your podcasts. If you’d like to submit a question you can record it as a voice memo on your smartphone and send it to oustidein@nhpr.org, OR call the hotline, 1-844-GO-OTTER, OR submit it here.