Why do we domesticate some animals but not others? It all comes down to the four Fs.

Humans have domesticated several kinds of animals over the millennia, from trusty horses and mules to livestock for milk and meat to our favorite furry companions. But why those specific animals and not others? What is it that led us to those particular choices? Why can we ride horses but not zebras? Why don't we purposefully breed "war bears" to fight for us?

That last question comes straight from the always-interesting and often-hilarious CGP Grey, whose YouTube videos explore all kinds of things we wonder about but don't necessarily take the time to research. In the video "Why Some Animals Can't Be Domesticated," Grey explains the four main elements that make an animal a good candidate for domestication, which excludes bears (and many others) from the list.

Grey alliterated the four elements to make them easier to remember: Friendly, Feedable, Fecund, and Family-Friendly. Let's dig into what those mean.

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Domestication requirement #1: Friendly

This one is fairly self-explanatory, but basically an animal has to not post an inherent, immediate threat. We have to be able to catch them if we're going to domesticate them, so that eliminates all of the "carnivores whose day job is murder" as Grey puts it, as well as the large, nervous prey animals that are too afraid of us to let us get anywhere near them.

Two kudus with long spiral horns stand on a grassy hillside.Photo credit: Canva

To that end, many animals that look adorable (like antelope, moose, hippos…) have stress responses so intense that captivity can cause them to injure themselves or lash out. Kind of cute in cartoons, not so cute IRL.

Domestication requirement #2: Feedable

Every animal is feedable, of course, but that doesn't mean it's easy or cheap to feed them, especially in large numbers. This category pretty much eliminates pure carnivores and some omnivores, leaving mostly herbivores (and some unpicky omnivores) that are easy and cheap to feed. And that aren't dangerous (see #1).

Person in hat feeding cows in a farm enclosure.Photo credit: Canva

Pure carnivores also require huge hunting territories and enormous calorie intake. At scale, it would have drained early human settlements more than it helped. This is why we'll likely never see domesticated lions, wolves (beyond proto-dogs), or big cats.

Domestication requirement #3: Fecund

This requirement is all about breeding and babies. Some animals are extremely slow to breed, like pandas and elephants, making them undesirable candidates for domestication. Animals that have mate frequently and have relatively short gestation times and/or large litters are more suited to domesticated life. They also need to grow up quickly, which also takes elephants out of the pool.

However, as Grey points out, humans can still tame other animals like elephants. But taming is not the same as domesticating. The basic rule is: If it's on a farm, it's domesticated. If it's in a circus, it's tamed.

There’s also a wild side effect: as animals become domesticated over generations, they develop floppy ears, curlier tails, smaller teeth, and rounder faces. Scientists call this “domestication syndrome,” and it’s actually been observed in foxes, dogs, pigs, and even certain birds.

Domestication requirement #4: Family-friendly

This is where the horses and zebras question comes in. Horses were domesticated in Eurasia, but if humans started in Africa, why weren't zebras domesticated first? Grey explains that while horses tend to live in hierarchical herds, zebras are more independent with no family structure. Humans can capture the lead male horse and get the rest of the herd to fall in line. Zebra herds are more of a free-for-all and they're kind of jerks to even one another.

While it would seem that horses we see in the United States today are indigenous, the North American horse species went extinct around 10,000 years ago. They were reintroduced to America by 16th-century European colonialists who spread them throughout the country through Native American trade routes. Some ecologists argue that because of this, they effectively “re-wilded” an ecological niche they originally evolved in.)

Silhouettes of three horses galloping at sunset on a hill.Photo credit: Canva

Barnyard animals have inherent family structures that humans have figured out how to fit into. These animals learn to see the humans who own them as a lead cow, top chicken, or whatever.

Way back in the hunter-gatherer age, when humans were just figuring out animal domestication, animals had to have all four of these requirements. With this in mind, it makes sense that scientists estimate that of the millions of animal species on Earth, only about 30–40 have ever been truly domesticated. That’s shockingly few.

Today, we have the ability and technology to domesticate more animals if we want to, but we also have less of a need to. For example some breeds of foxes have recently been domesticated, bred to be friendly with humans. How fun would it be to have a pet fox? Foxes are incredibly cute but wildly intelligent. So good luck keeping one in your backyard for very long.

So sure, we could spend a few centuries trying to domesticate a zebra or raise a herd of war bears. But honestly? We’re probably better off thanking the species that already acquiesced.

You can follow CGP Grey on YouTube for more fun and informative videos.

This article originally appeared in April.