The 3rd largest “land” mammal on Earth spends 2/3rds of its life in the water, and that’s just the start of the enigma. They have canine tusks like a narwhal, the hooves of a pig, the BMI of a killer whale, the hip-to-waist-ratio of a manatee, and the harems of an elephant seal. So what exactly are hippos?

The answer to that question has been changing for thousands of years. The Ancient Greeks gave them the name hippopotamus, meaning “river horse.” And sure, they’re pretty blatantly not horses, but centuries later Marco Polo famously mistook a rhinoceros for a unicorn - humans spent a long time being bad at classifying animals.

Of all the things we’ve tried to classify the hippo as there are really only two that stood up to scientific criticism for any significant time:

1. Pigs
2. Whales

The Argument for Hippos Being Pigs

Until the early 1900s, it was a commonly accepted fact that hippos were pigs, or at least closely related—and it made sense. I mean look at Okja, the so-called “super pig” was just a big-eared hippo without the cool teeth.

Even as recently as 2007, scientific papers have described hippos as being most closely related to pigs. This is the result of the longstanding way people learned to deduce phylogeny (the evolutionary history of an organism). For centuries, we looked at the fossils of creatures to try and show how they are linked.

It’s only over the past few decades that we’ve really gotten any good at looking into DNA evidence. And even now, fossils are still important in phylogeny. Some of the main arguments to prove evolution are based on evolutionary chains linked to the fossil record. 

In short, even though DNA is the current reigning champion of biological evidence right now, we shouldn’t dismiss the stories that fossils tell. And the story for hippo and pig fossils is pretty compelling.

Here’s a hippo skeleton:

And here’s a pig skeleton:

Okay, that’s pretty close. There are some pretty clear differences, the rib:body ratio, the head shape, the crook of the neck, and those front legs. But that’s a modern pig. Domesticated and bred by humans for over 9000 years, on top of the millions of years of natural evolution from when hippos and pigs supposedly diverged.

Even if we go with a wild boar skeleton, it starts to look a lot more like a hippo, albeit a little less thicc (granted not as thicc as a humpback whale). 

Still, the wild boar only dates back about 10 million years and, even then, this is based on a modern skeleton. Hippos branched off about 55 million years ago. So what happens if we take a look at an ancient pig. Here’s an entelodont, aka Hell Pig, aka Terminator Pig. These guys popped up probably more like 20 to 30 million years ago:

Now that’s a straight-up hippo-looking skeleton. Picture those little hippo ears and fill it out with a whole bunch of blubber and you’ve got yourself a hippo. 

But here’s where it all breaks down. The entelodont, it turns out, wasn’t a pig. Despite all appearances, it wasn’t even related. And we know that now the same way we discovered that hippos and pigs aren’t related, DNA evidence.

It’s like they say in DNA research: If it looks like a duck, it walks like a duck, and it quacks like a duck - you still have to check the lab results.

The Argument for Hippos as Whales

DNA evidence hammered the nail into the coffin of the “hippos are pigs” argument. But like most things that aren’t pigs, not being a pig doesn’t automatically qualify them as whales. DNA evidence doesn’t show that hippos are whales either. Instead, what we’ve discovered is that hippos are the closest living relatives to whales.

Whales have branched off a long way from hippos, but the other branches have all slowly faded from existence. The long, slow move from four-legged, landbound mammal to obligate marine predator is one of the most stunning examples of macroevolutionary change. And despite it all, the hippo is still hanging around.

Sure, the hippopotamus may hang out in the water all day, but it’s no whale. Their webbed feet are topped off with hooves, they have four sturdy legs, and no use for a blowhole.

So Are Hippos Whales or Pigs?

A hippo is a hippo, is a hippo. Whether they’re pygmy or hungry hungry, a hippopotamus is its own thing. Despite their similarities, they’re certainly not pigs. And even though they are close relatives, they aren’t whales either.

What are hippos? They’re big, they’re blubbery, they’re adorable, and, despite all appearances, deadliest land mammal on the planet.