A favorite subject of nature documentaries, the hippopotamus is huge, heavy, and herbivorous. Hippos are the third largest land animal—only elephants and white rhinoceroses are bigger—and surprisingly agile in an aquatic environment. But don’t get in a hippo’s way: They are extremely aggressive and kill about 500 people a year, making them the world’s deadliest mammals (after humans). Here are some more unexpected facts about hippos.

1. There are two species of hippopotamus.

Both species are classified in the family Hippopotamidae, but belong to two genera. The familiar Nile or common hippo, Hippopotamus amphibious, is more abundant and can be found in aquatic habitats across sub-Saharan Africa. The smaller pygmy hippo, Choeropsis liberiensis, numbers fewer than 2500 individuals in the wild and is considered endangered. They live in rainforests in Sierra Leone, Liberia, and Côte d’Ivoire.

2. Whales and dolphins are the hippo’s closest relatives.

Despite their name, derived from Greek for “horse of the river,” hippopotamuses aren’t related to horses. Hippos belong to the order Artiodactyla, which comprises even-toed ungulates like pigs, camels, and deer, as well as cetaceans (whales, dolphins, and porpoises). Hippos and whales had a common ancestor that lived about 55 million years ago, after which hippos and cetaceans branched out. Even today hippos share many features with cetaceans, like nearly hairless skin and underwater birth. Scientists have even found that hippos make clicks while moving underwater, similar to a cetacean echolocation.

3. Hippos are built for the water.

With their rotund bodies, big heads, and small legs, hippos are basically submarine-shaped. They spend most of their time resting in lakes, rivers, and wetlands. At night, they emerge onto land and graze on grasses and reeds, creating “hippo paths” through the vegetation to their favorite spots. They even mate and give birth while submerged.

4. Hippos can’t swim.

Surprisingly, these semi-aquatic animals can’t swim. They have dense bones that cause them to sink in deep water, but are perfect for walking, galloping, or bouncing in shallow rivers and lakes. Their bones offer just the right amount of weight to achieve optimum buoyancy, effectively turning the river into a microgravity environment. Hippos can be deceptively fast when moving underwater and on land, where they can run up to 24 mph.

5. Fish help hippos keep their skin clean.

Hippos and fish enjoy a symbiotic relationship in their watery habitat. African fishes including cichlids and barbels nibble the dead skin, algae, and parasites on the hippo’s skin and inside their mouths. The fish eat this crud as a food source while removing potentially harmful pathogens from the hippo’s body. Fiona, the Cincinnati Zoo’s celebrity hippo, enjoys these spa treatments from the tilapia in her enclosure.

6. Hippos's red “sweat” is actually sunscreen.

Hippos are often slathered in pinkish “sweat,” which is not actually sweat or blood. They secrete two substances that turn red (hipposudoric acid) and orange (norhipposudoric acid) and act as sunscreens. The pigments also have antibacterial properties against Pseudomonas and Klebsiella, which can cause infections.

7. Hippos are so dangerous that Nile crocodiles steer clear of them.

Hippos are the deadliest mammal in Africa, even more dangerous to humans than lions or elephants. Their victims are often fishermen or boaters, whom they charge from underwater. Hippos topple boats, trample and drag people into lakes, and bite with incredible force. They can amputate limbs, fracture bones, and mangle soft tissues. In 2014, 13 people in Niger died when a hippopotamus overturned their boat, and in 2018, a hippo attacked an American woman on a safari in Zimbabwe after it upended her canoe. (She survived with a broken leg.)

8. People have hunted hippos for hundreds of thousands of years.

Homo sapiens and our ancestors hunted hippos as a source of meat and bones for making tools. One of the oldest known bone hand-axes is 1.4 million years old and made from a hippo’s femur. Hippopotamus meat has been an important human food source since the dawn of Homo erectus. Archaeologists investigating a 700,000-year-old human habitation site in Ethiopia found numerous hippo bones that bore marks from butchering tools. Other archaeologists working in Turkana Basin in Kenya found similar disarticulated hippo bones bearing butchery marks at a 1.9-million-year-old human habitation.

Ancient Egyptians hunted hippos for their meat, skin, and teeth, and carved spiritually protective designs into their tusk-like canines. Because hippo ivory is denser and stronger than elephant’s, it was a popular material for dentures in the 18th century. George Washington’s dentures had some teeth carved out of hippo ivory, and Paul Revere is believed to have used hippo ivory in his dental practice.

9. President Calvin Coolidge was given a hippo as a gift.

Coolidge was given a number of potentially dangerous pets during his time in the White House, like two lions named Tax Reduction and Budget Bureau. Another was William Johnson Hippopotamus, a.k.a. Billy, a pygmy hippo from Liberia given to Coolidge in 1927 by rubber magnate Harvey Samuel Firestone.

Coolidge promptly donated Billy (and his other exotic animals) to the National Zoological Park, where biologist paired him with a female pygmy hippo named Hannah. Billy eventually sired 18 calves, all named Gumdrop followed by Roman numeral (I though XVIII). All the pygmy hippos now in U.S. zoos are believed to be descended from Billy.

10. A Louisiana Congressman wanted to import hippos as livestock.

In 1910, the U.S. faced a beef shortage, and what was to be done about the crisis was dubbed “the meat question” by the media. On March 24, 1910, a bill was introduced by Representative (later Senator) Robert Broussard of Louisiana to import hippos from Africa and introduce them into the bayous of Florida, Mississippi, and Louisiana. Hippos would not only provide tons of meat, but also eat water hyacinth, an invasive species that blocked streams and rivers in the South. The New York Times asked readers to think of the fatty flesh as “lake cow bacon.” Broussard's bill failed.

11. Invasive hippos are running wild in Colombia.

Colombia is the only country outside Africa where hippos are found in the wild. The drug lord Pablo Escobar imported one male and three female hippopotamuses for his private zoo, but after he was killed in 1993, the hippos were left to fend for themselves. Soon, these so-called "cocaine hippos" escaped, bred, and colonized the Magdalena River, Colombia’s chief waterway. The hippos found themselves in an environment that had plentiful food, no competition, and zero drought. Their population exploded.

Now, 80 to 100 hippos are running wild in Colombia, and the number is expected to increase to over 1400 by 2034. The hippos are considered an invasive species and present a major challenge for the Colombian government. Ecologists predict the animals will cause environmental damage like altering river chemistry and driving away native species like manatees and otters, but so far, public opinion is with the hippos.