Hippopotamus: The Whale's Cousin That Sweats Red and Cannot Swim

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The hippopotamus of board games and cartoons is chubby, sleepy, and funny. The real animal is a one and a half ton fighter with an evolutionary secret. Its Greek name means river horse, and until 1909 naturalists grouped it with pigs because of its molar teeth. Molecular evidence overturned both ideas: the hippo's closest living relatives are whales, dolphins, and porpoises, with a shared semi-aquatic ancestor that split off roughly 60 million years ago. Hippos are the last survivors of a once diverse family called anthracotheres.

This episode walks through the body and habits of that survivor. Dense bones let it sink and trot along the riverbed instead of swimming. Its jaws open 100 to 110 degrees, and its lower canines, which never stop growing, can reach 50 centimeters and are used only for combat. By day hippos crowd together in pods of up to a hundred. At dusk they leave the water and walk three to five kilometers alone to eat about 40 kilograms of grass. The account closes with their long, violent history with humans and their vulnerable status today.

• The so-called blood sweat is a secretion containing two acidic pigments, hipposudoric acid and norhipposudoric acid, that work as both sunscreen and antibacterial agent.
• Hippos sleep underwater and surface to breathe every few minutes by reflex, without waking.
• Males mark territory by muck spreading, spinning the tail to spray dung. In the dry season the sheer volume can strip oxygen from the water and cause fish kills.
• In Virunga National Park, poaching for meat and ivory cut the population from around 29,000 in the 1970s to just 800 or 900 by 2005.
• Hippos that Pablo Escobar imported for a private zoo in Colombia escaped after his death in 1993 and have multiplied into an invasive population of more than a hundred.