Oh the hippopotamus, famous for escaping the ranch of a notorious drug lord, fighting lions, and having poop so toxic it can kill fish. Apparently, the ancient Greeks also believed that hippos were sweating blood so let's dive in.

Hippos (Hippopotamus amphibius) spend much of their time lying in the water, eating grass and aquatic plants. Their 5-centimeter-thick (2-inches) skin is mostly hairless and is very sensitive to both drying out and sunburn, which is tricky to avoid in the heat of Sub-Saharan Africa. While hippos do have a red substance oozing out of their pores, if you’re close enough to see it, your more immediate problem is likely to be one of the world’s heaviest land animals barrelling toward you. 

Why is hippos' sweat red?

The red substance is neither blood nor technically sweat as it is secreted by the subdermal glands. However, it does offer a similar function to sweat as it helps control the body temperature of the hippo. Initially colorless, the viscous liquid changes to a red color in minutes and eventually turns brown as the pigment polymerizes. While blood would be washed away thanks to the hippos' semi-aquatic lifestyle this substance sticks to the skin due to a high concentration of mucus. 

In 2004, a team of researchers took it upon themselves to find out exactly what this substance was. They collected the alkaline liquid by wiping it off the face and back of a hippo with gauze (don't ask how they found a hippo willing), and found two pigments responsible for the color reaction: one red and one orange. 

They found these unstable pigments are non-benzenoid aromatic compounds and are surprisingly acidic. They named the red pigment hipposudoric acid and the orange one norhipposudoric acid. Both of these pigments absorb ultraviolet light, acting as sunscreen, while the red pigment also acts as an antibiotic inhibiting the growth of pathogenic bacteria. 

So no, hippos aren't sweating blood, they are secreting antibiotic sunscreen.