An African hippopotamus produces a red-orange skin secretion often called “blood sweat.”
The secretion is not blood and is not ordinary sweat. It contains pigments called hipposudoric acid and norhipposudoric acid, which are produced by specialized skin glands. These compounds help absorb ultraviolet radiation and may also inhibit the growth of some bacteria.
The secretion begins colorless or pale and can darken toward reddish brown as it is exposed to sunlight and air. This contributes to the familiar appearance of a hippo seeming to sweat blood, especially after it has been in water and then emerges.
Hippopotamuses still rely heavily on water and mud for cooling and skin protection. They do not have conventional sweat glands like humans, so the red secretion should not be confused with perspiration or an injury.