People used to say that hippos sweat blood. When a hippo comes out of the water, its skin sometimes looks red. But it is not blood. The liquid comes from glands under the skin.

At first it has no color, but after a few minutes it turns red. Japanese scientists found two colors in it, red and orange. These colors may protect the hippo from the sun and from bacteria.

Hippos have long been said to sweat blood, because their skin is often covered with a reddish liquid. In fact, this liquid is not blood, and it is not even true sweat. It comes from special glands under the skin. It starts out colorless, turns red within minutes, and later becomes brownish.

In 2004, a team including Yoko Saikawa of Keio University analyzed the liquid from two zoo hippos. They discovered two pigments, a red one called hipposudoric acid and an orange one called norhipposudoric acid. Both absorb ultraviolet and visible light, and the red one also slowed the growth of some bacteria. So the liquid may act as both a sunscreen and an antiseptic.

A hippo climbing out of a river can look as if it is bleeding all over. Its skin glistens with a reddish fluid, which is why people long claimed that hippos sweat blood. The fluid is neither blood nor true sweat. Hippos have few sweat glands for cooling, and this liquid comes from other glands beneath the skin. It is colorless when it first appears, turns red within minutes and later goes brown.

In 2004, Kimiko Hashimoto and Yoko Saikawa collected secretions from the faces and necks of two zoo hippos. They identified two pigments and named them after the Latin word for sweat: red hipposudoric acid and orange norhipposudoric acid. Both absorb light across the ultraviolet and visible range, and the red one inhibited the growth of bacteria. All of this was measured in the laboratory, using material from just two captive animals, and nobody has tested how well the pigments work on wild hippos.

Even so, the chemistry suits the animal's life. Hippos often carry wounds and spend their days in bacteria-rich water, so a built-in sunscreen and antiseptic would be useful. The team synthesized both pigments in 2006 and refined their structures with computer calculations in 2015.