Why do hippos "sweat" with a mysterious red liquid similar to blood?

Hippos spend most of their time lying in the water and eating grass and aquatic plants. Their skin is 5 centimeters thick, mostly devoid of hair and is very sensitive to both drying and sunburn, which is difficult to avoid in the hot countries of sub-Saharan Africa. At the same time, a red substance is released from their pores, which gave rise to the myth that hippos "sweat blood".

However, this composition is neither blood nor sweat. At the same time, it performs the same function as sweat, since it helps to control body temperature.

The initially colorless, viscous liquid changes color to red in a matter of minutes and eventually turns brown as the pigment polymerizes. While the blood would be washed out thanks to the semi-aquatic lifestyle of hippos, this substance sticks to the skin due to the high concentration of mucus.

In 2004, a group of researchers undertook to find out exactly what this substance is. Scientists collected an alkaline liquid from the hippo's muzzle and back with gauze and found two pigments responsible for the color reaction: one red and one orange.

These unstable pigments are non-benzoid aromatic compounds and have amazing acidity. The researchers called the red pigment hipposudoric acid, and the orange pigment norhipposudoric acid. Both absorb ultraviolet light, acting as a sunscreen filter, and the red pigment also functions as an antibiotic, suppressing the growth of pathogenic bacteria.

Thus, hippos do not "sweat blood", but exude an essentially unique antibiotic sunscreen.