It is protected from the sun by a secreted substance that is natural sunscreen. This secretion, neither blood nor sweat, is initially colorless; after a few minutes, it becomes reddish-orange and eventually brown. It is made up of two pigments (red and orange), which are acidic solid chemical compounds, and the red pigment has additional bacteriostatic properties and is probably an antibiotic. Light absorption from both pigments has a maximum in the ultraviolet band, which protects hippos from excessive heat. Because of the color of the secretions, hippos are said to "sweat blood."
