Hippopotamuses are one of the largest extant mammals in the world.Hippos are considered megafauna, but unlike all other African megafauna, hippos have adapted for a semi-aquatic life in fresh water lakes and rivers.For additional protection from the sun, their skin secretes a natural sunscreen substance which is red-colored. The secretion is sometimes referred to as “blood sweat,” but is neither blood nor sweat. This secretion is initially colorless and turns red-orange within minutes, eventually becoming brown. Two distinct pigments have been identified in the secretions, one red and one orange. The two pigments are highly acidic compounds. They are known as red pigment hipposudoric acid and orange pigment norhipposudoric acid. The red pigment was found to inhibit the growth of disease-causing bacteria, lending credence to the theory that the secretion has an antibiotic effect. The light absorption of both pigments peaks in the ultraviolet range, creating a sunscreen effect. All hippos, even those with different diets secrete the pigments, so it does not appear that food is the source of the pigments.