The hippopotamus or hippo mystified ancient Greeks because it appeared to sweat blood. Although hippos do sweat a red liquid, it isn't blood. The animals secrete a sticky liquid that acts as a sunscreen and topical antibiotic.

Color Change Perspiration

Initially, hippo perspiration is colorless. As the viscous liquid polymerizes, it changes color to red and eventually brown. Droplets of perspiration resemble drops of blood, although blood would wash away in water, while hippo perspiration sticks to the animal's wet skin. This is because the hippo's "blood sweat" contains a high amount of mucous.

Colored Pigments in Hippo Sweat

Yoko Saikawa and his research team at Kyoto Pharmaceutical University, Japan, identified non-benzenoid aromatic compounds as the orange and red pigment molecules. These compounds are acidic, conferring protection against infection. The red pigment, called "hipposudoric acid"; and the orange pigment, called "norhipposudoric acid", appear to be amino acid metabolites. Both pigments absorb ultraviolet radiation, while the red pigment also acts as an antibiotic.

Reference: Yoko Saikawa, Kimiko Hashimoto, Masaya Nakata, Masato Yoshihara, Kiyoshi Nagai, Motoyasu Ida & Teruyuki Komiya. Pigment chemistry: The red sweat of the hippopotamus. Nature 429, 363 (27 May 2004).