Hippo skin is nearly hairless and lacks true sweat glands, which leaves it vulnerable to sunburn and to infection in the wounds males inflict on each other. Subdermal glands secrete a viscous, initially colourless fluid that turns red within minutes and brown over hours. Japanese chemists isolated the responsible compounds in 2004 and named them hipposudoric and norhipposudoric acid.
Both pigments absorb strongly in the ultraviolet range, and laboratory tests showed the red pigment inhibited the growth of two bacterial species at concentrations below those found on the animal. The colour change is a spontaneous polymerisation of the acids, not oxidation of iron, which is why the popular phrase blood sweat is doubly wrong.
What remains unsettled is how much of the benefit is sun protection and how much is antisepsis, and whether the secretion also plays a role in thermoregulation. Field measurement is difficult, since hippos spend the sunlit hours in water where the film is repeatedly washed away.