After carefully collecting the fresh sweat of a hippopotamus, chemists have isolated and identified the unstable compounds that give hippo sweat its rusty hue [Nature, 429, 363 (2004)].
It took a few months of wiping gauze on the head and back of a hippo two times a week for Kimiko Hashimoto, a professor of chemistry at Kyoto Pharmaceutical University, and his colleagues Yoko Saikawa and Masaya Nakata at Keio University to collect enough of the substance for analysis. They had to be sure to keep it cool, dilute, and wet because the reactive molecules, which are initially colorless, easily oxidize to red, then polymerize into brown, pigments.
After purifying the mixture by gel filtration and ion-exchange chromatography, the Japanese group analyzed it by several spectrometric techniques. They pinned down the source of the color to two conjugated three-ring structures: a red pigment they dubbed hipposudoric acid, and an orange one, norhipposudoric acid.
Because the compounds absorb light in the UV-visible range (200–600 nm), the researchers suspect that the pigments protect the hippo's dermis like a sunscreen does. (The hippopotamus has a thin epidermis, Hashimoto says, which exposes the sensitive dermis to UV rays.) In addition, the scientists found that hipposudoric acid, at low concentrations, inhibits the growth of bacteria.
Hipposudoric and norhipposudoric acid are highly hydrophilic and unexpectedly acidic, Hashimoto says. NMR spectrometry shows that the pigments tautomerize and exist in water mainly as resonance anions stabilized by hydrogen bonds with water molecules. In hippo mucus, the compounds are further stabilized by an unknown agent that keeps the compounds from fully polymerizing for several hours even after hippo sweat dries.