Studies on the red sweat of the Hippopotamus amphibius

The secretion from the hippopotamus' skin changes its color from colorless to red, and then brown by polymerization of its pigments. The responsible pigments for the coloring reaction were isolated and denoted as hipposudoric acid (the red pigment) and norhipposudoric acid (the orange pigment). The syntheses of these pigments and the related derivatives were performed, and the latter were of use to elucidate the structures of these pigments including their tautomeric structures in aprotic and protic solvents. These pigments were estimated to be medicines for the hippopotamus, having the effect of both protecting the skin from sunburn and preventing infection by some microbes.