Red sweat: hippo skin oozes antibiotic sunscreen.

The hippo version of sweat is red-orange, and Japanese researchers have now isolated two pigments responsible for the colorful glow.

Tests so far confirm years of speculation that this skin secretion can block microbial growth as well as some ultraviolet light, says Kimiko Hashimoto of Kyoto Pharmaceutical University. The pigments are highly acidic compounds containing carbon rings, Hashimoto, Yoko Saikawa of Keio University in Yokohama, and their colleagues report in the May 27 Nature.

The researchers named the newly identified red pigment hipposudoric acid and the orange one norhipposudoric acid. If chemists could make more-stable derivatives, Hashimoto says, "these pigments would be seeds of pharmaceutically important compounds."

Hippo biologist Keith Eltringham of the University of Cambridge in England explains that the secretions are not technically sweat because hippos don't have the small sebaceous glands that produce it. Instead bigger, deeper glands release liquid through skin holes that are visible to the naked eye. Eltringham is skeptical that the secretions play much of a role in regulating body temperature because a hot hippo just lumbers into water to cool off.

Eltringham traces the speculations about the secretions' antibiotic powers to observations that hippo wounds tend not to get infected, even though male hippos fight viciously over territory. "The hippos turn nose to tail and just slash each other," he says.

For the new experiments, keepers at the Ueno Zoo in Tokyo swabbed hippo skin with gauze to collect the secretions. Then, Hashimoto, Saikawa, and their colleagues figured out the pigments' structure and properties. Lab tests showed that the red pigment inhibits growth of two disease-causing bacteria.

Each pigment's light absorption peaks in the ultraviolet range. "There's no question that the hippo is getting its sunscreen," says photobiologist Gavin Greenoak of the University of Sydney in Australia.

Hashimoto says that hippos all over the world secrete pigments, so she doesn't think they depend on pigments ready-made in their food. She suspects instead that the animals synthesize the pigments from common precursors such as the amino acid tyrosine.

The secretions start out colorless, turn red-orange within minutes, and then gradually go brown. Hashimoto's team reports that the darkening results when the pigments form long chains.

Greenoak notes that natural-product chemists have found other substances, mostly from plants, that block UV light. In general, they haven't been successful as sunscreens for people. "The problem with all of them is they're horribly unstable," he says.

He adds that a sunscreen probably wouldn't sell if it turned people red-orange.