Red Sweat: Hippo skin oozes antibiotic sunscreen

Susan Milius

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.

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Letters:

The Japanese researchers who dubbed a pachyderm secretion to be "hipposudoric acid" seem to know more about biochemistry than about etymology. The word hippopotamus is a synthesis of Latin hippo (horse) and potamus (river), apparently because of the resemblance of the face of a submerged hippopotamus to the head of a horse. Hipposudoric implies a connection to horses, which is obviously not the case.

Harry Pottol
Sunnyvale, CA

References:

Saikawa, Y., K. Hashimoto, et al. 2004. The red sweat of the hippopotamus. Nature 429(May 27):363. Abstract available at http://dx.doi.org/10.1038/429363a.

Further Readings:

For an introduction to hippo biology, go to http://animaldiversity.ummz.umich.edu/site/accounts/information/Hippopotamus_amphibius.html

Sources:

Keith Eltringham
Department of Zoology
University of Cambridge
Downing Street
Cambridge CB2 3EJ
United Kingdom

Gavin E. Greenoak
Australian Photobiology Testing Facility (APTF)
Suite 204-205
Ross Street, Building AO3
University of Sydney
New South Wales 2006
Australia

Kimiko Hashimoto
Kyoto Pharmaceutical University
1 Shichono-cho
Misasagi, Yamashina-ku
Kyoto 607-8412
Japan

Yoko Saikawa
Department of Applied Chemistry
Faculty of Science and Technology
Keio University
3-14-1 Hyoshi, Kohoku-ku
Yokohama 223-8522
Japan

From Science News, Volume 165, No. 22, May 29, 2004, p. 341.