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How does a hippo make its own sunscreen?

The SPF of Hippo Sweat: Sterile, Pigmented Fluid
Hippo teeth can inflict some serious damage. It's a good thing the animal makes its own antiseptic.
Hippo teeth can inflict some serious damage. It's a good thing the animal makes its own antiseptic.
Richard Du Toit/Minden Pictures/Getty Images

Though Barr's heroic adventure in the bulky hippo suit proved fruitless, some other scientists have had better luck. Instead of weighing themselves down in armor, a team of Japanese researchers led by Professor Yoko Saikawa collected sweat samples by wiping a hippopotamus's face and back with absorbent gauze and then extracting the chemical components with water.

What they found were two unstable and highly acidic compounds -- one red, which they named hipposudoric acid, and one orange, which they named norhipposudoric acid. Although the two chemical pigments are unstable on their own, when they dry on the animal's skin in the presence of mucus, they harden and stick around for hours. Thus, the thick, sticky mixture is tough enough to survive the hippos' daylong soaks, all the while absorbing sunlight in both the ultraviolet and visible range [source: Saikawa].

When Saikawa and his team tested the pigments, they found that hipposudoric acid is also a powerful antibiotic. At concentrations even lower than those normally found on the animal's skin, the pigment's high acidity -- hundreds of times more powerful than vinegar -- inhibits the growth of pathogenic bacteria [source: Saikawa, Arthur].

­The antiseptic powers of hippo sweat help to explain how the beasts manage to remain largely infection-free despite the wounds the males often inflict upon one another with their tusks -- long ivory teeth that can grow up to 20 inches (51 centimeters) long. These canine teeth are so imposing that when males want to threaten one another, sometimes all they have to do is stand face to face while opening their mouths an impressive 150 degrees in a process called gaping [source: San Diego Zoo]. If the sight of one another's jaws doesn't scare the other off, they'll duke it out by slashing at each other with their teeth or swinging their giant heads back and forth like wrecking balls.

Despite extensive (and valiant) attempts to research it, much remains a mystery about the hippo's gooey antibiotic sunscreen. Saikawa's team hypothesizes that the animal synthesizes it from amino acids (the building blocks of protein) in the presence of oxygen. Since all hippos seem to produce the pigments, scientists don't believe their "sunscreen" is linked to diet. Some scientists think it may even act as a bug repellent since flies seem averse to landing on it [source: Grossi].

One thing's for sure though. Despite its initial appeal, this is one sunscreen you may not want to slather on -- according to Saikawa, it really stinks [source: Arthur].