How Do Hippos Produce Their Own Sunscreen?

On a scorching day when most people are cooling off by the pool or relaxing under a powerful fan, Dr. Brady Barr found himself dressed in a 196-pound (89-kilogram) protective suit covered in mud and dung. In the name of science, he was on a quest to collect a wild hippo's sweat before it dried up.

You’re probably wondering why anyone would risk their life by getting close to one of Africa's most dangerous and aggressive creatures. After all, hippos are responsible for more deaths than any other animal on the continent—a perfectly valid question [source: Harlow].

This isn't just any ordinary sweat we're discussing. This mucus-like substance—initially mistaken for blood due to its deep red hue—does more than regulate the body temperature of these 5,000- to 8,000-pound (2,300- to 3,600-kilogram) creatures. It also serves as a powerful sunscreen and antibiotic [source: Hughes, Saikawa].

It turns out that fair-skinned humans aren’t the only ones who need to consider SPF when they go outdoors. The two hippo species—the common hippopotamus from central and southern Africa, and the rare pygmy hippopotamus, a smaller species native to West Africa, weighing around 440 to 605 pounds (200 to 247 kilograms)—structure their days around the fierce rays of the African sun [source: African Wildlife Federation].

These semi-aquatic mammals spend as much as 16 hours a day submerged in rivers or lakes to keep cool, emerging only after dark to graze [source: Hughes]. They feed on short grasses until dawn, at which point they return to the cooler waters.

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Although the water helps prevent these lumbering animals from overheating, it doesn’t do much to protect their skin. That’s where the blood-red sweat comes into play, creating the hippo sunscreen. Though technically not sweat, as it’s produced by glands under the skin rather than in it, this gelatinous, oily secretion functions much like sweat but with added benefits.

The SPF of Hippo Sweat: Sterile, Pigmented Fluid

While Barr’s brave attempt in the heavy hippo suit didn’t succeed, other scientists have had more success. Rather than encasing themselves in armor, a Japanese research team led by Professor Yoko Saikawa gathered sweat samples by wiping a hippopotamus's face and back with absorbent gauze, then extracting the chemical components with water.

The researchers discovered two unstable and highly acidic compounds—one red, named hipposudoric acid, and one orange, named norhipposudoric acid. Though these pigments are unstable alone, they solidify and stick to the animal's skin when they dry in the presence of mucus, lasting for hours. This thick, sticky combination can withstand the hippos' long hours in water, while simultaneously absorbing sunlight in both the ultraviolet and visible spectrums [source: Saikawa].

When Saikawa’s team tested these pigments, they found that hipposudoric acid also acts as a potent antibiotic. Even at lower concentrations than those found on the animal’s skin, its high acidity—hundreds of times stronger than vinegar—prevents the growth of harmful bacteria [source: Saikawa, Arthur].

The antiseptic qualities of hippo sweat help explain how these animals remain mostly free from infections, despite the wounds they often sustain from the tusks of males. These long, ivory teeth, which can grow up to 20 inches (51 centimeters), are so formidable that when males want to intimidate each other, they sometimes only need to stand face to face and open their mouths an impressive 150 degrees in a display called gaping [source: San Diego Zoo]. If the sight of each other's jaws doesn’t scare the other off, they’ll battle by slashing with their tusks or swinging their massive heads like wrecking balls.

Despite numerous (and brave) efforts to study it, much about the hippo’s thick, antibiotic sunscreen remains a mystery. Saikawa's team suggests that the animal might produce it from amino acids (the building blocks of protein) when exposed to oxygen. Since all hippos seem to create the pigments, scientists don’t believe their ‘sunscreen’ is linked to their diet. Some researchers even think it could serve as a bug repellent, as flies appear to avoid landing on it [source: Grossi].

One thing is certain, however. Despite its initial appeal, this is one sunscreen you may want to avoid applying—according to Saikawa, it really smells [source: Arthur].