How Does a Hippo Make Its Own Sunscreen?

Hippopotamus on Rufiji River of the Selous Game Reserve. — The red tint of a hippo's light-absorbing sweat lends to their characteristic flush. Giampaolo Cianella/Getty Images

On a sweltering day, when most people are cooling off at the pool or lying motionless under a high-powered fan, Dr. Brady Barr found himself decked out in a 196-pound (89-kilogram) armored suit smeared with mud and dung. In the name of scientific discovery, the scientist was on a mission to collect a wild hippo's sweat before it dried.

You're probably wondering why anyone in their right mind would risk their life by approaching one of the most aggressive and dangerous animals in all of Africa. Considering that hippos cause more deaths than any other animal on the continent, it's a reasonable question [source: Harlow].

But this isn't just any run-of-the-mill sweat we're talking about. This mucuslike secretion -- which initially led people to believe that the animal sweat blood because of its deep red color -- not only helps to control the body temperature of these 5,000- to 8,000-pound (2,300- to 3,600-kilogram) animals, it also acts as a potent sunscreen and antibiotic [source: Hughes, Saikawa].

It turns out that fair-skinned humans aren't the only ones who need to worry about SPF when they venture outside. The two species of hippo -- the common hippopotamus found in central and southern Africa, and the rare pygmy hippopotamus, a smaller species found in West Africa, weighing around 440 to 605 pounds (200 to 247 kilograms) -- structure their days around the harsh glare of the African sun [source: African Wildlife Federation].

The semiaquatic land mammals spend up to 16 hours a day submerged in rivers or lakes to stay cool, venturing out to graze only after nightfall [source: Hughes]. They nibble on their main food of short grasses until dawn, when they return to their refreshing sanctuaries.

While the water prevents the lumbering beasts from getting overheated, it doesn't offer much in the way of skin protection, which is where the blood-red sweat comes in and hippo sunscreen is created. Though it's not technically sweat since it's produced by glands underneath the skin rather than in it, the gelatinous, oily secretions act much the same way but with a few extra perks thrown in.

The SPF of Hippo Sweat: Sterile, Pigmented Fluid

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].

Hungry, Hungry Hippos?

It made for a great game, but aside from the obvious stretches of truth (hippos don't eat marbles and they certainly don't come in shades of blue, pink and purple), hippos aren't very big eaters. The average 88 pounds (40 kilograms) of vegetation they consume each day isn't that much when you consider they weigh more than 30 times that [source: San Diego Zoo]. In fact, the largely sedentary hippos (which are closely related to whales) only eat around 1 to 1.5 percent of their body weight each day. Cows, on the other hand, may eat up to 2.5 percent [source: San Diego Zoo, UC Berkeley].

Hippo FAQ

Do hippos eat meat?

Generally no, hippos are herbivores that eat up to 40 kilograms of grass per day. There are a few reported cases of cannibalism or hippos eating other scavenged carcasses, but itâ€™s not common.

Why are hippos so dangerous?

Hippos are naturally aggressive and dangerous. These semi aquatic land mammals are known to cause more deaths than any other animal on the continent. Theyâ€™re huge (up to 3,200 kg), have massive incisor teeth to bite with and can run incredibly fast.

Why is hipposâ€™ sweat red?

The red or pink color of the hippo's sweat is due to the presence of two unstable and highly acidic compounds - hipposudoric acid, which is red, and norhipposudoric acid, which is orange. Although the two pigments are unstable on their own, they mix with mucus on the animalsâ€™ skin, leaving a colorful coating.

What do hippos use as sunscreen?

The mucus secretion on a hippo's body not only helps control their body temperature, but also acts as a potent sunscreen and antibiotic. The thick and sticky mixture is tough enough to survive the hippos' daylong soaks as it continues to protect against sunlight in both the ultraviolet and visible range.