Do Hippos Sweat Blood? The Truth About Red Hippo Sweat

Where does the “blood sweat” idea come from?

For centuries, people watching hippos in Africa and in zoos noticed that their skin seemed to ooze red fluid, especially when the animals were out of the water in hot weather or after fights. It looked like a mixture of blood and sweat, and the name “blood sweat” stuck. Early naturalists and zookeepers repeated the idea, and it still appears in books and online today.

The truth is different. The fluid comes from glands in the skin and contains no blood. Its red color appears only after it reaches the surface and reacts with air and light.

What is hippo sweat made of?

In 2004 a team of Japanese chemists led by Yoko Saikawa analyzed the secretion, collected from hippos at a zoo, and published the results in the journal Nature. They found two pigments that give the fluid its color:

Both pigments are strongly acidic and are unusual compounds not known from other animals. Later work suggested that hippos build them from simple chemicals made when the body breaks down amino acids such as tyrosine. All hippos produce them, whatever they eat, so the color does not come from their food.

Why does hippo sweat change color?

When the fluid first appears on the skin, it is clear or slightly cloudy. Within a few minutes it turns red and orange as the pigments form and react with oxygen in the air. Over several hours the color darkens to brown as the pigment molecules join together into larger chains.

The thicker the layer and the longer it sits on the skin, the darker it looks. That is why a hippo resting in the sun can look streaked with red, while one fresh from the water shows little color.

What does red sweat do for a hippo?

Scientists think the secretion helps the hippo in at least two ways:

• Sun protection. The pigments absorb ultraviolet light, the part of sunlight that burns skin. Hippos have very little hair, and although their skin is thick overall, the outer layer is thin and sensitive. The secretion forms a natural sunscreen on the bare skin.
• Protection against infection. In laboratory tests, the red pigment slowed the growth of some disease-causing bacteria. Hippos often have wounds from fights and live in muddy, dung-rich water, so a skin coating with antibacterial effects may help keep cuts from becoming infected.

The secretion also helps keep the skin moist. Hippos lose water through their skin much faster than many other mammals, and they can suffer if they stay out of water too long in strong sun.

How is hippo skin built?

Hippo skin is a mix of tough and delicate. Over much of the body it is about 5–6 cm (2 in) thick, which helps it withstand the slashing tusks of rivals. But the outer layer, the epidermis, is thin and almost hairless, and it dries out and cracks easily when exposed to air and strong sun for long periods.

This combination explains why the secretion matters so much. Thick skin protects the hippo in fights, while the red coating shields the delicate surface from sunlight and germs in between trips to the water.

Do hippos sweat to cool down?

Not in the way humans do. People cool off by producing watery sweat from millions of sweat glands; as it evaporates, it carries away heat. Hippos lack true sweat glands of this kind. Their skin glands produce the thick, oily secretion described above, which mainly protects the skin.

Hippos keep cool mainly by spending the hot hours of the day in water or mud and by feeding at night. Read more about their nightly grazing in What do hippos eat? and about their life in the water in Can hippos swim?

Myths linked to hippo sweat

Myth: hippos sweat blood. As explained above, the fluid contains no blood.

Myth: hippo milk is pink. This popular claim, widely shared online, is false. Hippo milk is white. The story seems to come from the idea that the red secretion mixes with milk, but there is no evidence for pink hippo milk.

Myth: the red color comes from their food. Studies found that hippos produce the pigments whatever they eat, so the color is made by the hippo’s own body chemistry.

Do pygmy hippos have red sweat too?

Much less is known about the skin chemistry of the pygmy hippopotamus of West Africa, because it is rare and hard to study in the wild. It lives mainly in shady forests and swamps, but like its larger cousin it has bare skin that loses water easily, so it stays close to rivers and wallows to keep its skin from drying out.