Do hippos really sweat blood?

Watch a hippo haul itself out of a river on a hot afternoon and you might notice something alarming. Its skin starts to glisten pink, then red, as if the animal were quietly bleeding all over. For centuries people assumed exactly that. The truth is stranger and a lot more useful to the hippo.

The short answer

Hippos do not sweat blood. The reddish film is a skin secretion that contains two pigments, which scientists named hipposudoric acid (red) and norhipposudoric acid (orange). Lab tests suggest the mixture works as a natural sunscreen, and the red pigment also slows the growth of certain disease-causing bacteria.

The real story

In 2004 a team led by chemists Yoko Saikawa and Kimiko Hashimoto published a short paper in Nature titled, wonderfully, The red sweat of the hippopotamus. Their samples came from two hippos at Ueno Zoo in Tokyo, whose keepers gently wiped the animals’ skin with gauze swabs.

The secretion starts out colourless. Within minutes it turns red-orange, and over the following hours it slowly darkens to brown as the pigment molecules link together. The researchers found the pigments are surprisingly acidic and very unstable on their own, and they think mucus in the secretion helps them hold their colour for longer.

When the team measured how the pigments soak up light, both absorbed strongly in the ultraviolet range. That is exactly what you want from a sunscreen, and a hippo needs one. Hippos spend a lot of time in and around water, often out in the open under the strong African sun, with almost no hair to shade their skin.

The red pigment also held back the growth of two bacteria, Pseudomonas aeruginosa and Klebsiella pneumoniae, in lab tests. That fits an old observation from people who work with hippos: despite living in muddy water and fighting with those enormous teeth, their wounds rarely seem to become badly infected.

And it is not quite sweat either. Hippos do not sweat the way we do. According to hippo expert Keith Eltringham, the liquid comes from larger glands that sit deeper in the skin.

Why people get it wrong

The colour does most of the work. A large animal that suddenly looks streaked with red is going to make anyone think of blood, and the idea of a beast that bleeds when it is angry or hot made for a great traveller’s tale. Before anyone could analyse the chemistry, "blood sweat" was the most obvious explanation for what people saw.

The second reason is that the real answer sounds too clever to be true. A huge animal producing its own sun protection and antiseptic in one handy squirt seems more like a skincare advertisement than biology. But the chemistry has been checked, and the hippo was simply ahead of the beauty industry by a few million years.

“Built-in sunscreen and antiseptic, all in one. I’ve been under desks for thirty years and never once thought to make my own.”