Hippos Make Their Own Sunscreen

Picture a hot afternoon on an African river. A dozen hippos lie in the shallows with only their eyes, ears and nostrils showing above the water, like a row of smooth grey boulders that now and then yawn. Hippos spend almost the whole day like this, and for a good reason: out in the open sun their skin would soon dry out and burn. So when one of them finally hauls itself up the bank in the evening, it is not heading out unprotected. It is already wearing sunscreen, and it made that sunscreen itself.

Glands in a hippo's skin release a thick liquid that spreads across its body. When it first appears it is completely colourless. Within a few minutes of meeting the air it turns orange-red, and over the following hours it slowly darkens towards brown. On a big animal standing in the late sun, the effect is a glossy, faintly coppery shine, as though the hippo had been freshly polished for a special occasion.

That shiny coat does several jobs at once. It forms a layer that keeps the skin moist and protects it from sunburn, which matters a great deal to an animal that is mostly bare skin and where a large male can weigh more than two family cars. It also sees the hippo through its night out. Once the sun has set, hippos leave the water to graze, often for around six hours, and a single hippo may walk three to five kilometres and eat some forty kilograms of grass before it is back in the river by dawn.

The coat also holds back germs. In laboratory tests the red pigment in it slowed the growth of bacteria that cause illness, even in small amounts. So the same film works as a sunscreen and a mild antiseptic at once, which is handy for an animal that spends its days in warm, muddy water shared with fish, birds and a great many other hippos.

Then there is the milk. A popular claim on the internet says that this coloured film turns hippo milk pink. It makes a lovely picture, a strawberry milkshake served straight from the river, but sadly it is not true. Hippo milk is white or beige, like the milk of other mammals. The pink milk story seems to come from mixing up two quite different things: the colour on a hippo's back and what a mother hippo feeds her calf.

The chemistry was worked out in 2004, when a team of chemists published their results in the journal Nature. They collected the liquid from hippo skin and found two pigments in it: a red one, which they named hipposudoric acid, and an orange one, named norhipposudoric acid. Both are acidic. When the team measured how the pigments absorb light, both turned out to soak it up most strongly in the ultraviolet range, the part of sunlight that causes sunburn, which is exactly what a good sunscreen does.

Where do the pigments come from? Whatever a hippo eats, it produces the same coloured film, so the colour does not seem to come from its food. Scientists think hippos build the pigments themselves from ordinary ingredients already in the body, such as the amino acid tyrosine. So the next time a hippo in a nature documentary climbs out of the river with a coppery shine on its back, you will know what you are looking at: sunscreen and antiseptic in one, made on the spot in its own skin, with no bottle to carry and nobody needed to reach the awkward bit in the middle of its back. That is more than most of us manage on a beach holiday.