Wallowing in sunscreen sweat is secret of hippos' silky skin regime

Hippos protect their hairless skin from the sun by sweating a sunscreen similar to commercial products used by humans, researchers said yesterday.

Hippos protect their hairless skin from the sun by sweating a sunscreen similar to commercial products used by humans, researchers said yesterday.

They said the animals secrete a colourless, viscous liquid - which also acts as an antibiotic - that gradually turns red and then brown as it turns into a plastic-like structure.

The key to the protective nature of the secretion, which enables the aquatic mammal to keep its skin smooth despite the equatorial heat and dust, lies in its later stages, after it turns brown.

A team at the Kyoto Pharmaceutical University in Japan tested the "red sweat" collected from a specimen at the Ueno Zoological Gardens in Tokyo by "wiping [its] face and back with gauze".

To their surprise, the team led by Yoko Saikawa discovered that although the sweat is alkaline when it is secreted, as it turns brown it becomes a strongly acidic substance - hundreds of times more powerful than vinegar - that works as a strong antiseptic. That could be useful, scientists suggest, to neutralise infection in any open wounds that three-ton fighting males might inflict on each other with their tusks.

But the thick layer also acts as a sunscreen and further studies discovered that it absorbs light, particularly in the ultraviolet range, just like commercial sunscreens. Being such a thick, sticky substance, it tends to stick to the skin for the day-long baths that hippos prefer, when they will linger with only their eyes, ears and nose above the waterline.

But there are a couple of drawbacks that might stop cosmetics companies from marketing hippo sunblock in the near future. The first is the rarity of the animals: only about 174,000 are thought to be alive, though they are notoriously difficult to count because they stay underwater where they can. That means that it would be hard to get enough of the "red sweat" for a commercial operation - even with teams of brave gauze-wipers.

The other problem is a far more basic one that is key to the red substance itself. According to Professor Saikawa, it really stinks.