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Michael Brooks: Hippo fossils offer clues about swimming

Fossilised guides to what the earth was like millions of years ago are rare, and understanding water tracks can make a difference.

Water horses: a mother and baby hippo swim at a zoo in Mexico City. Photo: Getty
Water horses: a mother and baby hippo swim at a zoo in Mexico City. Photo: Getty

Swimming is unlikely to be something you think about in scientific terms. If you’re doing it at the beach, it’s meant to be fun. And if you’re at a pool, it’s relaxing, or part of an exercise routine. To many scientists, though, swimming is an aide to understanding our place in the world.

Three British scientists have just published their observations of hippopotamus swimming. And not by any old hippo – they were interested in the swimming style of a now-extinct species called Hippopotamus gorgops.

Their interest was piqued by some peculiar tracks in what was once a small lake in northern Kenya. The tracks suggest that 1.4 million years ago a hippo had pushed itself off from the edge, before half walking and half swimming across the lake. It would have been the first observation of fossilised swim tracks made by mammals, so the scientists went to watch modern hippos swimming in a shallow pool to see whether the tracks made sense. They did: the observed hippos paddled through a pool, leaving similar hoof marks on the bottom.

Fossilised guides to what the earth was like millions of years ago are rare. We have to take advantage of every available clue, and understanding water tracks can make a difference. Last year, fossilised tracks that had been associated with a dinosaur stampede were reinterpreted as a river crossing.

We are unlikely ever to find out exactly how land-based dinosaurs swam, but it’s a good bet they looked nothing like we do in the pool. Most four-limbed land creatures swim doggy-paddle. When researchers took chimps and orang-utans for a swim, however, they found breaststroke was the natural instinct for staying afloat. It is probably easier for creatures built with shoulders and hips which have evolved to reach for branches to move limbs parallel to the surface of the water.

Not that our fastest swimmers use breaststroke any more. The front crawl has been in human use for thousands of years, but our observations of the natural world have made it even more efficient. Watching the swimming events at the Commonwealth Games, you’ll see several tricks copied from more natural swimmers.

At the turn, for instance, the competitors will empty their lungs, making them less buoyant. Doing this enables them to use their energy to move forward rather than maintain depth, which is why marine mammals and birds exhale deeply before diving into water.

It is important to remain streamlined when under water, so the leg kick has to be kept to no more than one-third of the body’s length – which is how much the dolphin’s tail moves naturally. Competitors are allowed to swim no more than 15 metres of each length under water and they will make the most of this, as the water there offers one-fifth the resistance of the drag just beneath the surface, research shows.

Swimmers can gain an advantage by mimicking penguins’ and dolphins’ streamlined shape and smooth surface. Hence, swimsuits are engineered to hold the human body in an optimum shape and to allow water to move past with minimal resistance.

That said, you can’t mimic everything in nature. Studies of the clawed frog showed the importance of a long push-off from the end of the pool: its pelvis actually slides along its backbone to give extra length to its legs. But not even the Olympic champion Michael Phelps could manage that. 

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