A pair of evolutionary biomechanics experts from the Royal Veterinary College, University of London, discovered that hippos have all four legs in the air at all times when they are running at full speed. In their paper published in the journal Spouse JJohn Hutchinson and Emily Pringle describe how they filmed hippos running at full speed using high-speed cameras and what they discovered by analyzing the results.

The hippo has earned a reputation as one of the most dangerous animals in the world to humans. They are known to attack when provoked and to chase when needed. They are also quite large: males weigh an average of 1,500 kg, while females weigh 1,300 kg. They can run surprisingly fast for such a large animal; some can reach speeds of up to 30 km/h.

In this new study, Hutchinson and Pringle wondered how such a heavy creature could reach such speeds, and they noted that little research had been done on hippos’ gaits because they were so dangerous to walk around.

To find out, they travelled to the Flamingo Land resort in North Yorkshire and used high-speed cameras to study hippos’ gaits from a distance as they ran. They managed to capture 169 running cycles of 32 individual hippos on video.

Two researchers who examined the video found that hippos only ran at high speeds when they were being chased by another hippo, usually as a sign of dominance.

They also found that when they ran at top speed, they lifted all four feet off the ground at the same time at certain points in their gait—about 15 percent of the time during the run. Each occurrence lasted about 0.3 seconds. Further studies showed that hippos trot, not gallop, as rhinos do.

A pair of researchers also discovered that hippos run in water in a similar way; they can’t swim from place to place, so they trot along the bottom at high speed.