So many hippos congregate to feed in Zambia's Luangwa Valley that they threaten the survival of other species in the park, so the authorities cull around 200 of them every year.

The cull offers veterinary scientist Mark Evans and comparative anatomist Joy Reidenberg the opportunity to dissect one of these magnificent animals.

Hippos are often mistakenly seen as Africa's laziest giant, lolling around all day in the mud. But as the team discover, at night they're surprisingly active.

The first obstacle in the dissection is the hippo's inch-thick skin. This acts as a protective shield against the foot-long canines of rival hippos. Mark and Joy are amazed to discover that the skin produces its own sun cream.

As they delve deeper into the guts and weigh the contents of the stomach, the vast quantities of half-digested vegetation confirm the hippo's reputation as a gluttonous feeding machine.

Meanwhile, Simon Watt searches for hippo dung to find out why these grazers incessantly flick their muck using their short tails.

Richard Dawkins reveals the surprising fact that the hippos' closest living relatives are whales. As the dissection draws to a close, Joy finally succeeds in extracting the hippo's voice-box and finds a remarkable similarity with their ocean-dwelling cousins.

Film Duration: 47 min