Zambia's Luangwa Valley holds so many hippos that park managers run an annual cull to keep the population from crowding out other species, and one carcass becomes the subject of a full dissection led by veterinary scientist Mark Evans and comparative anatomist Joy Reidenberg. Cutting through skin roughly an inch thick, they find it secretes its own natural sunscreen, one of several adaptations that keep an animal built like a barrel alive in equatorial sun and water. The stomach contents, thick with half-digested grass, confirm how much a hippo eats in a night of grazing that most people never see, since the animals are largely nocturnal despite their reputation for lounging in rivers by day. Simon Watt tracks down dung to work out why hippos flick their tails while defecating, spraying waste as a territorial signal. Richard Dawkins explains the evidence linking hippos to whales, and the dissected larynx turns out to share structural features with a whale's, physical proof of a shared ancestor most viewers would never guess from looking at either animal.