Rodents are the most numerous mammals on the planet, comprising an incredibly diverse variety of species. They range from the naked mole rat, which spends its entire life below ground, to the world's largest rodent, the capybara, which grazes in herds across the vast grasslands of South America. In the fourth of his ten-part epic series David Attenborough shares his fascination with these animals, which stop only at his pet hate - rats.