Overview: Ecologist Professor Sue Hartley continues to show how the epic 300-million-year war between plants and animals has shaped us and the world we live in. The life of a herbivore is not a happy one. For a start, plants are the wrong sort of food for animals: they are low in essential nutrients and getting any of those nutrients in the face of flora defences is even harder. In this programme, Professor Hartley reveals the many different ways plant-eating animals, from sloths to aphids, have evolved to overcome these problems. Herbivores use all sorts of tricks: they employ "friendly" bacteria in their gut to extract as many nutrients as possible from indigestible plants. They also have continuously growing teeth to grind down tough plants like grasses. Professor Hartley also reveals some of the many ways herbivores cope with plant poisons, and that some herbivores even steal plants' poisons to use in their own defence against their predators