This book examines the food chain, from solar radiation and photosynthesis to food sources for herbiviores and for carnivores, and compares the merits of different designs of digestive system, and of different strategies for finding and choosing food. The energy costs of motion and the energetic demands of growth and reproduction are then discussed. A chapter on body temperature shows how the processes of life go faster at higher temperatures, and discusses how animals regulate their temperature. A final chapter draws all of these aspects of energy use together, and considers the energy budgets of several different animals, assessing the different energy gains and costs of their everyday activities in the wild. The book is truly comparative and contains much practical information from relevant research.