Designed to give students a deep understanding of the central topics of introductory biology, E. O. Wilson’s Life on Earth is a captivating iBooks Textbook presented as a seven-unit collection. This fourth unit explores the physiology of animals, shining a light on complex systems of interacting structures and organs. We examine how animals sense their environments, move around, process energy, regulate their internal systems, protect themselves, and reproduce. Each unit of the engagingly interactive E. O. Wilson’s Life on Earth features video, photo galleries, and Multi-Touch images and illustrations that inspire students to study, enjoy, and help protect the wonders of life on Earth.