This book examines four examples of animal physiology that illustrate emergent properties in whole organisms. The first example shows how mammals coordinate the activity of all their cells using a daily rhythm. The second case explains an apparent contradiction that happens every time a woman gets pregnant and delivers a healthy baby—how the immune system tolerates a foreign tissue such as the fetus. The next case study in this book shows how bodies regulate the amount of fat using a complex in­teraction of proteins that function as a lipostat, a self-regulating fat maintenance system. Finally, the book provides an understanding of why some species live long lives while others die after very short lives, and under what conditions each situation is favored. What is evolutionarily adaptive about death? These four case studies provide sufficient evidence to understand how animals regulate many of their own metabolic functions.