Many mammals and birds are remarkable athletes; mice work hard to dig burrows for protection and sparrows fight gravity with each flap of their wings. In order to have the energy to sustain vigorous exercise, the body’s tissues need a steady supply of oxygen, and red blood cells (RBCs) are the center of the oxygen delivery system. Size matters, too; athletic mammals and birds have much smaller RBCs than other vertebrates with lesser capacities for exercise. Biologists have long been puzzled over the evolutionary origins of RBC size. Were predecessors of mammals and birds—including dinosaurs—athletes and did they have tiny red blood cells? How do you measure the blood of extinct animals?