A 19th century claim has fueled a 21st century debate about how a warming climate might reshape animals. Beginning in the early 1800s, biologists identified multiple “rules” describing the ecological and evolutionary impacts of temperature. One rule held that animals have bigger appendages (ears, beaks) in hot climates, to help dissipate body heat. Another said that, within any group of animals, the biggest generally reside closer to the poles—think of polar bears towering over midlatitude brown bears—because larger bodies help retain heat.