A new evolutionary theory could explain the mystery of shrinking animals

In animal evolution, a classical yet controversial idea posits that bigger is better. Bigger animals can reign supreme as hunters, fighters, and mates by simply out-bulking the competition. Living large enables easier survival, so the long-held theory goes. Called “Cope’s Rule,” after paleontologist Edward Drinker Cope, the concept first emerged among paleobiologists in the 19th century. Initially, Cope’s Rule seemed like a good explanation for the mega mammals like wooly mammoths and saber tooth tigers that once roamed North America. For dinosaurs, too, Cope’s Rule appears to fit. But exceptions to the trend soon became apparent.