Cold is better for polar predators Generally, biodiversity is higher in the tropics than at the poles. This pattern is present across taxa as diverse as plants and insects. Marine mammals and birds buck this trend, however, with more species and more individuals occurring at the poles than at the equator. Grady et al. asked why this is (see the Perspective by Pyenson). They analyzed a comprehensive dataset of nearly 1000 species of shark, fish, reptiles, mammals, and birds. They found that predation on ectothermic (“cold-blooded”) prey is easier where waters are colder, which generates a larger resource base for large endothermic (“warm-blooded”) predators in polar regions. Science, this issue p. eaat4220; see also p. 338 Marine mammal and bird diversity is highest in polar regions, owing to the availability of cold, slow prey. Marine mammal and bird diversity is highest in polar regions, owing to the availability of cold, slow prey.