Being able to fly opens up a whole new range of habits and lifestyles. A flyer can seek food from the tops of trees and airborne insects. It can avoid ground-based predators and rear its young safely out of reach. But flying is the most demanding form of locomotion, requiring a lot of power. ‘Flying mammals’ describes the single mammalian Order that has evolved flight—Chiroptera—with almost 1,200 species of bat worldwide, often living in colossal colonies. It also describes other airborne mammals that glide passively (using a membrane stretched between the limbs) rather than by actively flapping wings (e.g. colugos of Southeast Asia, Australian opossums, and flying squirrels).