Most mammals respond to the demands of exercise in the same way. As they undertake low-intensity exercise, their bodies convert stored lipids (fats) into energy; when their physical activity ramps up, they start burning large amounts of carbohydrates from recent meals. But every good rule deserves an exception. Vampire bats, for example, feed only on blood: an energy drink low in lipids and carbohydrates, and rich in protein. This realisation led Giulia Rossi and Kenneth Welch at the University of Toronto to question how these animals were able to sustain intensely energetic activities like flight. There was a possibility that the bats were transforming their blood meals into carbohydrates which were then being burned. But Drs Rossi and Welch were happy to entertain a wilder hypothesis: that the bats might, instead, be able to feed off proteins in the way certain bloodsucking insects do.