How blood-sucking vampire bats get their energy

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.Tsetse flies and female mosquitoes fuel their energetically costly flying activities by directly burning the amino acids that make up the proteins found in their food. It is an extraordinary metabolic trick that only these insects were thought to have mastered. To find out if vampire bats were doing it too, Drs Rossi, Welch and their fellow stakeholders went vampire-hunting.The researchers travelled to a tropical forest in Belize, captured 24 adult vampire bats and then took them back to a nearby lab. Their goal was to feed the bats blood, get them to exercise and then monitor their breath for chemical signals of the metabolic processes at play. Putting the bats in wind tunnels where their breath could be monitored as they flew was too challenging. Instead, Drs Rossi and Welch made use of the unsettling fact that vampire bats are actually pretty good runners and built a customised bat treadmill.To work out where bats were getting their energy, the animals were fed a meal of cow’s blood roughly eight minutes before being placed in the centre of the treadmill chamber. Most bats consumed blood that had one of two amino acids (either leucine or glycine) chemically labelled beforehand. Three were kept as controls and fed unlabelled blood. The researchers report in that the exhalations contained the chemical labels. This means that the bats were using the amino acids from the blood they had recently consumed as their primary source of fuel as soon as they began exercise. Had they been burning lipids and carbohydrates, as all other mammals do, their exhalations would have been label-free.These findings suggest that vampire bats and blood-feeding insects have metabolic systems that are very much alike. In an extraordinary example of convergent evolution, it is likely that natural selection led both groups of animals to develop very similar solutions to the same problem.