They pull off a trick previously thought unique to a few insects
Most mammals respond to the demands of exercise in the same way, by breaking down carbohydrates and stored lipids (fats) in their bodies, releasing the energy they need to keep functioning.
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 blood 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 some 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 Biology Letters 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. From an evolutionary perspective, it is impossible that these evolved in the insects and were then retained by bats without appearing in other mutual relatives. What is much more likely is that natural selection led both groups of animals to develop similar solutions to the same problem.
This article appeared in the Science & technology section of the print edition under the headline “Out for blood” (Nov 6th 2024)
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