Putting vampire bats on treadmills reveals an unusual metabolism

Vampire bats have become such specialized bloodsuckers that they metabolize their food more like some blood-feeding flies than like other known mammals, a new experiment shows.

The common vampire bat (Desmodus rotundus) doesn’t stick to flying for moving around. It also scurries along the ground in targeting cattle, horses and other mammals to steal blood. Thus researchers, who were curious about what fuels the animals use, could coax the vampires — at least for a little while — to run on a bespoke-for-bats treadmill.

A substantial portion of the carbon dioxide the bats exhaled during a workout (up to 60 percent) came from metabolizing fuel other than the carbs or fats that typically power a running mammal, the team reports November 6 in Biology Letters. Instead, much of the energy bats were using came from a recent high-protein meal of cow blood; the gases contained telltale traces from amino acids, the building blocks of proteins.

Another clue to the fuel a runner is using (the ratio of carbon dioxide exhaled to oxygen used) stayed steady as researchers pushed the treadmill pace from walking to running. That steadiness is a sign that bats weren’t fueling their exertions the usual mammal way. Gas ratios typically change as exercise gets more strenuous and mammals shift from burning mostly lipids to relying more on carbs.

The discovery means the vampire bats power along in large part by metabolizing a recent blood meal, much like blood-feeding tsetse flies or some female (and therefore bloodsucking) mosquitoes. That’s a first for mammals, thinks Ken Welch, an ecological physiologist at the University of Toronto. He did the experiments along with physiologist Giulia Rossi, now at McMaster University in Hamilton, Canada.

The basic bat chemistry isn’t that unusual. “In most of us mammals,” Welch says, some kind of biochemical pathways can break down amino acids for fuel or some other use, but “they’re built to slowly churn along in the background.”

This fuel source might matter to hibernating bears or overwintering penguins, he says, “but these are extreme examples.” And even then, the penguins and bears “are not using amino acids they ingested in a meal only minutes before.”  

The vampire bat blood diet seems relatively poor in the kinds of nutrients that let most mammals build up their stores of more conventional mammal fuels such as lipids. So it makes sense to Welch that any ancestral ability to use sugar by-products and fats weakened over time in vampire bats. Yet such heavy reliance on amino acids, which aren’t as easy to access quickly, means the bats could be more susceptible to starvation. These vampires get thirsty often.