The bat that never carb-loads
A team led by Kenneth Welch at the University of Toronto Scarborough, working with Giulia Rossi, ran 24 adult common vampire bats (Desmodus rotundus), all captured along known flight paths in the forests of Belize, on tiny treadmills. The bats were tested at three paces, 10, 20, and 30 meters per minute (up to about 1.1 mph), while researchers measured every breath. Not every bat cooperated. Welch has described some of them hooking their thumbs into crevices on the treadmill to keep from running at all, holding still the way a person might dig in their heels, an image that says more about the animals’ personalities than about their metabolism.
What the team tracked was the ratio of oxygen taken in to carbon dioxide breathed out, a number that in humans, dogs, horses, and every other mammal tested this way climbs steadily as effort increases: the body burns stored fat at rest and shifts toward carbohydrate as the pace rises. The study, published November 6, 2024, in the journal Biology Letters, found that in these bats the ratio sat at roughly 0.8 to 0.9 and stayed there, from the slowest walk to the fastest run. The ratio never climbed and never shifted. Whatever was fueling that run, it wasn’t fat and it wasn’t carbohydrate.
Breakfast to breath in ten minutes
To find the real fuel, the team held the bats without a meal for up to 18 hours to make sure any earlier feeding was fully digested, then fed them cow’s blood spiked with one of two isotopically labeled amino acids, either glycine or leucine, and put them straight back on the treadmill. Labeled carbon started turning up in their exhaled breath within about 10 minutes, and in large amounts. That is a direct trace of digestion becoming exhaust in real time: at peak effort, the labeled meal accounted for as much as 60% of everything the bats were burning. The amino acids weren’t stored as fat or converted to glycogen for later use; they were stripped down and oxidized in the muscle almost as fast as the gut could hand them over. In most mammals running on a treadmill, amino acids cover less than 5 to 10% of the fuel bill, with carbohydrate and fat doing the rest. In these bats, protein did almost all of it.
Why bother evolving this at all
Blood is the reason. It is nearly pure protein, with almost no fat and almost no carbohydrate in it, so an animal that drinks nothing else has nothing else worth stockpiling. Welch has pointed out that these fuels simply aren’t abundant in a vampire bat’s diet, which is likely why the bats never evolved to lean on them the way most mammals do. It’s even possible, as Welch suggests, that whatever ancestral ability vampire bats once had to burn fat and carbohydrate efficiently has quietly weakened over generations of living on blood alone, though that idea is a hypothesis, not something the study measured directly.
The common vampire bat is also the only one of the three vampire bat species able to run on the ground, using a bounding gait powered by its front limbs, more like a gorilla’s knuckle-walk than anything else in the bat world, to close in on a sleeping animal before feeding. Running hard on protein alone isn’t a mammal habit. It’s an insect one: tsetse flies (Glossina) power flight by oxidizing the amino acid proline, and female Aedes aegypti mosquitoes, the ones that actually take blood meals, run the same amino-acid economy the males never need. A mammal ended up solving its fuel problem the way a fly and a mosquito already had, because all three are stuck eating the same lopsided meal.
The fat-then-carbs rule wasn’t a rule
Exercise physiology has one pattern that has held up across essentially every mammal studied: burn fat at rest and light effort, shift toward carbohydrate as the pace picks up. It shows up in humans, in dogs, in horses, and it’s taught close to a universal law of mammalian metabolism. Common vampire bats break it outright. There’s no partial shift, no delayed climb across the 10, 20, and 30 meter-per-minute paces the team tested; the rise in fuel-use ratio that should appear as the treadmill speeds up simply never happens. The rule was never wrong about the animals it was built on. It just assumed every mammal has both a fat store and a carbohydrate store worth shifting between, and a bat living entirely on blood has neither.
The gap nobody’s filled yet
Two things remain genuinely open. Whether vampire bats burn amino acids to power flight the same way they do running hasn’t been tested directly, since flight costs far more energy than running and is much harder to measure on a treadmill; Welch and Rossi think it’s likely, but that’s a hypothesis, not a finding. And the researchers themselves have flagged a limit to their own data: a correction published in August 2025 noted an error in the originally reported cost-of-transport figures, which the team says weren’t central to their main conclusions but is a reminder that even a clean result gets revised.
There’s also a cost to living this way. Relying so heavily on amino acids, which aren’t as quick to mobilize as stored fat, may leave vampire bats more vulnerable to starvation if a meal doesn’t come through in time. It’s a fitting trade for an animal whose entire diet is a single, narrow, high-stakes commitment: one bad night without blood, and there’s no fat reserve waiting in the wings to cover for it.