Why scientists put vampire bats on treadmills

It might sound like something from a popular horror novel, but vampire bats running on treadmills is a serious research piece that aims to gain a better understanding of a mystery that has surrounded vampire bats; what is going on with their metabolism that lets them keep up such an active lifestyle, on a diet so heavily propped up by blood? Ken Welch at the University of Toronto explains…

Ken - Blood is very rich in proteins. Think of all the haemoglobin that's floating around in our red blood cells and all of the albumin and the other proteins that make up part of the plasma in our blood. But the other components of the diet that we think about other than proteins are carbohydrates, sugars, and starches, and then fats. And in a traditional mammalian diet or a typical mammalian diet, it is a diet that's relatively well-rounded, having lots of carbohydrates in it, some fats and some proteins, some amino acids. So their diet is just kind of really lacking almost all of the carbohydrate and lipid components and has this excess of protein and an excess of iron because a lot of that protein is the haemoglobin that has the iron in it.

Will - So something must be going on for them to still be able to run so efficiently and quickly. How does that go from you theorising that, to a bat ending up on a treadmill?

Ken - <laugh> Yeah. Well, if we wanted to investigate the metabolism of flying things like bats and hummingbirds and other birds, for most of those animals, you need to let them fly in a wind tunnel. But it's really hard to study the physiology of an animal that's flying around freely. And it's very difficult to get access to a large flight tunnel where you can really interrogate these things. But with vampire bats, they don't hover. But we knew that they were actually pretty good on the treadmill. And it was in conversation with those scientists in Belize in between other experiments that I kind of became convinced that it would be really quite possible to get them to run on the treadmill for extended periods of time so that we could actually measure their metabolism while they were doing this.

Will - So by analysing what comes out when they respire, you're able to see what they're using as fuel for this running.

Ken - So the oxygen that you and I are taking in from the environment around us, and we're breathing out CO2, other animals are doing that as well. And so just like we can put a human on an exercise bike or on a treadmill so that we can capture their exhaled breath. Well, we can't necessarily convince a vampire bat to wear a mask, but if we make the treadmill small enough, we can stick it inside of a chamber and we can measure the air coming out of the backside of that chamber where that bat is running, which includes some of its breath.

Will - So what did you find out?

Ken - First and foremost, we found out that they're oxidising chemicals, that they're producing energy, to sustain running at a very high rate. But in addition to that, we wanted to actually say what fuel source are they burning? And for that we needed a way to kind of label something and look for it to be produced. And what we're looking for in this case is a certain, if you will, flavour of carbon dioxide, carbon dioxide that has a carbon-13 molecule in it rather than a carbon-12 molecule. These are isotopes of carbon. Your audience might be familiar with carbon-14. That's the radioactive form of carbon that's used in radiocarbon dating. Well, carbon-13 is not radioactive, so we don't need extra permits to work with it. But if we can buy certain chemicals, be they sugars or lipids or amino acids that are very, very enriched in this rare form of carbon, we can use a special laser technique to measure that and tell the difference between those two forms of carbon. So we gave the bats a blood meal just prior to running them on the treadmill, to which we had added a small amount of the amino acid glycine or the amino acid leucine that was enriched with this carbon-13 molecule. And if the bats started to use those amino acids as well as the amino acids in their blood meal as their fuel, then when they're oxidising those things in their muscle mitochondria, that's what's becoming the CO2 that's being breathed out. So we look for the signature of that special heavy form of carbon dioxide coming out in their breath. And we saw that very, very rapidly. That signature went very high, as high as 60%, meaning 60% of the CO2 molecules that they were breathing out had the signature consistent with the amino acids in the meal we'd just given them. It looked like they were relying mostly or exclusively even on the amino acids, the building blocks of the proteins that were there in that blood meal. So not only were they, yes, seeming to power their running essentially exclusively with protein oxidation, but they were doing so using amino acids that they'd ingested just moments before. So they were bringing these into their system and using them extremely quickly.

Will - Do we have any idea how they might be doing this?

Ken - So what we think is going on is that in their cells, in their muscle cells especially, the biochemical pathways that utilise carbohydrates or that utilise lipids to burn them for energy, are expressed at lower levels potentially. And they've really ramped up the biochemical pathways that do break down amino acids and have just sort of supercharged those. That said, we want to understand if the way that vampire bats metabolism has been supercharged to use amino acids, is it exactly the same as the way that, for example, the tsetse fly or a female mosquito that feeds on a blood meal, is it the same way that their pathways are enhanced or are there differences? Are all amino acids equally good as fuel if you're a vampire bat? Lots of those questions are still left to be explored and reasons to go down to Belize the next time around and study them again.

Will - And just one last question. And not that I'm worried about it at all, but I feel compelled to ask how quickly can they run, just out of interest?

Ken - Well we were pushing them to 30 metres per minute, that's almost a hundred feet per minute. Now, you and I can probably outrun that, but we didn't get them to their maximum. They can potentially go faster. We didn't have time to test it. And back in 2005, the treadmill that Dan Riskin and John Hermanson used was run with a drill to make the rotor spin and they couldn't get it to go any faster than the maximum that they hit. So these guys still have some surprises up their sleeves as it were.