JOHN DANKOSKY: This is Science Friday. I’m John Dankosky. Ira is away. Later this hour, we’ll talk about the ethics of research on brains.

But first, do you shudder at the thought of a blood-drinking vampire bat? Pretty creepy, right? Well, it turns out that a diet of blood isn’t exactly the best way to eat, and so vampire bats have to consume more than their body’s weight in blood every single night. In fact, vampire bats are the only mammals on Earth where blood is the only thing on the menu. So how do they evolve to rely on this nocturnal nourishment?

My next guest went looking at their genes for clues and found some surprising changes. Dr. Michael Hiller is Professor of Comparative Genomics at the Center for Translational Biodiversity Genomics. He’s based in Frankfurt, Germany. Welcome to Science Friday, Dr. Hiller. Thanks for being here.

MICHAEL HILLER: Hi, John.

JOHN DANKOSKY: So we’ve talked about vampire bats on the show before. In fact, they’re one of our charismatic creatures that we’ve spoken of. But tell us how does drinking blood require vampire bats to be different from other bats.

MICHAEL HILLER: So drinking blood is a very unique diet that among mammals evolved only once in this lineage that comprises these three living vampire bat species. And so blood is– as you already said– not a really good diet. It has a low caloric value, most of it is fluid. Of the nutrients, most of what these vampire bats get is protein. So it’s really a challenge to live and survive with such a diet.

Typically, you need sugars, for example, to nourish your brain and other organs. Fatty acids are very powerful or very energy-rich nutrients, and proteins are not so much. So it challenges really that these vampire bats get these highly biased diet. So essentially, of the dry mass of blood, people estimate about 93% is protein and only about 1% each is carbohydrates, and sugars, and fat.

JOHN DANKOSKY: They have to drink an awful lot of blood to get what they need to be able to survive, to fly around all night.

MICHAEL HILLER: Yeah, that’s right. This is something– they likely need to get enough nutrients to actually make it through the day until the next night when they are flying out again to feed. Their stomach, which is typically a digestive organ that secretes acid, has been converted into a dispensable storage organ that keeps the blood that they ingest until the gut is able to process it.

JOHN DANKOSKY: Interesting. So with all that in mind, what exactly were you looking for in the genome of vampire bats?

MICHAEL HILLER: So we were interested in how this very specialized diet actually evolved, and what are the genomic underpinnings to adapting to this really challenging diet. And we had a hunch that if you live on such a very specific diet, maybe some genes that other animals need that have more mixed diets would no longer be important for a vampire bat, and that we find inactivations or losses of these genes in the genome of the vampire bat.

JOHN DANKOSKY: So why exactly would gene losses be interesting? Why would that help to explain how this animal came to be?

MICHAEL HILLER: So there are two main explanations for why a gene can get lost in evolution. What we thought would likely happen is the so-called use it or lose it explanation. So if you have a certain gene and you require this, selection will preserve also the gene sequence, and this will encode a functional protein because the protein function is, at the end, what really matters.

The other explanation in a way is so-called less is more. We were hoping to find also a few of these cases when we look at the vampire bat. And less is more describes the case when you inactivate a gene that this can contribute to an adaptation. So it could be beneficial for certain species that adapted to certain environmental conditions, or in this case, this very specific diet.

And if you think about this less is more, this is, at first glance, counterintuitive because we hear about gene inactivations and mutations in genes typically in the context of disease. But in evolution, sometimes it can be that taking away a gene is actually beneficial. We found examples of this less is more in other bats and other species in the past, and so we were also curious about whether we can find such adaptive or beneficial gene losses in the vampire bat.

JOHN DANKOSKY: All right, so let’s talk about some of these interesting gene losses you found in vampire bats. And the first one we want to talk about relates to how they process all of this iron in their diet. What exactly is happening here?

MICHAEL HILLER: Yeah. So a blood is not only biased in terms of nutrients. It also contains a lot of iron. And it’s estimated that the dietary iron intake of vampires is about 800-fold higher than in humans. And that’s something those vampires [INAUDIBLE] we have to deal with. They have to cope with this challenge.

And so what we found is that one of these 10 previously undescribed gene losses, one of these is a gene that is specifically expressed in the gastrointestinal tract. And the function of the gene, which we know from studies in mice, is to inhibit iron uptake from the bloodstream into these intestinal cells.

The master’s student, Moritz Blumer, who was working on the project, he developed the hypothesis that maybe what’s going to happen here is that if you lose this gene, these intestinal cells are able to take up more iron from the bloodstream. And because these cells have a very short half-life and get frequently replaced by new cells, these cells, and with it the iron they contain, would get excreted out of the body. So the gene loss could facilitate a more efficient iron excretion mechanism in vampire bats.

And to our big surprise– and this was, for me, some kind of Eureka moment– Moritz then found a study from the 1980s that had analyzed the distribution of iron in the gastrointestinal tract of the vampire. And in this paper, they were showing that the outermost cells of the intestine, they are packed full of iron.

So we can really visualize that these cells contain a lot of iron. And they could also visualize that these cells– and with it, the iron– are present in the feces of vampire bats. And that really provides evidence that this iron excretion mechanism exists in vampire bats, and that’s certainly one part of how they deal with this, in a way, iron challenge.

JOHN DANKOSKY: That’s so interesting. So instead of some sort of a super gene that allows them to take in iron and process it in a way that’s better than other animals, this is essentially a gene loss that lets them just slough off this extra iron in a really unique way.

MICHAEL HILLER: Yeah. So it’s certainly the case that this gene loss is not the only genomic change that contributes to the ability to handle and to cope with this large amounts of iron. But in this case, we believe the losing this inhibitory gene like a built in break in a system, that also makes a contribution to coping with this iron challenge. And this then would be an example of this less is more principle that describes when losing genes can be beneficial.

JOHN DANKOSKY: That’s very cool. Well, let’s move on to another one of these genes. And on our program, we’ve talked before about how smart vampire bats are. So tell us what you found in the genes associated with these bats’ brains.

MICHAEL HILLER: One of the other genes that we found is an enzyme that is expressed in the liver. The other place where this gene is expressed is the brain. And in the brain, it’s the only enzyme that is able to degrade, to metabolize a certain cholesterol compound. And this cholesterol metabolite is very interesting because this metabolite stimulates receptors. These receptors mediate learning, memory, and social behavior.

And there were a number of studies that showed if you increase the levels of this cholesterol compound, then memory, and learning, and social behavior is enhanced. And if you experimentally reduce these levels in rodent models like mice or rats, then memory, learning, and social ability is reduced.

If the only enzyme in the brain that can break down this important cholesterol metabolite, if that gene no longer exists in a vampire bat, we would expect that the levels of this cholesterol metabolite are increased. And with this, we can predict that this should then have a positive effect on social behavior and cognitive function.

And I should say, when we started this project, I was mainly interested in the dietary adaptation and what kind of solutions to the physiological challenges vampire bats have evolved. What I didn’t know about that they are maybe among the most socially advanced bats that are out there. They are very starvation-sensitive so they really have to ideally feed every night. Otherwise, they are risking starvation.

And to alleviate this problem, they are helping each other out. So one bat that was able to feed can regurgitate blood, and therefore, feed another individual that was not able to feed that night. And they tend to share their food with other individuals that were helping them out in the past. And this requires some long term social memory. So we have to remember individuals and what they did to you in the past. And that’s a pretty unique behavior among mammals.

What probably– maybe enables these special cognitive functions and this social behavior is that among 270 bats where such measurements exist, the vampire bats have the largest brain volume compared relative to the body size. So these social behaviors are likely linked to a larger brain, and we think that losing this enzyme that metabolizes this important cholesterol metabolite, that enhances memory and learning is something that is maybe connected to this advanced social behavior that these species show.

JOHN DANKOSKY: You seem pretty enthusiastic about studying these bats.

MICHAEL HILLER: Oh, yeah. Absolutely. For me, it was fascinating to learn more about these species, learn more about the genes they lost, what they do, what they do in other animals, and how we can connect them to traits, to something that has changed in a vampire bat. But I think there’s much more to learn. So gene loss is certainly only one type of evolutionary change.

We’ve sequenced the other two living vampire bat species, and with this, we would like to conduct a broader, more comprehensive study to look into the genes and maybe also the regulatory elements that determine where and when these genes are expressed to learn more about the genomic basis of adaptations to blood feeding, and, in a way, to get a more complete picture on what happened at the molecular, at the genomic level that allowed these adaptations.

JOHN DANKOSKY: Dr. Michael Hiller is Professor of Comparative Genomics at the Center for Translational Biodiversity Genomics in Frankfurt, Germany. Dr. Hiller, thank you so much for joining us.

MICHAEL HILLER: Thank you very much.

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