For most mammals, blood's not typically the only item on their menu. It doesn't have much in the way of nutrients and it can harbour all sorts of nasty blood-borne diseases.
But a bacterial army and "jumping genes" help vampire bats get everything they need from blood meals, new research shows.
Key points:
- Common vampire bat (Desmodus rotundus) genome and gut microbiome were mapped to study the evolution of its diet
- Genes and microbes were found that help the bats digest blood, synthesise vitamins and stave off infections
- Studying the immune system of bats could help treat viral infections in humans
In work published today in Nature Ecology & Evolution, researchers found blood-eating bats have distinct genetic and gut microbe differences from their vegetarian and meat-eating counterparts that help them live on their grisly meals.
The creatures' extreme diet means they need loads of adaptations to survive, said Lisandra Zepeda Mendoza, a geneticist at the University of Copenhagen and co-author of the study.
"Sanguivory [blood-eating] is a very, very rare diet in the animal kingdom, especially if it's 'obligate sanguivory', meaning that they can only eat [blood]," she said.
Get the blood flowing
Only three species of mammal feed solely on blood, and they're all vampire bats.
They've evolved a few tricks to get better at feeding on blood, which they hunt for under the cover of darkness.
After sidling up to sleeping horses or cattle — or, sometimes, humans — they use heat-sensors around their nose to home in on a blood vessel under the skin.
A quick nip gets blood trickling out, which they slurp up. Anticoagulants ooze from the bat's tongue to prevent the blood clotting, and keep it flowing.
But blood is a challenging food source on which to survive. It contains very little carbohydrate and vitamins — and runs the risk of carrying all sorts of diseases.
To find out how vampire bats manage to get by, Dr Zepeda Mendoza and an international team of researchers mapped the common vampire bat (Desmodus rotundus) genome and the microbes that live in its gut, called the microbiome.
Genes that jump around
Part of the common vampire bat's ability to live with an extreme diet could be thanks to transposons — stretches of DNA that can multiply and shift around the genome.
It's why they're dubbed jumping genes.
Human DNA is rife with transposons. For instance, the "Alu element" is a DNA sequence about 300 letters long, but is repeated so often, it comprises around 10 per cent of our total genome.
The phenomenon was first spotted in the 1940s by American geneticist Barbara McClintock, when she noticed some maize genes seemed to be mobile.
(She scored the 1983 Nobel Prize in Physiology or Medicine for her efforts.)
In the common vampire bat, Dr Zepeda Mendoza and her colleagues found 1.6 to 2.26-fold more transposons in genes responsible for defending against viruses and metabolising fats and vitamins.
This particular type of transposon — called MULE-MuDR — has a "high mutagenic capacity", Dr Zepeda Mendoza said.
And by more readily mutating immunity and metabolism genes, MULE-MuDR transposons could have helped vampire bats adapt to the challenges of a blood diet faster.
"We're hypothesising that this is one of the mechanisms why that lineage was able to adapt or change some of its genes more rapidly than some of its other ancestors," study co-author and Curtin University geneticist Mike Bunce said.
But, Dr Zepeda Mendoza stressed, "This is our suggestion based only on their presence. Other types of analyses [are needed] to go deeper into this observation".
Microbial helpers
The bats' microbial residents, too, seem to lend a hand.
When Dr Zepeda Mendoza and her colleagues screened bacterial DNA found in bat faeces, they found signs that microbes help stop blood clotting as it works its way through the digestive system.
These were in addition to anti-clotting genes in the vampire bat genome.
"You'd expect to see changes in some key genes, but we've also seen some of that [anti-clotting] has been taken up by the bacterial community as well," Professor Bunce said.
Compared to the other bat microbiomes, the common vampire bat gut contained most enzymes that manufacture vitamins, such as carotenoid, that they can't get from blood alone.
Incredibly, the vampire bat gut microbiome also contained more than 280 bacterial species, many transmitted by blood-sucking ticks and fleas, that cause disease in other mammals.
So why don't they get sick and die — particularly if they also ingest diseases from blood?
Their secret seems to hinge on a first-line defence called the interferon response, said Michelle Baker, a bat immunologist at the CSIRO's Australian Animal Health Laboratory.
"In humans, if we're exposed to a pathogen, proteins called interferons are switched on," she said.
"That results in a cascade that switches on hundreds of other genes. It's those genes that are the anti-viral players that actually deal with the infection."
Our interferons are switched off when we're not infected.
"The reason we don't want it switched on all the time is it can be quite toxic. It's an inflammatory response so you only want to switch it on when you need to deal with something."
Too much interferon activity can cause autoimmunity in humans, where the immune system attacks healthy cells.
But bat interferons don't seem to have an "off" switch.
"In the bats that we focus on, black flying foxes, they have their interferons switched on all of the time, whether they're infected or not," Dr Baker said.
"You can see how this is an advantage as you can respond [to disease] far more rapidly, but the question is, 'Why don't they have autoimmunity or lots of inflammation going on?'"
Dr Baker suspects bats switch on a subset of genes that aren't as inflammatory as those in humans.
Examining bats' immune systems could help control viruses in humans, she added.
"If we can work out how to ramp up interferons in humans but ramp up only that subset of genes that are less inflammatory, you can imagine that would be a really good thing for dealing with viral infection."
