Many mammals consume blood as part of their diet, but blood is actually a pretty poor source of energy. Only bats (order Chiroptera) include species that feed exclusively on blood.
So how do vampire bats manage to survive on such low-grade nourishment? A recently published article in Nature Ecology & Evolution provides part of the answer. The bats had to evolve in tandem with their microorganisms.
The challenges were clear. Blood consists of 78 per cent liquid. The remainder is 93 per cent proteins and only one per cent carbohydrates. Blood provides very little in the way of vitamins. On top of all that, a blood-based diet exposes these animals to blood-borne pathogens.
To find the answer, researchers had to look at the vampire bat's genome.
So now the genes of the common vampire bat (Desmodus rotundus) have been thoroughly investigated. But not only its genes.
Professor Tom Gilbert is the senior author of the Nature article and has collaborated with PhD student and first author Lisandra Zepeda Mendoza on this research.
Gilbert works at the Centre for GeoGenetics at the University of Copenhagen, and also holds a part-time position as an adjunct professor at the Norwegian University of Science and Technology's (NTNU) University Museum.
"Coping with this kind of diet requires one species to co-evolve with other species," says Gilbert.
But exactly which other species vampire bats have co-evolved with may not be immediately apparent.
Some vampire bat characteristics are easy to recognize. Vampire bats have developed specialized adaptations that enable them to access blood and then make use of it.
All three species of vampire bats are native to the Americas. Anyone who has watched animal programs or scary movies is probably familiar with some of their adaptive behaviors.
Everyone knows about bats' razor sharp teeth and especially their striking incisors. The teeth are practical for penetrating the skin of their victim. The bats also have specialized cells, called thermoreceptors, that can detect heat and are useful in finding bare skin on a sleeping animal at night.
Substances in the bat's saliva prevent the blood in a wound from coagulating. The bat's inner adaptations are at least as interesting as the more obvious outer ones. Their kidneys are specially adapted to cope with high protein content. Their immune system helps deal with any pathogens.
However, none of these known adaptations explains how vampire bats have evolved to rely exclusively on blood for their nutrition. This is what the researchers behind the recent Nature article set out to investigate.
This development seems to require one species to develop in tandem with other species. The way this happens may alter the way we perceive evolution: it turns out that some of the other species we co-evolve with may be found within us.
According to the article, a diet this specialized requires a highly specific adaptation of the genome of the species itself. But it also requires a uniquely adapted microbiome.
A microbiome consists of the entire genetic material of all the microorganisms that live in our bodies, whether we're talking about viruses, bacteria or fungi.
You and I and everyone you know are full of other organisms, maybe around 100 trillion of them. The actual number is controversial and subject to debate - but a lot of organisms at any rate.
Some folks may feel disgust at this thought, but there's little to loathe about them. We are completely dependent on other organisms to survive, and the vast majority of them are useful, or at least don't pull any bad pranks that you would notice.
We have even co-evolved with several of these organisms and share evolutionary history with them. At least that's the way it works with vampire bats.
"Our results show that vampire bats became blood sippers after their own genome and microbiome co-evolved closely," the Nature article says.
The genes of the vampire bats thus evolved along with all the microbes in their bodies.
In other words, in order to understand vampire bats you have to look at the bat's own genes and all the genes in the microbiome as a whole. This is what researchers call the "hologenome" - the genes of the host plus all its symbiotic microbial guests.
The common vampire bat has a unique hologenome. The vampire bat's microbiome helps to compensate for the lack of vitamins and various fatty substances in what would otherwise be an imbalanced diet. Its microbes also help the body get rid of waste and maintain the cells' fluid balance through osmoregulation.
The article's researchers emphasize the value they found in studying both the host and its interaction with the microbiome as they tried to figure out the adaptations that underlie the vampire bat's radical diet.
The bat's ability to survive on a diet that would be inadequate for other mammals is in fact only made possible through the help of the microbiome.
"But the main finding probably applies to all animals in regards to their diet, whether we're talking about cows and grass, vultures and carrion or koalas and eucalyptus. We have to look at both the animal itself and the collective microbes to understand what's happening. Now we've arrived at a point where this is possible both technically and economically," says Professor Gilbert.