Only three mammalian species are sanguivorous—that’s blood feeding—and they are all bats. Blood, apparently, is not that nutritious. It has almost no carbs, fats, or vitamins; its high iron levels can disrupt heart, liver, and pancreas function; its obscenely high protein and salt levels can cause renal disease if nitrogenous waste products build up. It contains pathogens. It clots.

Vampire bats have some obvious adaptations to allow them to survive on their limited and macabre diet. They have sharp incisors and canines, clawed thumbs on their wings, and use infrared sensing to find blood vessels in their prey. Their kidneys are hyperactive so that they can effectively excrete the urea that comes from all the protein in blood, and they have enhanced immunity to deal with blood-borne pathogens.

How they manage all that, however, remains largely unknown.

To identify the genetic bases of these adaptations, an international consortium of geneticists just sequenced a reference vampire bat genome. The geneticists then compared it to the genomes of carnivorous bats, insect-eating bats, and fruit-eating bats. Their findings appear in Nature Ecology and Evolution.

Since the geneticists were looking for genetic changes that allowed for adaptation to a specialized diet, they knew that the microbiome (the microbes living in the animal's digestive system) was likely involved as well. So they treated the bats and their resident microbiomes almost as a single organism—a holobiont—and sequenced not just the bats’ genomes but also those of their resident microbes (from feces, obviously) as well. Together, these sequences comprise each organism's hologenome.

Since selective pressures act on all of these genomes simultaneously, the authors argue that evolutionary studies can no longer consider them in isolation but must include them all.

As expected, they found some genomic adaptations to feeding on blood, or sanguivory. People had previously found some evidence that the vampire bats’ bitter-taste receptor gene had lost some of its capability and posited that it wasn’t under evolutionary selection anymore. The new work, however, shows that variations in the gene are still being selected for.

Most of the genomic changes they found helped deal with the nutritional challenges of the blood diet, especially the lack of sugars and fats. But these genomic changes could not completely account for the bats’ ability to thrive solely on their dubious food source. Vampire bats rely on their resident bacteria for that.

The taxonomic composition of the vampire bat microbiome was different—but not entirely distinct—from that of bats with other diets. They shared some species in common in their guts, but, at the functional level, they are almost completely distinct; their bacterial species are doing very different things biochemically. The vampire bat microbiome helps with the nutritional aspect of blood sucking by aiding with carbohydrate metabolism and synthesizing essential cofactors and vitamins lacking in blood.

The microbes also deal with most of the challenges of coagulation by providing pathways for degrading the coagulant heparan sulfate. They manage iron toxicity as well by enriching the iron-storing protein ferritin. Amazingly, these bacteria also contribute to the fight against pathogens by producing antiviral compounds.

Surviving on blood can be done, but it is not easy. Perhaps that’s why the ability to do it evolved only once in mammals, even with all of the help from their bacterial residents. And it happened in the American tropics—not Transylvania.

Nature Ecology and Evolution, 2018. DOI: 10.1038/s41559-018-0476-8 (About DOIs).